• Author / Uploaded
• Erick Espinosa

Citation preview

20th

Modern Refrigeration and Air Conditioning

Edition

by

Andrew D. Althouse, BS, (ME), MA Carl H. Turnquist, BS, (ME), MA Alfred F. Bracciano, BS, M.Ed., Ed. Sp. Daniel C. Bracciano, BSME Gloria M. Bracciano, BA, MA, Ed. Sp.

Publisher

The Goodheart-Willcox Company, Inc. Tinley Park, IL www.g-w.com Copyright Goodh art Wrllcox Co, Inc, 2017

Copyright© 2017 by The Goodheart-Willcox Company, Inc. Previous editions copyright 2014, 2004, 2000, 1996, 1992, 1988, 1982, 1979, 1975, 1968, 1960, 1958,1956, 1950, 1944, 1943, 1939, 1936,1933 All rights reserved. No part of this work may be reproduced, stored, or transmitted in any form or by any electronic or mechanical means, including information storage and retrieval systems, without the prior written permission of The Goodheart-Willcox Company, Inc. Manufactured in the United States of America. Library of Congress Catalog Card Number 2015039667 ISBN 978-1-63126-354-5 1 2 3 4 5 6 7 8 9 - 17 - 20 19 18 17 16 The Goodheart-Willcox Company, Inc. Brand Disclaimer: Brand names, company names, and illustrations for products and services included in this text are provided for educational purposes only and do not represent or imply endorsement or recommendation by the author or the publisher. The Goodheart-Willcox Company, Inc. Safety Notice: The reader is expressly advised to carefully read, understand, and apply all safety precautions and warnings described in this book or that might also be indicated in undertaking the activities and exercises described herein to minimize risk of personal injury or injury to others. Common sense and good judgment should also be exercised and applied to help avoid all potential hazards. The reader should always refer to the appropriate manufacturer's technical information, directions, and recommendations; then proceed with care to follow specific equipment operating instructions. The reader should understand these notices and cautions are not exhaustive.

The publisher makes no warranty or representation whatsoever, either expressed or implied, including but not limited to equipment, procedures, and applications described or referred to herein, their quality, performance, merchantability, or fitness for a particular purpose. The publisher assumes no responsibility for any changes, errors, or omissions in this book. The publisher specifically disclaims any liability whatsoever, including any direct, indirect, incidental, consequential, special, or exemplary damages resulting, in whole or in part, from the reader's use or reliance upon the information, instructions, procedures, warnings, cautions, applications, or other matter contained in this book. The publisher assumes no responsibility for the activities of the reader. The Goodheart-Willcox Company, Inc. Internet Disclaimer: The Internet resources and listings in this Goodheart-Willcox Publisher product are provided solely as a convenience to you. These resources and listings were reviewed at the time of publication to provide you with accurate, safe, and appropriate information. Goodheart-Willcox Publisher has no control over the referenced websites and, due to the dynamic nature of the Internet, is not responsible or liable for the content, products, or performance of links to other websites or resources. Goodheart-Willcox Publisher makes no representation, either expressed or implied, regarding the content of these websites, and such references do not constitute an endorsement or recommendation of the information or content presented. It is your responsibility to take all protective measures to guard against inappropriate content, viruses, or other destructive elements.

Cover images: Emerson Climate Technologies; Arkema, Inc.; Stride Tool Inc.; Danfoss; Tempstar Back cover image: Ritchie Engineering Co., Inc. - YELLOW JACKET Products Division

Library of Congress Cataloging-in-Publication Data Names: Althouse, Andrew D., author. I Turnquist, Carl H., author. I Bracciano, Alfred F., author. I Bracciano, Daniel C., author. I Bracciano, Gloria M., author. Title: Modern refrigeration and air conditioning/ by Andrew D. Althouse, Carl H. Turnquist, Alfred F. Bracciano, Daniel C. Bracciano, Gloria M. Bracciano. Description: 20th edition. I Tinley Park, IL: The Goodheart-Willcox Company, Inc., [2017] I Includes index. Identifiers: LCCN 2015039667 I ISBN 9781631263545 Subjects: LCSH: Refrigeration and refrigerating machinery. I Air conditioning. Classification: LCC TP492 .A43 2017 I DDC 621.5/6--dc23 LC record available at http:/ /lccn.loc.gov/2015039667 Copyright Goodheart-Willcox Co., Inc. 2017

Preface Modern Refrigeration and Air Conditioning is the standard for a new generation of learner. This classic is an excellent blend of theory, skill development, and service techniques to help you learn how to install and service refrigeration and HVAC systems. Modern Refrigeration and Air Conditioning delivers comprehensive and authoritative content on the basic and advanced principles of refrigeration and air conditioning, provides excellent instruction and training in the skills and techniques essential for servicing and troubleshooting, and emphasizes career opportunities, workplace skills, and safety. The content in Modern Refrigeration and Air Conditioning is correlated to the curriculum guides and competencies used for HVAC Excellence and PAHRA program accreditation. The accreditation curriculum dovetails with entry-level and professional certification exam requirements. Thus, Modern Refrigeration and Air Conditioning is a valuable resource as you begin your journey toward entry-level certification, employment, professional certification, and career advancement. Modern Refrigeration and Air Conditioning has been carefully designed and crafted to make your learning experience effective and efficient. Concepts are explained clearly and simply, with text narrative supported by numerous engaging and attractive illustrations. The preview and review features in each chapter-Chapter Outline, Technical Terms list, Review of Key Concepts, and Summary-help you quickly master HVACR concepts and topics.

This 20th edition incorporates many changes: New technical updates include added information on variable refrigerant flow (VRF) systems, microchannel heat exchangers, variable frequency drives, thermostat diagnostics, HC and HFO refrigerants, and additional Code Alert features. New and updated content focusing on energy efficiency includes air-side economizers and multistage and zoning thermostats. • Over 400 new images and illustrations have been added throughout the textbook. • A new Careers and Certification chapter and new Service Call Scenario features provide you with an overview of career opportunities available in the HVACR industry and an introduction to workplace skills that will help you succeed in your career goals. • A new Safety chapter provides an overview of safety-related topics to complement the strong, existing contextual safety information located throughout the chapters. In the coming years, the number of new positions in the HVACR industry combined with open positions due to retirements is expected to be significantly greater than the number of new employees entering the field. This will create a shortage of trained workers and a surplus of employment opportunities. You are entering the HVACR field at an ideal time, and Modern Refrigeration and Air Conditioning will be a fantastic resource for you as you build your career! •

Copyright Goodheart-Willcox Co., Inc. 2017

iii

About the Authors Andrew D. Althouse received his engineering degree from the University of Michigan. He was the Assistant Director of the Vocational Education Department at Cass Technical High School in Detroit and later became the Supervisor of Vocational Education for Detroit. As a leader in the field, he and his coauthor Carl Turnquist created one of the first training programs in refrigeration while at Cass Technical High School. Andy, as he was known, created the first technical training course in refrigeration for industry. The instructional materials used in this first course became the foundation for the textbook Modern Electric and Gas Refrigeration, which was published in 1933. Mr. Althouse was a Member of the American Society of Refrigerating Engineers. Carl H. Turnquist earned his engineering degree from Wayne State University in Detroit and, along with Mr. Althouse, developed one of the earliest training programs to provide instruction for mechanical refrigeration for the automotive industry and for railroad passenger cars. With industry support, Carl's program flourished as the demand for skilled technicians in this new field expanded. The Modern Electric and Gas Refrigeration book was revised every three to five years as new equipment was developed. The title of the book was eventually changed to Modern Refrigeration and Air Conditioning. Mr. Turnquist was an Associate Member of the American Society of Refrigerating Engineers. Alfred Bracciano received a bachelor of science degree in Industrial Education with Certification in Vocational Education from Wayne State University in Michigan. He also earned a master's degree in Secondary Education and a Specialist degree in Administration and Supervision. Mr. Bracciano was employed as a teacher of Refrigeration and Air Conditioning for twelve years. He then became Director of Career and Technical Education for Warren Consolidated Schools in Warren, Michigan. He taught Community Resources Workshops for Michigan State University and presented at conferences throughout the country.

Mr. Bracciano is a life member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), Refrigeration Service Engineers Society (RSES), Association for Career and Technical Education (ACTE), and the American Technical Education Association (ATEA). Dan Bracciano graduated from Oakland University in Rochester Hills, Michigan, with a bachelor of science degree in Mechanical Engineering. He began his career in HVACR at the Warren Schools Career Center, graduating in HVACR, and worked in the HVACR field performing residential and commercial HVACR installations and service. Dan has over twenty-five years of experience working in design development and manufacture of HVAC systems for Fiat/Chrysler, General Motors, Mitsubishi Climate Control, and Alternative Energy Corporation. He holds several patents in the field, including a patent for a Modular Hermetic HVAC system. Dan is a member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the Mobile Air Conditioning Society (MACS). Gloria Bracciano received a bachelor's degree in Education, a master's degree in Curriculum and Instruction, and an education specialist degree in Administrative Leadership. She completed coursework in Heating, Ventilation, Air Conditioning, and Refrigeration through Oakland and Macomb Community Colleges. Ms. Bracciano has worked in the field of education for over twenty-five years and has held positions as both university professor and administrator. She has also served as the Provost of Gulliver Schools. Ms. Bracciano specializes in development and implementation of innovative curriculums and has presented at local, state, and national conferences. Ms. Bracciano is a member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), Refrigeration Service Engineers Society (RSES), and the American Technical Education Association (ATEA).

The authors and publisher wish to thank Connie Habermehl, Administrative Assistant for Associated Technical Authors, for her contributions to this and previous editions of Modern Refrigeration and Air Conditioning.

iv

Copyright Goodheart-Willcox Co., Inc. 2017

The authors and publisher wish to thank the industry and teaching professionals listed below for their valuable input into the development of Modern Refrigeration and Air Conditioning. The authors and publisher wish to express particular gratitude to the following individuals:

Dr. Christopher Molnar, of Porter and Chester Institute, for writing three sections for and providing a detailed technical review of the thermostats chapter. Greg Jourdan, of Wenatchee Valley College, for his detailed technical review of the electrical controls systems and energy management content.

Don Crawshaw, of Salt Lake Community College, for his detailed review of commercial refrigeration chapters. Randy F. Petit, Sr., of HVAC Excellence, for his extensive review and suggestions. Howard Weiss, of HVAC Excellence, for his detailed review of certification and industry association and general guidance.

Jerry Weiss, Thomas Tebbe, and Coy Gibson, all of HVAC Excellence, and Warren Lupson, of PAHRA/AHRI, for their frequent and generous contributions of guidance and wisdom.

Anthony L. Baham

Danny Burris

George Frank

South Central Louisiana Technical College Morgan City, Louisiana

Eastfield College of the Dallas County Community College District Mesquite, Texas

British Columbia Institute of Technology Burnaby, British Columbia, Canada

David Blais Ivy Tech Community College Indianapolis, Indiana

Terry Bradwell Midlands Technical College West Columbia, South Carolina

Stevan Brasel Moraine Valley Community College Palos Hills, Illinois

Michael Brock Florida Coast Career Tech/Florida State College at Jacksonville Jacksonville, Florida

Mark R. Buller British Columbia Institute of Technology Bumabay, British Columbia, Canada

Terry Carmouche

Rod Fronk

South Central Louisiana Technical College Reserve, Louisiana

Wichita Technical Institute Topeka, Kansas

Tennessee Technology Center at Pulaski Pulaski, Tennessee

David W. Fuller Clovis Community College/ Air One HVAC Sales and Service Clovis, New Mexico/Portales, New Mexico

James Conway

Tim Gohdes

Lindsey-Cooper Refrigeration School Irving, Texas

Central Texas College Killeen, Texas

Rick Dorssom

Front Range Community College Fort Collins, Colorado

Michael Chandlee

Hillyard Technical Center St. Joseph, Missouri

Brad Guthrie

Patrick Duschl

Marvin J. Hamel

Fortis College Cincinnati, Ohio

Locklin Technical Center Milton, Florida

Copyright Goodheart-Willcox Co., Inc. 2017

V

vi

Modern Refrigeration and Air Conditioning

James H. Hanway

James Martini

Doug Sallade

Northland Career Center Platte City, Missouri

Henry Ford Community College Dearborn, Michigan

Cypress College Cypress, California

Gary L. Harrison

Todd Matsuba

AG&H Contractors LLC Baton Rouge, Louisiana

Thomas E. Shiflet

Northern Alberta Institute of Technology Edmonton, Alberta, Canada

Greenville Technical College Greenville, South Carolina

Patrick Heeb Long Beach City College Long Beach, California

John Henry Diablo Valley College Pleasant Hill, California

John P. Ingram Northwest Community College Senatobia, Mississippi

Gordan Jacoby Milwaukee Area Technical College Oak Creek, Wisconsin

Robert Johnson Amarillo College Amarillo, Texas

Nick Kyriakopedi Laney College Oakland, California

Richard McDonald Santa Fe College Gainesville, Florida

John L. Mulder

Allen Smith College of Lake County Grayslake, Illinois

Stephen V. Spletzer

Roanoke-Chowan Community College Ahoskie, North Carolina

Arkema Inc. King of Prussia, Pennsylvania

Patrick Murphy

Grand Rapids Community College Grand Rapids, Michigan

Quinn-Murphy Consulting, LLC Spring Lake, New Jersey

Keith J. Otten Southwestern Illinois College Belleville, Illinois

Joseph G. Owens Antelope Valley College Lancaster, California

Jeffrey Patronek

Donald Steeby

Richard C. Taylor Pennsylvania College of Technology Willamsport, Pennsylvania Mark Tyrrell Franklin Technical Center Joplin, Missouri

Glenn Walsh British Columbia Institute of Technology Burnaby, British Columbia, Canada

Robert Morgan Educational Center Miami, Florida

Alfred State/SUNY College of Technology Alfred, New York

Mark Loan

Joseph Pellecchia

Chad Wheat

Platt Regional Vocational Technical School Milford, Connecticut

Georgia Northwestern Technical College Rome, Georgia

Whit Perry

Gerald L. Williamson Montgomery College Rockville, Maryland

Aaron Latty

Red River College Winnipeg, Manitoba, Canada

Raul Lopez Houston Community College Houston, Texas

Barbara MacQueen Vancouver Island University Cowichan Campus, British Columbia, Canada

Northwest Mississippi Community College Senatobia, Mississippi

Jesse R. Riojas Oakland Community College Auburn Hills, Michigan

Rick Marks Cisco College Abilene, Texas

Gary Marowske Flame Heating, Cooling, Plumbing & Electrical Warren, Michigan

Terry Robinson Lincoln Technical Institute Grand Prairie, Texas

Harold Wynn Wichita Technical Institute Joplin, Missouri

Robert G. Young Autry Technology Center Enid, Oklahoma

Terry Rogers

Brian Youngblood

Midlands Technical College West Columbia, South Carolina

Atlantic Technical Center Coconut Creek, Florida

Copyright Goodheart-Willcox Co., Inc. 2017

The authors and publisher would like to thank the following companies, organizations, and individuals for their contribution of resource material, images, or other support in the development of Modern Refrigeration and

Air Conditioning. A-1 Components Corporation AAON ABB Stal Refrigeration Corporation Abbeon Cal, Inc. ACCA - The Indoor Environment & Energy Efficiency Association

Carrier Corporation, Subsidiary of United Technologies Corp.

Flame Heating, Cooling, Plumbing & Electrical

CarrierTransicold Division, Carrier Corp.

Fluke Corporation

CCI Thermal Technologies Inc.

Frigidaire

CertainTeed Corporation

Frigidaire Company

Climate Master

Fujitsu General America, Inc.

Aeroquip Corporation

CMP Corporation

Fusite

AICoil, Inc. Alerton

Comfortmaker GNJ, International Comfort Products Corporation

Gates Corporation

Alfa Laval Inc. All American Heating & Cooling

Continental Industries, Inc. Control Resources, Inc.

General Filters, Inc. Goodman Manufacturing Company

Allanson Inc. Alto-Shaam, Inc.

Control4 Corporation

Goodway Technologies Corp.

Amana Refrigeration, Inc. American Saw & Mfg. Company Amprobe Andersen Corp. A. 0. Smith

Cooper Tools, Nicholson

Grasslin Controls Corporation

Copeland Corporation

GrayWolf Sensing Solutions, LLC

Corken Steel Products

Haier America

Cyber Prodigy LLC

Hampden Engineering

Daikin Applied

Appian, Inc.

Danfoss

Arkema, Inc.

DENSO Sales California, Inc.

Armacell LLC

Dial Manufacturing, Inc.

Arzel Zoning Technology, Inc.

Dispensed Water Div. of Elkay Mfg. Co.

Bacharach, Inc.

DiversiTech Corporation

Bally Refrigerated Boxes, Inc. Baltimore Aircoil Company

DuctSox Corporation

BernzOmatic

DuPont Company

Bitzer Blissfield Manufacturing Bosch Thermotechnology Corp. BouMatic Braeburn Systems LLC Bristol Compressors, Inc. Cadet Manufacturing Co.

GEA Heat Exchangers

Dunham-Bush, Inc. DuPont Energy Management Co., Inc.

Harris Group Hartford Compressors, Inc. Heat Controller, Inc. Henry Technologies, Inc. Hill Phoenix, Inc. hilmor Hi-Velocity Systems Honeywell, Inc. Hoshizaki America, Inc. Hussmann Corporation Ice Energy, Inc.

Dwyer Instruments, Inc.

lce-0-Matic

Dynatemp International, Inc.

Ideal Industries, Inc.

Earthlinked Technologies, Inc.

Imperial

Elite Soft Inc.

INFICON

Emerson Climate Technologies

lnsteon

Caleffi North America, Inc.

Emerson Electric Co.

Invensys Climate Controls Americas

CALMAC Manufacturing Corporation Camfil Farr Co.

Extech Instruments Corp.

ITT McDonnell & Miller

Fedders North America, Inc.

ITWVortec

Carel Industries

Fenwal Controls

Jackson Systems, LLC

Carlin Combustion Technology, Inc.

Field Controls, LLC

Jenn-Air

Copyright Goodheart-Willcox Co., Inc. 2017

vii

viii

Modern Refrigeration and Air Conditioning

Johnson Controls, Inc. Jug Lugger KE Fibertec NA, Inc. Kenmore Kennametal, Inc. King Electrical Mfg. Co. Klein Tools, Inc. Kysar/!Warren LA-CO Industries Inc. Lancer Corporation Lennox Industries Inc. LG Appliances Lordan A.C.S. Ltd Ludeca, Inc. Luvata Manchester Tank Mastercool Inc. Maytag Corporation McQuay International Mestek Machinery Micro Switch, Div. of Honeywell, Inc. Midco International, Inc. Milwaukee Electric Tool Corp. Mitsubishi Electric, HVAC Advanced Products Division Mueller Industries, Inc. Mueller Refrigeration Company, Inc. National Air Duct Cleaners Association (NADCA) National Cancer Institute National Weather Service Nest Labs, Inc. NexRev Inc. NORA North American Technician Excellence OSHA Owens Corning Ozone Solutions, Inc. Pacific Transducer Corp. Packless Industries Paragon, Invensys Climate Controls Americas Parker Hannifin Corporation PB Heat, LLC Peerless of America, Inc. Malcolm Prather QMark, A Division of Marley Electric Heating

R.W. Beckett Corporation Ranco, Invensys Climate Controls Americas Raymon-Danco Corp. Raypak, Inc. RectorSeal Reed Manufacturing Co. Refrigeration Technologies REMIS AMERICA, LLC RenewAire Rheem Manufacturing Company Ritchie Engineering Co., Inc. YELLOW JACKET Products Division Robinair, SPX Corporation Robur Corporation RTCA-Radon Testing Corp of America, Inc. Runtal North America Scale Free International Schneider Electric Scotsman Ice Systems Sealed Unit Parts Co., Inc. Selca Products Company Sherwood Valve Siebe Environmental Controls, Invensys Climate Controls Americas Skuttle IAQ Products Snap-On Inc. Spectronics Corporation SpeedClean Sporlan Division - Parker Hannifin Corporation Sprinkool Systems International, Inc. SPX Corporation SRC Refrigeration SSAC, LLC Standard Refrigeration Co. Steinen Nozzles Steril-Aire, Inc. Steven Shepler Stride Tool Inc. Suburban Manufacturing Sub-Zero/Wolf Appliance Sun Spot Solar and Heating, Inc.

Copyright Goodheart-Willcox Co., Inc. 2017

Superior Refrigeration Products Sweden-Alco Dispensing Systems, a Div. of Alco Foodservice Equipment Co. TEC (The Energy Conservatory) Tecogen, Inc. Tecumseh Products Company Tempstar Texas Instruments, Inc. The Coleman Company, Inc. The Energy Conservatory The Trane Co. Thermo King Corporation Thermostat Recycling Corporation TIF™ Instruments, Inc. Tjernlund Products, Inc. Trane, a brand of Ingersoll Rand Transcold Distribution, Ltd. Transducers Direct, LLC. Traulsen Refrigeration TSI Incorporated Tutco, Inc. U.S. Cooler Company Uline Ullman Devices Corporation United States Federal Trade Commission Uniweld Products, Inc. Uponor, Inc. US Department of Energy-DOE Veco NA - Coastal Climate Control, Inc. Venstar Virginia KMP Corp. WaterFurnace International, Inc. Webster Fuel Pumps and Valves Westermeyer Industries, Inc. Westwood Products, Inc. Whirlpool Corporation White-Rodgers Division, Emerson Climate Technologies Wittern Group Women in HVACR Worthington Industries Xylem Inc. York International Corp. Zero Zone, Inc. Zettler Controls, Inc. ZONEFIRST

G-W Integrated Learning Solution Together, We Build Careers At Goodheart-Willcox, we take our mission seriously. Since 1921, G-W has been serving the career and technical education (CTE) community. Our employee-owners are driven to deliver exceptional learning solutions to CTE students to help prepare them for careers. Our authors and subject matter experts have years of experience in the classroom and industry. We combine their wisdom with our expertise to create content and tools to help students achieve success. Our products start with theory and applied content based on a strong foundation of accepted standards and curriculum. To that base, we add student-focused learning features and tools designed to help students make connections between knowledge and skills. G-W recognizes the crucial role instructors play in preparing students for careers. We support educators' efforts by providing time-saving tools that help them plan, present, assess, and engage students with traditional and digital activities and assets. We provide an entire program of learning in a variety of print, digital, and online formats, including economic bundles, allowing educators to select the perfect mix for their classroom.

Student-Focused Curated Content Goodheart-Willcox believes that student-focused content should be built from standards and accepted curriculum coverage. Standards from HVAC Excellence and PAHRA/AHRI were used as a foundation for this text. Modern Refrigeration and Air Conditioning also uses a building block approach with attention devoted to a logical teaching progression that helps students build on their learning. We call on industry experts and instructors from across the country to review and comment on our content, presentation, and pedagogy. Finally, in our refinement of curated content, our editors are immersed in content checking, securing and sometimes creating figures that convey key information, and revising language and pedagogy.

Curriculum Correlations Modern Refrigeration and Air Conditioning aligns with curriculum standards for HVAC Excellence and PAHRA accreditation. HVAC Excellence is a not-for-profit organization that serves the HVACR industry with the goal of supporting and improving HVACR education and training. HVAC Excellence provides many services to HVACR education and training, including awarding program accreditation, professional certifications, and instructor credentials. The Partnership for Air-Conditioning, Heating, and Refrigeration Accreditation (PAHRA) is an independent, third-party organization that is a partnership between HVACR educators and the HVACR industry. PAHRA awards accreditation to programs that meet or exceed industry-validated standards developed by AHRI.

To see how Modern Refrigeration and Air Conditioning correlates to HVAC Excellence and AHRI standards, please visit www.g-w.com/modernrefrigeration-air-conditioning-2017 and click on the Correlations tab. For more information on PAHRA and HVAC Excellence, please visit www.pahrahvacr.org and www.hvacexcellence.org.

A

~ PAHRA IIYAC cxce~nce ~

AcCond;t;on;ng. Heohng, Refrige,ot;onAcuedtot;on

Copyright Goodheart-Willcox Co., Inc. 2017

ix

Features are student-focused learning tools designed to help you get the most out of your studies. This visual guide highlights the features designed for the textbook.

Technical Terms list the key terms to be learned in the chapter. Review this list after completing the chapter to be sure you know the definition of each term.

Review of Key Concepts states previously covered facts related to the topics in the chapter. A chapter reference is provided so you can go back and review the topic in more detail.

Ti ch

IT,

rm•

be'k-dn\"1f1Jmf'll'W)f

R

1100'W:'r n ~ w 1.""1tnf~o1J u:impn>-.""

p~111np1n

u-r,nr~;illon,r,

fJl:)lunr1n,:

c!NrM'l(I"~

r~rehAmuuon

11Jffll~l(lflfllll

IU'll"IN'11fltrud

rn1nkc:o1Sf"

jlt'ilOO

1--yl1nt!Mht-.:icl

1"11Ulll-'t,ll'nlin'-1,'-0i

.-n11C'i.-r

mufilt-r o.lnng 011ls~

Clfl"n-dn- IT.-rl/llT"lo!o(JI

KNIMM"SI •

rN:1pruc:o1l1rttiompff"l.'IOf MM)"C-t.M"ll~sot

Sco«:h)'>b,

hr'mwitc C:tJl"lll l ~

·-.,....

•

The combned sn LM• s1•~ lholl 1~ r.MIO bM:Wtt'n jll'!'"V'f', Vl"'flM", ,1nd"""11....-ol!l.ll"fNl111111'

,I~-~

ultbl.ill'll.(~plt-r51

JMrt'W'.ill

"""'"" ul~'f'l"-t.e

A~ J i;,;tS, e QJft1f1ff'SM.'d,. ib flfl.~Wll' .iml M'fflf,C,f.itutto ns,c_ Th,s .s due tu 1ht- fflf'fg)' .iddt-d "' tht- ps 1,y c:omJl~.Thto~y.iddNftjoll,.nlefflW'dthr

hr.io(c(mpn...,,lflln .iCha111rrSJ

lof'R"WUlfflflA~

~1-h.:-tmr-1•c-um1N'i:",,Wlt spl~hsy,.kffl

Co

• A J:-•~ ft .i phywi .al '°ll)lo1nn- w11h no dN,ntir ,h,-s•· or wh.1m~. whilh "Jlilnt.'ll ~ ~ nt d•l\"'"11lqoldl\X\'J\\.")'

t ,-.,m,uffl~\.'l'~d~n.ifN~t.-ratlfwt~,.'frl 2. Cl'Mll.'(11~•)'SK'ffl•~sn,o,,.ninAg1,11tll"9 3, fllllt\.'\'\~CA'\'ffiJl1,i~rofl"'l'or0il 111\e 0..t1.'f·n,111lki1IJJ"ltn\l\)'Nt\l"1ifion-

E.lectrlc F"urnaceOrlentatlan ~ ...etAC "-!Jnjj · - - · ...

tral illlr hitncHer \'ollh duct ~ 1£n hiwlng lnclhldUil l

"°' ~

NnlllM ~eonwolMYION,&uel'llltfMfe~COft,,

tKtataandaome1ype1olP'IWfa.1npo..-on-Mn1-11Jve and need to bil lnl1llled Ir! • oar1ain OMflliltlon to o~r-

... ~per!r Altc,

aofM f!\,1111Jfai;1JJJen1

m.., requu•

(lffUin cottlfOI ~ t lO tMl loclli.d be!oworlOona MNlol l'IN\JftOMtTIINIII. rallltlftlnlllorHwtw•tM1 nwy U1S1 oft 11MI aytl*I' UTINCNNI~ B4klNI W'lltalng •n4'tctnc tumlo. M'lado'lllffll'bw0f\4dt.'\i-po"-.•rnotor 0 "Pflfl/,;h.:fls«in

J.1Jnaton.w1...,.,,.t"°'-'d!>)'~1.a_o,>ens wtx,n lt.'mJ){'r.lh111" nght ,1bow rhZl'5f•I

f\,fn,:.,.'l'"i,nlr(•/11,(,.al\'(•

O.z.,nt.•damf)('r

.lS.A(t"JfJ\-..kl11~"·hh1~ro.,1t..,._,., ~,::i1la -k.'M lei' all fnt ltou~,».;, • fol\."t\J..;11r Ulnt'd))'M1._'fTlby _

A. tpointafh.'ra

10. A ~ih fu.m«"(>wltha mu/rbf~"t'th.:\Hk:handroiwn1'h1gheum'nthl'll'-\t•lrag" cl.,.'\ il"l'>.1"111Chllj, fan,-.,,d

0..

::s p, ::s

s·

"'~

/l)

(JQ. "'

~ ....,

"'::s

/l)

0..

0

a,

0)

(") 0

--J

~

I\)

3" ri

?

(")

8 X

~

;cl.

Ill

(1)

a.

0 :::,

0

G)

;:?;

~ tl5"

CFC azeotropic blend

HFC azeotropic blend

R-502

R-507A

46% R-23 / 54% R-116

50% R-125 / 50% R-143a

48.8% R-22/ 51.2% R-115

73.8% R-12 / 26.2% R-152a

50% R-32 / 50% R-125

CHF 3 + CF 3 CF3

CHF 2 CF 3 + CH 3 CF 3

CHF 2 CI + CCIF 2 CF3

CCl 2 F2 + CH 3 CHF 2

CH 2 F2 + CHF 2 CF 3

-

CJ

CJ

CJ

-126 (-88)

-52 (-47)

-50 (-46)

-28 (-33)

-61 (-52)

256.1 psig

40.0 psig

35.9 psig

16.4 psig

54.9-55.1 psig

N/A

198.3 psig

176.6 psig

112.9 psig

257.7-258.6 psig

R-13, R-503

R-22, R-502

R-402A, R-404A, R-408A, R-507A

R-401A, R-409A

R-22

POE

POE

AB/MO

AB/MO

POE

Goodheart-Willcox Publisher

Low temperature medical freezers and environmental chambers

Commercial refrigeration systems, supermarket display cases, ice machines

Supermarket freezers, refrigerated cases, frozen food processing plants

Residential and commercial air conditioning, domestic refrigeration, commercial chillers

Residential and light commercial heat pumps and air conditioners

Figure 9-21. This chart serves as a comprehensive reference for identifying and comparing the properties and applications of some commonly used refrigerants.

***POE = Polyol ester/ AB= Alkylbenzene /MO= Mineral oil/ PAG = Polyalkylene glycol

**Pressures are given for a 5° evaporator temperature and an 86° condenser temperature-these are referred to as Standard Ton Conditions

*The boiling points listed are for atmospheric pressure

HFC azeotropic blend

CFC azeotropic blend

R-500

R-508B

HFC zeotropic blend

R-410A

....,.

0)

--

Pressure activated

Light

---0---

Engaged

-------1>--

Temperature activated (NO)

-0---

4 Conductor

Meter

""""T""""""< ...............

*Denote usage

Diode

-----t>I--

Resistor

Shielded cable Multiple conductor cable

=9=

}---+-}---+-}---+--

Horn

Buzzer

Temperature activated (NC) Flow activated (NO)

Alarms

Bell

--~

---+-----) ---+-----) ---+-----)

0

~

0

0

o

0,

Any number of transmission paths may be shown

Segment contact

Thermal relay

Jlo

o~ or

8=8,000008=8,

.......efXp-

-'

~ I

0---

~ 0--......L..

0

0

ili

o¾

1_ _ _

o+a

"2S" ___ J

Motors

=O

General

Windings

Main : 0 Aux.

Conductors

No spring return

(NO)

(NC)

°t

~

Power (factory wired)

---

Control (factory wired)

---

1°

Power (field installed)

----

[

Control (field installed)

----

)2 Close on rising

Open on rising

Close on increase

°t

Transistors

s

PNP type

6?>

~

NPN type

@

8

Flow activated (NC)

Open on increase

[[JJ

Liquid level (NO)

Close on rising

Liquid level (NC)

Open on rising

\[]

Symbol 0

General selector switch

Switches

Single throw

Component

Goodheart-Willcox Publisher

Figure 12-14. Electrical symbols commonly used in wiring diagrams.

Copyright Goodheart-Willcox Co., Inc. 2017

I

280

Modern Refrigeration and Air Conditioning

Pro Tip

Electrical Symbols in Diagrams Manufacturers of HVACR equipment sometimes use their own symbols for electrical components. HVACR service technicians should always check the symbol for a component by comparing the wiring diagram with what is physically in the unit.

12.4.2 Series Circuits A circuit having only a single path for current is called a series circuit. The same current flows through all the electrical loads in a series circuit, Figure 12-15. If one of these devices is open or does not conduct, the circuit is broken and will not conduct current. Since failure of a single component causes the whole of a series circuit to fail, a series circuit may be used as a safety circuit. For instance, when a power switch, a safety limit switch, and a fuse are wired in series with a compressor, any one of these devices can open and stop compressor operation in an unsafe situation. However, series circuits are rarely used in power circuits for the same reason. One component can cause the failure or apparent failure of every other component in series. Remember that Ohm's law states that current is determined by the amount of voltage divided by the amount of resistance applied across a device. Since the voltage applied to a series circuit treats the entire circuit as a single resistance, the current flowing through each device is calculated by dividing the total applied voltage by the total circuit resistance. Using Figure 12-15 as an example, the total resistance of the circuit is 12 Q

(3 Q + 4 Q + 5 Q = 12 Q). Therefore, the current throughout the whole circuit is 1 A, as 12 V + 12 Q = 1 A. While current stays the same through each device in a series circuit, the voltage across each device will differ, depending on the resistance of each device. Using Figure 12-15 as an example again, the first lightbulb has a resistance of 3 Q. According to Ohm's law, the voltage across it is 3 V because voltage equals resistance multiplied by current (3 Q x 1 A = 3 V). Each lightbulb in this series circuit has a different voltage across it because each bulb has a different amount of resistance.

12.4.3 Parallel Circuits A parallel circuit allows current to flow to and from a power source along two or more electrical paths, each of which has only one electrical load, Figure 12-16. The electrical wiring in a house is an example of a parallel circuit. If one of the bulbs burns out, the rest continue to light because their paths to and from the power source remain complete. If the lights in a house were wired in series, then the failure of any one of the lights would cause all the lights in the house to go out. This is why power circuits are wired in parallel. Whereas the current is the same across each load in a series circuit, the voltage across each load is the same in a parallel circuit. This means that in a parallel circuit, current changes based on the resistance of each load. The load with the lowest resistance receives the highest current, and the load with the highest resistance receives the lowest current. Looking at Figure 12-16, you can see that the lightbulb with the lowest resistance (3 Q) has the most current (4 A), and the lightbulb with the highest resistance (6 Q) has the least current (2 A). 1A

t

Battery ( 12 V) -----+-

Goodheart-Willcox Publisher

Figure 12-15. In a series circuit, current only has one path to follow. Therefore, current is equal at each point of the circuit. Notice

that the current is 1 A at each point between the lightbulbs. However, the voltage is different across each lightbulb because each lightbulb's resistance is different. Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 12 Basic Electricity

4A

Battery (12V) -

3A

t

t

__.

-+------

281

2A

t Gaadheart-Willcax Publisher

Figure 12-16. These lightbulbs are wired in a parallel circuit. If one of the bulbs burns out, the rest of the bulbs will continue emitting light. Note that each bulb receives the same voltage, regardless of its resistance.

12.4.4 Series-Parallel Circuits A circuit that has some electrical loads in series and some in parallel is called a series-parallel circuit. Electronic control boards are often wired using a series-parallel circuit. It is rare that an HVACR technician will need to calculate expected voltages on a series-parallel circuit that has more than one electrical load in series. A theoretical example of a series-parallel circuit is shown in Figure 12-17. The same principles of series and parallel circuits apply but in a complex combination. Start with the farthest combination of electrical loads and work backward to determine total resistance, so that total current can be calculated. Then begin applying the principles discussed. Electrical loads in series have the same current. Electrical loads in parallel have the same voltage.

While knowledge of electrical circuits is important to know, calculating all the variables of series-parallel circuits is not normally done in day-to-day HVACR work.

12.4.5 Voltage Drop A voltage drop (VD) is the voltage applied across an electrical load that is causing current to flow through it. Electrical loads are devices that offer some resistance to current passing through. Most electrical loads are intentional and perform some work or function, such as a motor or a relay coil. Others are unintentional and may cause a circuit to malfunction, such as dirty contacts, a poorly made connection, or wire that is too small for its application. Some electrical loads are so small within a circuit that they make virtually no difference. Examples would be clean contacts, correctly sized wire, and a properly

Battery (12 V) ~__,,.._

t

Gaadheart-Willcax Publisher

Figure 12-17. This diagram shows a series-parallel circuit. Two loads are wired in series with three loads that are wired in parallel.

Copyright Goodheart-Willcox Co., Inc. 2017

I

282

Modern Refrigeration and Air Conditioning

functioning switch. Such small electrical loads may not always be easily measurable, even with a multimeter. The value of most voltage drops can be read with a voltmeter connected across an electrical load. Voltage drop (VD) is equal to the resistance of a load (RJ multiplied by the current (IJ passing through that load: VD= RLxIL The total voltage drop (VDT) of a circuit (the total applied voltage, VT) in a series circuit equals the sum of the voltage drops of the electrical loads in the circuit: VDT or VT= Vm + VD2 + VD3 + ·· · Electrical loads in a parallel circuit will all have the same voltage drop, which equals the total voltage drop (VDT) or the total applied voltage (VT) of that parallel circuit: VDT or VT= Vm = VD2 = VD3 = . . . Using Figure 12-18 as an example, the total voltage supplied by the power source is 120 V, and the ammeter indicates that the current is 5 A. This is a series circuit, so current only has one path. This means that each of the electrical loads will have an identical current of 5 A flowing through it. Using this value for current, we can multiply by the resistance of each load to find each load's voltage drop. Resistance of circuit wiring is 0.1 Q: V01 = 5 A x 0.1 Q = 0.5 V Resistance of thermostat switch is 0.1 Q: VD2 = 5 AX 0.1 Q = 0.5 V Resistance of starting relay contacts is 0.1 Q: VD3 = 5 AX 0.1 Q = 0.5 V Resistance of motor compressor is 23.7 Q: VD4 = 5 AX 23.7 Q = 118.5 V By adding the individual voltage drops across each load, we can confirm the total voltage drop, which should equal the applied voltage. Thermostat switch (0.1 Q)

i

Starting relay contacts (0.1 Q)

r-l

rv

VDT= VDl + VD2 + VD3 + VD4 VDT = 0.5 V + 0.5 V + 0.5 V + 118.5 V VDT= 120.0V Measuring and calculating the current, resistance, and voltage across the individual loads in a circuit is critical to troubleshooting and problem solving. There is always some electrical resistance across any electrical switch, relay contacts, or circuit wiring. However, most voltage drops across such components are so low that they are usually negligible. When voltage drops across these components become higher, problems with the rest of the circuit can develop. Voltage drops of importance are those measured across motors, relay coils, and other higher resistance loads. Important values for current are those measured through motors and other significant loads.

12.5 Magnetism All magnets have a north pole and a south pole. Like poles repel each other (try to move apart). Unlike poles attract (pull toward each other). The attraction and repulsion of magnetic poles is shown in Figure 12-19. There are lines of magnetic force connecting the north and south poles of a magnet. These lines of force are called magnetic flux. The space in which a magnetic force is operating is called a magnetic field. Magnetic flux will flow through most substances. It is not stopped by glass, mica, wood, air, or any other material used for electrical insulation. Some substances, particularly soft iron, are better conductors of magnetic flux than other substances. This is why certain parts of electric motors and generators are made of soft iron. Instruments can be shielded from a magnetic field by placing them inside a soft iron case. Because soft iron is a good conductor of magnetic flux, the magnetic field will pass around the instrument inside the soft iron case and not through it.

12.5.1 Permanent and Induced Magnetism

120V

t Resistance of circuit wiring: 0.1 Q

A

Ammeter (5 A)

Compressor (23.7 Q) Goodheart-Willcox Publisher

Figure 12-18. To calculate voltage drop, measure the circuit's current and the resistance of each electrical load. These values can then be used to calculate each component's voltage drop.

Permanent magnets are usually made of hardened steel. Once magnetized, they remain magnetized. Some patented alloys of iron, aluminum, nickel, and cobalt make strong permanent magnets. Magnetic lines (flux) tend to become as short as possible. This shortening force has many industrial applications. Permanent magnets are used in some controls to provide a snap action for electrical contacts. They are also used in small control motors.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 12 Basic Electricity

Any material capable of being magnetized becomes a magnet if it is placed in a magnetic field. This is called induced magnetism. For example, if a piece of soft iron, which is a good magnetic conductor, is placed in a magnetic field, then the piece of soft iron becomes magnetized. Induced magnetism forms the basis for how an electromagnet is constructed.

A

283

12.5.2 Electromagnetism If an electric current is passed through a conductor, the conductor becomes surrounded by a magnetic field. If the current is turned off, the magnetic field will disappear, Figure 12-20. If a conductor is wound around a piece of soft iron and current is passed through the conductor, the soft iron becomes a magnet. This is an example of induced magnetism. Turning off the current (opening the circuit) stops the magnetic effect. This magnetic effect caused by current is called electromagnetism. Magnets formed in this manner are called electromagnets. Electromagnets are used in motors, relays, solenoids, and in many other electromagnetic applications. The iron part is called the core. The current-carrying conductor is called the winding, Figure 12-21. The strength of an electromagnet is based on four factors: • Number of turns in the winding. • Strength of the current. • Core material and construction. • Length of the coil. The more coil turns there are in the winding and the higher the current is, the stronger the electromagnetism.

+

-

B Magnetic field

t

_....--

Electron flow

Switch

Battery

C Goodheart-Willcox Publisher

Figure 12-19. Attraction and repulsion of magnetic poles. ALooking at the end of a horseshoe magnet, the magnetic flux around each pole is shown. B-Magnetic flux provides a force that pulls unlike poles of magnets together. C-Magnetic flux provides a force that pushes like poles away from each other.

Goodheart-Willcox Publisher

Figure 12-20. A conductor is passed vertically through the center of a sheet of cardboard with iron filings sprinkled over the cardboard's surface. When the ends of the vertical conductor are connected to a battery, the iron filings form circular patterns, demonstrating the magnetic field around the conductor.

Copyright Goodheart-Willcox Co., Inc. 2017

I

284

Modern Refrigeration and Air Conditioning Magnetic field

!

----

Wire conductor (winding)

--

l.

J)

))' Iron core

Goodheart-Willcox Publisher

Figure 12-21. A simple electromagnet has several turns of conductor (wire) placed around a soft iron core. When current is passed through the conductor, circular magnetic fields that form around the wire are combined in the core to form a single magnetic field.

Since materials react to electricity and magnetic flux differently, core material matters. A solid core results in fluctuations in the magnetic field that weaken the field. Therefore, a laminate core is used to create a stronger magnetic field than a solid core. The closer together a given number of windings are, the more concentrated (stronger) the magnetic field will be. The number of turns in the coil and amount of current passing through them are the two most important factors in determining electromagnetic strength. These two factors determine the magnetomotive force (MMF), which is measured in ampere-turns. The magnetomotive force is the amount of energy used to generate a magnetic field. To calculate magnetomotive force, multiply the number of turns in the winding by the amperes flowing through the winding. The magnetomotive force is directly proportional to the strength of the magnetic field. However, the strength of a magnetic field can vary for any given magnetomotive force depending on core composition, style of coil winding, and other factors.

12.6.1 AC Generator When a loop rotates through a magnetic field, emf is generated. The amount of emf generated is dependent on the direction the legs of the loop are moving. Refer to Figure 12-22. When the legs of the loop are moving mostly perpendicular to the lines of magnetic force, a greater amount of emf is generated, resulting in more current in the loop. When the legs of the loop are moving mostly parallel to the lines of magnetic force, less emf is generated, resulting in less current in the loop. The rise and fall of emf during the rotation of the wire loop is represented by a sine wave of alternating current (ac), Figure 12-23. In order to maintain an electrical connection to the rotating wire loop, ac generators have slip rings and brushes. A slip ring is a cylindrical piece of electrically conductive material that rotates with the wire loop. Brushes are electrically conductive materials that remain stationary as the slip rings rub against them. Electricity flows from the wire loop, through the slip rings, across the brushes, and into the circuit, Figure 12-24.

Magnetic flux

i

~

N

Wire loop (conductor) Less EMF

Magnetic flux

12.6 Electrical Generators Just as electricity flowing through a conductor can be used to create electromagnetic force, magnets can be used to generate electricity. If a conductor is moved across a magnetic field, an electromotive force (emf) will be generated that induces current in the conductor. This can be done by forming a wire loop and rotating the loop in a magnetic field. See Figure 12-22.

Current flowing

Wire loop (conductor) More EMF Goodheart-Willcox Publisher

Figure 12-22. When a loop is rotated through a magnetic field, current is induced in the loop. The amount of emf generated is dependent on the position of the loop in the magnetic field.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 12 Basic Electricity

285

AC Generator EMF

Angle of Rotation

Commutator

Gaadheart-Willcax Publisher

Figure 12-23. The graph of this sine wave illustrates how the rise and fall of emf in the wire loop of an ac generator produces ac current. To circuit

12.6.2 DC Generator

Gaadheart-Willcax Publisher

A direct current generator creates electricity just as an ac generator does. Induced electricity in a generator is normally an alternating current, but a de generator rectifies the electricity, so the output does not reverse its directional flow. To prevent the electricity from reversing direction, de generators use a commutator. A commutator is a split slip ring that forces the current to flow in one direction only, generating direct current (de). A commutator is a slip ring that is split in half by an insulating material, with each end of the wire loop (rotor) attached to one half of the ring, Figure 12-25. Each half of a split ring commutator contacts one of the brushes during a half rotation. For the next half

To circuit Gaadheart-Willcax Publisher

Figure 12-24. An ac generator has slip rings attached to each end of the wire loop. The slip rings rub against the brushes, transferring electricity to the circuit.

Figure 12-25. While the wire loop rotates within the magnetic field, the commutator causes each half of the split ring to change the brush that it connects to at every half rotation of the loop. This creates direct current.

rotation, each half of the split ring contacts the other brush. By constantly reversing which brush each half of the split ring connects to, a commutator provides direct current to an external circuit. Electricity flows from the wire loop, through the commutator, across the brushes, and into the circuit.

12.7 Transformer Basics A transformer operates on the two basic principles that electricity can be used to generate a magnetic field, and a magnetic field can be used to induce electricity. A transformer transfers an alternating current from one coil of wire to another coil of wire through a magnetic field. The process of transferring electricity using a magnetic field is called induction. Two coils of wire are placed near each other with a small gap of air between them. An ac electrical source is connected to the first coil of wire called the primary coil. The primary coil generates a magnetic field that is picked up by the second coil of wire called the secondary coil. The magnetic field generated by the primary coil grows and shrinks repeatedly due to the alternating current flowing through it. The secondary coil converts the changes in the magnetic field into electricity. The amount of voltage coming out of the secondary coil of a transformer is dependent on the voltage entering the primary coil and the same number of turns in each of the coils. If the primary coil has 100 turns and the secondary coil has 50 turns, the voltage exiting the transformer

Copyright Goodheart-Willcox Co., Inc. 2017

286

Modern Refrigeration and Air Conditioning

will be half of the voltage entering the transformer. A transformer with more turns in its primary coil than in its secondary coil is a step-down transformer, Figure 12-26. If the primary coil has 50 turns and the secondary coil has 100, the voltage coming out of the transformer will be twice that of the voltage entering the transformer. A transformer with more turns in its secondary coil than in its primary coil is a step-up transformer. See Figure 12-27. The formula for calculating the number of turns required for a given voltage is as follows: vs= NS VP Np

VP Vs NP

= primary voltage = secondary voltage = number of turns of wire in the primary coil winding

N s = number of turns of wire in the secondary

coil winding Example:

How many turns of wire are required in a secondary coil if the desired output is 24 volts, the primary voltage is 120 volts, and the primary coil has 100 turns of wire?

Solution: Solve for Ns (number of turns in the secondary winding) by isolating that variable:

Transformer core

~=~

Primary coil

VP

Np

Begin by plugging in the values that are known.

~= NS

____.

____.

60 volts out

120 volts in

120 100 Isolate N s on one side of the equal sign. To do this, multiply the fractions on each side of the equal sign by 100 (the value of Np)100 1

24 120

NS 100

100 1

--x-- = --x-100 turns

Magnetic flux Goodheart-Willcox Publisher

Figure 12-26. This is a step-down transformer because there are more turns in the primary coil than there are in the secondary coil.

Transformer core

2400 = 100N5 120 100 After calculating these amounts, reduce to whole numbers. 2400 = 100 N 5 120 100 20 1 120 I 2400 = 100 I 100 NS -2400 - 100 -

Primary coil

0

____,.

120 volts in

____.

240 volts out

50 turns

100 turns Magnetic flux

Goodheart-Willcox Publisher

Figure 12-27. This is a step-up transformer because there are fewer wire turns in the primary coil than there are in the secondary coil.

0

20 =Ns N s = 20 turns of wire in the secondary coil

In later chapters, you will learn about the different types, sizes, and uses of transformers. This information is important when deciding which motor to use for a forced air system, what voltages to expect out of a transformer, and most importantly, what safety precautions to take.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter Review •

Summary •

•

•

•

•

•

•

•

•

•

The three components of an atom are electrons, protons, and neutrons. Electricity is the flow of electrons moving from one atom's orbit to another. The interdependent relationship of voltage, current, and resistance can be explained mathematically using Ohm's law: E = I x R. In HVACR systems, capacitors are used to store electrical energy to help start motors, increase motor efficiency, and improve a circuit's power factor. There are two types of electricity: static and current. Static electricity is stored electricity, like the charge in a capacitor. Current electricity is electrons in motion and can be divided into two types: direct current and alternating current. Electrical and electronic systems utilize three types of materials: conductors, insulators, and semiconductors. A closed circuit provides a complete path for electrons to follow. An open circuit is an incomplete path in which current cannot flow. A basic electrical circuit has three components: a power source, a conductor, and a load. The total voltage drop in a series circuit equals the sum of the voltage drops of the electrical loads in the circuit. Electrical loads in a parallel circuit will each have the same voltage drop. Comparing applied voltage to the sum of the measured voltage drops can help in troubleshooting electrical circuits. If an electric current is passed through a conductor, the conductor becomes surrounded by a magnetic field. An electromagnet is made by winding a conductor around an iron core and connecting it to a power source. The strength of an electromagnet is affected by the number of turns in the winding, the strength of the current, the core material and construction, and the length of the coil. If a conductor is moved across a magnetic field, an electromotive force (emf) will be induced that generates current in the conductor. Electrical generators use this concept to create electricity by rotating a wire loop in a magnetic field. Electricity flows from the wire loop, through the slip rings, across the brushes, and into the circuit.

A transformer transfers an alternating current from one coil of wire to another coil of wire using the following two principles: electricity can be used to generate a magnetic field, and a magnetic field can be used to induce electricity. The voltage coming out of the secondary coil of a transformer is dependent on whether it has more or less turns than the primary coil.

Review Questions Answer the following questions using the information in this chapter. 1. Which of the following is not a part of an

atom? A. Electron B. Coulomb C. Proton D. Neutron 2. The potential difference of atomic charges that forces electron flow is called _ _. A. inductance B. voltage C. resistance D. capacitance 3. Current is measured in A. coulombs B. amperes C. ohms D. farads 4. The electrical property that resists the flow of electrons is called A. inductance B. voltage C. resistance D. capacitance 5. Resistance is measured in A. coulombs B. amperes C. ohms D. farads 6. According to Ohm's law, if a circuit's resistance is 5 Q and the voltage applied is 100 V, what is the current? A. 500 A B. 0.05 A C. 115A D. 20A

Copyright Goodheart-Willcox Co., Inc. 2017

287

I

288

Modern Refrigeration and Air Conditioning 7. According to Ohm's law, if a circuit's resistance is 40 Q and the current is measured at 4 A, what is the voltage? A. 160V B. lOV C. 44 V D. 0.1 V

15. A circuit that allows the current to flow along two or more electrical paths at the same time is called a circuit. A. open B. series C. parallel D. series-parallel

8. The ability of a material to store a charge of free electrons or electrical energy is called

16. A circuit having only a single path for current is called a circuit. A. open B. series C. parallel D. series-parallel

A. B. C. D.

inductance voltage resistance capacitance

9. Capacitors are used in HVACR systems to do all of the following except _ _. A. increase motor efficiency B. induce an alternating current from a magnetic field C. improve a circuit's power factor D. help to start motors

17. A circuit in which parts of it have only a single path for current and other parts have two or more electrical paths at the same time is called a circuit. A. open B. series C. parallel D. series-parallel

10. Electron flow along a conductor in one direction describes A. static electricity B. current electricity C. direct current D. alternating current

18. Which of these devices has a north and a south pole? A. Conductor B. Insulator C. Semiconductor D. Magnet

11. Electricity that flows in one direction and then in the other describes A. static electricity B. current electricity C. direct current D. alternating current

19. To construct an electromagnet, all of the following are necessary except a(n) _ _. A. current-carrying conductor B. power source C. iron core D. commutator

12. Which type of material resists electron flow? A. Conductors B. Insulators C. Semiconductors D. Magnets

20. Name the electrically conductive, cylindrical part of an ac generator that rotates with the wire loop. A. Brush B. Slip ring C. Commutator D. Primary coil

13. Which type of material allows electrons to flow easily? A. Conductors B. Insulators C. Semiconductors D. Magnets 14. Which type of material can be designed to manipulate by light, pressure, heat, or electricity to either conduct or resist electron flow? A. Conductors B. Insulators C. Semiconductors D. Magnets

21. Name the rotating part of a de generator that is connected to the wire loop. A. Brush B. Slip ring C. Commutator D. Primary coil 22. Name the stationary part of a generator that transfers electricity to the circuit. A. Brush B. Slip ring C. Commutator D. Primary coil

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 12 Basic Electricity

289

23. Alternating current is transferred between primary and secondary transformer coils by means of A. capacitance B. brushes C. a closed circuit D. induction 24. A transformer that has more turns in its secondary coil than in its primary coil is a transformer. A. series B. parallel C. step-up D. step-down 25. A transformer that has more turns in its primary coil than in its secondary coil is a transformer. A. series B. parallel C. step-up D. step-down

Copyright Goodheart-Willcox Co., Inc. 2017

I

CHAPTER 13

Electrical PovVer

Learning Objectives ;

Chapter Outline 13.1 Electrical Power 13.1.1 Root Mean Square Values 13.1.2 Power Loss 13.1.3 Power Factor 13.2 Power Circuits 13.2.1 Single-Phase and Three-Phase Power 13.2.2 Electrical Codes 13.2.3 Wire Sizes 13.2.4 Connectors and Terminals 13.2.5 Receptacle and Plug Configurations 13.2.6 Circuit Protection 13.2.7 Grounding and Bonding 13.3 Electrical Problems 13.3.1 Short Circuit 13.3.2 Ground Fault 13.3.3 Overload 13.3.4 Unintentional Voltage Drop 13.3.5 Open Circuit

Information in this chapter will enable you to: Use mathematical formulas to calculate root mean square values, apparent power, and power factor.

=• 5 = ~ ii

.

Summarize how resistance, inductive reactance, and capacitive reactance cause power loss and affect power factor in electrical circuits.

•

Understand the difference between single-phase and three-phase power.

•

Define a Class 2 circuit and identify the types of electrical connections an HVACR technician is permitted to make.

•

Recall wire size terminology and connect wires using wire terminals and crimping.

•

Explain the importance of properly grounding and bonding an electrical system.

•

Describe the purpose and operation of various overcurrent protection devices used in circuits.

•

Identify the different types and causes of common electrical problems.

Chapter 13 Electrical Power

Technical Terms American Wire Gage (AWG) apparent power bonding capacitive reactance circuit breaker Class 2 circuit fuse ground ground fault ground fault circuit interrupter (GFCI) grounded conductor grounding

inductance inductive reactance overload power factor root mean square (rms) short circuit single-phase three-phase true power ungrounded conductor unintentional voltage drop volt-amperes (VA) Watt's law wattmeter

291

Review of Key Concepts Reviewing these concepts from earlier chapters will help you understand new concepts presented in this chapter: • Voltage is electrical pressure that causes current (electron flow) in a closed circuit. Voltage is measured in volts. Current is the flow of electrons and is measured in amperes. (Chapter 12) •

Resistance is the name of the electrical property that measures how much a material resists the flow of electrons through it. (Chapter 12)

•

Alternating current switches its direction of flow at regular intervals. The regular intervals at which an alternating current switches its direction can be graphed to form a sine wave. (Chapter 12)

•

An electrical circuit has three main components: a power source, conductors, and an electrical load. (Chapter 12)

Introduction To understand electrical power, a technician must first understand how to calculate power in a circuit and how factors such as resistance and capacitance affect a circuit's power. A technician must also understand the types of power supplied by utility companies, the types and sizes of wire used in circuits, and the methods for properly connecting, grounding, and bonding an electrical system. In addition, being familiar with the different types of overcurrent protection devices and common electrical problems will make diagnosing electrical issues much easier for an HVACR technician. Understanding electrical power will enable a technician to install new components and troubleshoot malfunctioning components based on the power available in a given application.

I

292

Modern Refrigeration and Air Conditioning

13.1 Electrical Power When current flows due to a potential difference (emf or voltage), there is electrical power. Electrical power is measured in watts (W), kilowatts (kW), and megawatts (MW). A kilowatt is equal to one thousand watts, and a megawatt is equal to one million watts. A watt is the power produced when one ampere of current flows through an electrical component due to a potential difference of one volt. In other words, a load uses 1 W of power if a current of 1 A flows through the load when it is connected to a 1 V power source. The following formula can be used to calculate electrical power: P =lxE P = power (watts) I = current (amperes) E = electromotive force or voltage (volts)

P=

Example:

P= Ix E

Solving for Power

What is the power used by an electric motor that draws a current of 20 A from a 120 V power source?

Solution: P =lxE P = 20Ax 120V P = 2400W

E=

E=~

I

or, expressed in kilowatts,

p = 2400W 1000 P = 2.4kW Much like Ohm's law, the power formula makes it easy to solve for any three of these variables. This formula has been called Watt's law, Figure 13-1.

Solving for Voltage

I=

13.1.1 Root Mean Square Values Alternating current fluctuates from positive to negative values within a cycle, which means the values for voltage and current are always changing. This poses a problem when trying to calculate the power used by an ac circuit because there is no constant value for voltage or current. Root mean square (rms) values are used to equate the heat produced by alternating current to direct current values that would produce the same amount of heat. Thus, the root mean square voltage for an alternating current equals the voltage of a direct current that would produce the same amount of heat, Figure 13-2. Most voltmeters and ammeters measure the voltage and current of an ac circuit in root mean square values, so a technician does not always have to calculate them. The root mean square voltage of an alternating current is also called effective voltage or rms

Solving for Current Goodheart-Willcox Publisher

Figure 13-1. Similar to Ohm's law, Watt's law provides a formula that can be used to solve for three variables.

voltage (V ). To calculate the rms voltage of an alternating cu;;~nt, multiply the maximum voltage value (VmaJ in the alternating current's cycle by 0.707. Vrms = Vmax x0.707 Example:

What is the effective voltage (rms voltage) of an ac power source with a maximum voltage of 170 V?

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 13 Electrical Power Root Mean Square Voltage + Volts

/ Voltage peak

a, Cl

S

~

\

Root mean square voltage is constant

0- - - -Voltage - - - -.. . . . .- - - - - - -~ - changes during ac cycle

-Volts

Time Goodheart-Willcox Publisher

Figure 13-2. An alternating current produces voltage that rises and falls. Root mean square voltage equates the fluctuating voltage value to a direct current value that would produce the same heating effect.

= Vmax X 0.707 = 170 V x 0.707 Vrms = 120.19 V

are in phase when they both reach their positive and negative peaks at the same time. This in phase condition only occurs in a resistive circuit, Figure 13-3. Power loss for a purely resistive circuit can be calculated using the following formula: P=FxR P = power loss (watts) I = current (amperes) R = resistance (ohms) Example: What is the power loss through a circuit that has a 5 A current and a 24 Q resistance?

Solution:

P=PxR p = 52 X 24 p = 25 X 24 P = 600 W power loss

Vrms

Vrms

The effective current, or rms current (I ;), of an ac power source is calculated the same as r:rr{; voltage. The maximum current value (I ) in the alternating current's cycle is multiplied by 0~707. I rms = I max x 0.707 Example: If the maximum current of an ac power source peaks at 5 A, what is the effective current (rms current)?

= I max x 0.707 Irms = 5 Ax 0.707 Irms = 3.535 A

I rms

293

This formula can also be used to calculate the power loss through individual components in a circuit. To find a circuit's total power loss using this method, all the wattages of each component are added to get the total power loss of the circuit. Note that this formula (P = 12 x R) is equivalent to the Watt's law formula. This is because (12 x R) can be reinterpreted as E x I. Remember that I x R = E and I x E = P. Therefore, 12 x R = I x E.

Inductive Reactance Power loss can also occur due to inductance. Inductance is an electrical property that opposes a change in current. Therefore, in an ac circuit where current is changing constantly, inductance's opposition to current change creates a noticeable power loss. The opposition

13.1.2 Power Loss

In Phase Resistive Circuit

Power loss is the difference between power output and power input. In some cases, the cause of power loss is electrical resistance. Electrical resistance is comparable to mechanical resistance, much like brakes on an automobile. As an automobile's mechanical brakes use friction to slow or stop a wheel's motion, electrical resistance slows or stops the flow of current. As the current pushes through the resistance, heat produced from the resistance is released, indicating a power loss. Extra unintentional resistance may result from a bad connection, improperly sized conductors, or other conditions. A circuit that provides only resistance is called a resistive circuit. Since resistance limits or resists the flow of current, it creates a voltage drop, but it leaves both voltage and current in phase. Voltage and current

+

C

~:::,

~ C)

o----~---- ---~---~--

s

I I I I

0

>

Voltage - Current - -

I

I

I

1/4

1/2

3/4 cycle

cycle

cycle

cycle

Time Goodheart-Willcox Publisher

Figure 13-3. In a purely resistive circuit, voltage and current are in phase.

Copyright Goodheart-Willcox Co., Inc. 2017

I

294

Modern Refrigeration and Air Conditioning

of inductance to current change causes voltage and current to alternate out of phase. The alternating current lags behind the alternating voltage. This opposition to alternating current that causes current to lag behind voltage is called inductive reactance, Figure 13-4. Like resistance, inductive reactance causes power loss and is measured in ohms (Q). Examples of inductive components include motors, relays, transformers, and speakers. Inductors are generally devices with coils of wire.

Phase Shift in a Capacitive Circuit - - - - + - Voltage lag

+

c ~:::,

~

01'------,t-- - -- - - -- - - -....-- -

C)

I I I I

I I I I

I

I

I

1/4 cycle

1/2 cycle

3/4 cycle

.19

;g

Capacitive Reactance Capacitance is the ability to store an electrical charge in an electrostatic field. See Chapter 12, Basic Electricity. When ac voltage is applied to a capacitor, the plates of the capacitor charge and discharge repeatedly. As the voltage builds on one plate of the capacitor, electrons discharge from (current flows from) the other plate of the capacitor. As a result, a phase shift occurs in which alternating voltage lags behind alternating current. This opposition to alternating current that causes voltage to lag behind current is called capacitive reactance, Figure 13-5. Like inductive reactance and resistance, capacitive reactance produces resistance to the flow of alternating current and is measured in ohms (Q). Capacitive reactance also causes power loss in circuits. Most capacitive components are capacitors.

13.1.3 Power Factor As discussed earlier in this chapter, power can be calculated with the following formula: P = I x E. This calculated value is called apparent power, as it does not take into account the effects of inductive reactance

Phase Shift in an Inductive Circuit 1----+-

Current lag

C

~:::,

~

o,____,,_____ _________ _ _

C)

.19

;g Voltage - Current - -

I

I

I

1/4 cycle

1/2 cycle

3/4 cycle

cycle

Time Goodheart-Willcox Publisher

Figure 13-4. In an inductive circuit, voltage and current are out of phase. Current lags behind voltage because inductance resists a change in current.

Voltage - Current - -

I cycle

Time Goodheart-Willcox Publisher

Figure 13-5. In a capacitive circuit, voltage and current are out of phase. Voltage lags behind current because as voltage builds on one plate of a capacitor, current is peaking and being discharged from the other plate of the capacitor.

or capacitive reactance. The value of apparent power is always calculated in volt-amperes (VA). To find the actual power used by a circuit, which is called the circuit's true power, take a reading with a wattmeter. A wattmeter is an instrument that measures a circuit's true power, and true power is always measured in watts. The apparent power of a circuit can equal its true power, but this only occurs if the circuit is purely resistive. In other words, apparent power equals true power when the circuit only has resistive components and does not have inductive components (motors, relays, etc.) or capacitive components (capacitors). Without the influence of these two electrical properties, the voltage and current will alternate in phase and apparent power will equal true power. However, when inductive or capacitive components are in a circuit, two results will occur: • Voltage and current will alternate out of phase. • Apparent power and true power will differ. A circuit's power factor shows the relationship between a circuit's true power and apparent power. Power factor is the ratio of true power (a wattmeter reading) to apparent power (calculated power in voltamperes) and is given as a percentage. Power factor = ( True power ) x 100 Apparent power Example:

Connected to an ac circuit, a voltmeter reads 120 V, and an ammeter reads 10 A. Using the formula for power, we can calculate the apparent power:

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 13 Electrical Power

P =lxE P = 120Vx lOA P = 1200VA

295

Pro Tip

Electrical Equipment Variables

Connecting a wattmeter to the circuit measures its true power value. It reads 1000 W. To calculate the circuit's power factor, divide the true power by the apparent power, and multiply the result by 100. Power factor = ( True power ) x 100 Apparent power Power factor = ( lOOO W ) x 100 1200VA Power factor = 0.83 x 100 Power factor = 83% Because the true power and apparent power are not equal, the power factor of this circuit is below 100%. This means the circuit must have an inductive or capacitive component that is resisting the change in current. Thinking Green

Improving Power Factor To improve the power factor of inductive circuits, add capacitors to the circuit. In this way, the capacitive reactance of the capacitors will counteract the inductive reactance of the inductors. Based on the apparent power calculation, a technician can install a capacitor and compare the true power readings of a wattmeter with the apparent power to improve a circuit's power factor. The ideal power factor should be as close as possible to 100%. This will result in the most efficient and economical use of energy.

13.2 Power Circuits Electrical loads and their circuits must be compatible with the power provided by an electric utility company. Compatibility variables include voltage level, current capacity, frequency (in Hertz), and voltage phase. Wires must be large enough to carry the full or maximum current that electrical loads will use. Electrical loads must be designed to operate using a circuit's frequency, which is 50 Hz or 60 Hz, depending on location. The most commonly used voltage phases are single-phase and three-phase. Figure 13-6 shows some of the common voltage, frequency, and voltage phase options that electric utility companies supply.

Check with the electric utility company before installing equipment of any sizable horsepower. Remember that most of North America distributes electricity at 60 Hz, but many other countries distribute electricity at 50 Hz.

13.2.1 Single-Phase and Three-Phase Power The two most common voltage phase options used in HVACR are single-phase and three-phase: • 240 V single-phase power is usually supplied to residential homes. • 480 V three-phase power is usually supplied to commercial buildings. A single-phase voltage cycle has a single alternating current. The voltage starts at zero, rises to a positive maximum, falls to a negative maximum, and rises to zero again as the cycle repeats. There is no power produced during the instant that the voltage is zero. Most power circuits in a residence operate on 120 V single-phase power, Figure 13-7. More than one alternating current may be used in a single circuit. However, each alternating current is out of phase with the other alternating currents. Such an arrangement is called polyphase. The most widely used polyphase option is three-phase voltage, which has three separate voltage signals alternating in three separate phases, Figure 13-8. The separate voltage signals in a three-phase cycle are delayed so that they peak at different times.

Residential and Commercial Electrical Service Options Voltage

Frequency

Phase

115

60

Single

120

60

Single

208

60

Single

230

60

Single

230

60

Three

Caution

240

60

Single

AC/DC

240

60

Three

Never connect alternating current appliances or instruments into direct current circuits. Never connect direct current appliances or instruments into alternating current circuits.

480

60

Three Gaadheart-Willcax Publisher

Figure 13-6. This chart shows typical residential and

commercial voltages, frequencies, and phases.

Copyright Goodheart-Willcox Co., Inc. 2017

I

296

Modern Refrigeration and Air Conditioning Single-Phase Power I

170V

___ I ___ I ____ I __ I I I I

~ c,

;gs 0f-----.---------~---~-I I I

I

- - - -1- - - - t- - - -170V

I

I

I

1/4 cycle

1/2 cycle

3/4 cycle

cycle

Time Goodheart-Willcox Publisher

Figure 13-7. This graph represents one complete cycle of alternating current in a typical 120 V residential circuit. Note that for a 120 V circuit, the maximum voltage value is 170 V. By multiplying 170 V by 0.707, you can confirm that 120 Vis the root mean square voltage of the circuit.

Three-Phase Power

used in installing electrical systems. In addition, many cities and communities have supplementary local codes. All electrical installations must be made in conformity with national and local codes. For HVACR systems, most electrical circuits fall under the NEC definition of a Class 2 circuit. Class 2 circuits are defined as circuits supplied by a power source that has an output no greater than 30 V and 1000 VA In addition, a Class 2 circuit is defined as the portion of a wiring system between the power source and the connected equipment. Examples of Class 2 circuits include remote-control circuits with a relay or any other device that controls another circuit, such as a circuit for a thermostat. Class 2 circuits also include signal circuits, examples of which include circuits for a warning buzzer or signal light. HVACR service technicians are permitted to make Class 2 connections and installations. Examples include connecting electrical devices used to control furnaces or installing heat pumps and other HVACR equipment in residences. In addition, service technicians are allowed to install and service low-voltage components and wiring within HVACR equipment. Any component that is integral to the proper operation of HVACR equipment counts as being "within" the equipment, even if it is not physically inside it.

~

C)

S

;g

0t--"""Current switches directions

...,.._

Motor speed is initially calculated as synchronous speed. Synchronous speed refers to the speed of the rotating magnetic field in the stator. If the rotor rotates at the same speed as the stator's rotating magnetic field, then the motor runs at synchronous speed. To calculate a motor's synchronous speed (Ns) in revolutions per minute (RPM), use the following formula: 120 x

f

frequency (Hz) number of poles

P

Like poles repel each other

'--------------- +

t

NS

Example: What is the synchronous speed of a two-pole motor that operates at a frequency of 60 Hz?

~

Goodheart-Willcox Publisher

Figure 15-5. The magnetic polarity in the field po~es is . reversed as the direction of the current alternates in the field windings. This causes the rotor to turn as the polarities of the field poles change.

resistance to current drops. With such high current, the motor will then overheat. Continuous operation with too great a load is likely to burn out a motor. If the rotor is locked so it cannot turn and voltage is applied, current will be very high. A motor wi_ll quickly burn out when the rotor cannot turn o~ 1s locked. Locked rotors may occur due to a motor havmg to start under a heavy load or mechanical interference. Examples include motors connected to fans with worn bearings, which cause excessive strain on the motor, and compressors pumping against excessively high head pressure.

15.1.4 Motor Speed The speed of an ac electric motor is determined by two variables: • The alternating current's frequency (measured in Hz). • Number of magnetic field poles in the stator.

Solution:

t

NS

120 x

NS

6 120 x ;

Ns NS

120x30 3600RPM

The rotor in a two-pole motor rotates once with each cycle of alternating current (one half turn for each change of polarity), which means it turns 60 revolutions per second if the frequency is 60 Hz. Thus, a two-pole motor has a synchronous speed of 3600 RPM (60 revolutions x 60 seconds = 3600 RPM), Figure 15-6. If four poles are used in the stator, a motor's synchronous speed is 1800 RPM. The rotor only turns onehalf of a rotation for each cycle of alternating current. At a frequency of 60 Hz, the rotor completes 30 revolutions per second, which means it makes 1800 revolutions per minute (30 revolutions x 60 seconds = 1800 RPM). Most open-drive compressors and some hermetic compressors use four-pole motors, Figure 15-7. Two-pole motors are about two-thirds the size of four-pole motors that have the same power. The number of poles used in a motor is dependent on the application. For example, a high-speed blower will use a two-pole motor, while a much slower condenser fan will likely use a four-pole motor. It is also possible to

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 15 Electric Motors build motors having six, eight, or more poles. Many direct-drive compressors use a six-pole motor that operates at 1200 RPM.

Rated Full-Load Speed Because electric motors do not operate at exactly synchronous speed, they are not rated at synchronous speed. Instead, they are rated at their operating speed

under a full load, which is called rated full-load speed. The difference between synchronous speed and rated full-load speed is called slip. Figure 15-8 shows the operating speeds for two-, four-, and six-pole motors operating at different frequencies. Under actual conditions, a 3600 RPM motor operates at approximately 3450 RPM. An 1800 RPM motor operates at approximately 1750 RPM. This reduction

Stator Field windings

329

Stator Rotor

I + Motor shaft

+

Field pole

Four-Pole Motor

Two-Pole Motor

Stator poles switch

Current switches direction

Current switches direction

~+

Rotor makes half turn

\

Rotor makes quarter turn Half Cycle

Half Cycle

Rotor makes half turn

+

+

Full Cycle Goodheart-Willcox Publisher

Figure 15-6. In a two-pole motor, the rotor completes one full turn with each cycle (Hertz) of alternating current.

Full Cycle Goodheart-Willcox Publisher

Figure 15-7. With a four-pole motor, the rotor makes only onehalf of a rotation as alternating current completes one cycle.

Copyright Goodheart-Willcox Co., Inc. 2017

330

Modern Refrigeration and Air Conditioning

Motor Speed 60 Hz

50 Hz

Poles

2

Synchronous

Operational

Synchronous

Operational

3600 RPM

3450 RPM

3000 RPM

2850 RPM

4

1800 RPM

1750 RPM

1500 RPM

1450 RPM

6

1200 RPM

1150 RPM

1000 RPM

950 RPM Goodheart-Willcox Publisher

Figure 15-8. Synchronous and operational speeds for two-, four-, and six-pole motors at 60 and 50 Hz. Note that operational speed is approximate.

in speed is due to the slight magnetic slippage, which varies depending on the load. Generally, motor slip is between 4% and 5% of synchronous speed.

15.1.5 Motor Efficiency Motor efficiency is the mechanical energy produced by the motor shaft divided by the power input to the motor. Motors are not 100% efficient because of clearances, bearing friction, and imperfect windings. For a given voltage input, larger motors produce more mechanical energy at the shaft. While larger motors operating at a certain voltage may be up to 97% efficient, the efficiency of smaller motors at the same voltage is often only 50% to 60%.

15.2 AC Induction Motors Alternating current is the most commonly used operating current for HVACR motors. An ac motor is a motor that runs on alternating current. An ac motor can be further classified as either an induction motor or a synchronous motor. Induction motors, which are the type discussed earlier in this chapter, are ac motors that operate by using the magnetic field generated in the stator to induce current in the rotor. Induction motors are categorized in a number of ways. Motors differ from each other by the amount of starting torque and running torque that they generate. Torque is the work performed by a twisting or turning action, such as a rotating motor shaft. Induction motors can also be differentiated by their required input power: single-phase, two-phase, three-phase, and fourphase. Many small ac motors are single-phase, while many of the larger ac motors are three-phase.

15.2.1 Single-Phase Motors A single-phase motor is an ac motor that runs on a single phase of alternating current. Single-phase

motors are used in many residential applications because the power supplied by utility companies to residential homes is single-phase power. Most singlephase motors are rated for 120 V, 208 V, or 240 V power. Many single-phase motors can run either clockwise or counterclockwise. For many of these motors, the rotation direction can be changed by reversing the connections to the start winding. There are many different designs of single-phase motors. These motors differ based on the applications they are used in and the methods that are used to start and run them. The following single-phase motors are the most common, and each type is discussed in detail later in this chapter: • Split-phase motor. • Capacitor-start, induction-run (CSIR) motor. • Capacitor-start, capacitor-run (CSCR) motor. • Permanent split-capacitor (PSC) motor. • Shaded-pole motor.

Start and Run Windings Most single-phase induction motors have two types of stator windings: a start winding and a run winding. Remember that induction motors transfer electricity from the stator (start and run windings) to the rotor through induction, which is the same way a transformer transfers electricity between primary and secondary coils. Remember also that induction causes inductive reactance, which is the opposition to the flow of current that causes current to lag behind voltage in an ac cycle. Run windings are stator windings that are energized during the entire operation of the motor. They provide the bulk of the magnetic force for driving the rotor. Start windings are stator windings that are used for motor starting and additional torque. For each run winding there is a start winding. Start winding coil is made of a smaller diameter wire and has more turns than a run winding coil, Figure 15-9.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 15 Electric Motors

The start winding may be of smaller diameter than the run winding because it is only energized for a short time and is not required to handle a continuous current as the run winding must do. Being made of a smaller gage wire, the start winding has a higher resistance than the running winding. Having more coil turns, the start winding will also have a higher inductance than the run winding. Since the start and run windings have different inductance values, the current flowing through the start winding is out of phase with the current flowing through the run winding, Figure 15-10. This is called phase splitting. Phase splitting is the means by which single-phase motors are started. The split phases create a rotating magnetic field in the stator, causing the rotor to start turning. Most of the current in a single-phase induction motor is conducted through the run winding. When the motor is starting, however, current goes through both the start and run windings. When the motor reaches 60% to 75% of its rated full-load speed, the start winding circuit is opened by a centrifugal switch or a starting relay. See Figure 15-11. The motor then operates on the run winding only. If the start winding is left in the circuit, it may overheat.

331

When power is applied to the motor, electricity flows through the start and run windings. In the run winding, current lags behind voltage due to inductive reactance. In the start winding with a capacitor wired in series, voltage lags behind current due to capacitive reactance. This causes phase splitting, but with a much larger displacement between the phases than can be caused by just the start and run windings alone. A larger phase displacement leads to a higher starting torque, which means motors that use capacitors can start under heavier loads, Figure 15-12. There are two types of capacitors used with ac motors: start capacitors and run capacitors. A capacitor used only during motor start-up to provide initial starting torque is called a start capacitor. Start

Phase Splitting

--1 1-- Phase splitting caused by inductance +

Current in run winding Current in start winding

-~o~~------~ ~ - - - - - - - - C

:::,

0

Motor Capacitors In some cases, single-phase induction motors use capacitors to create a much larger phase displacement to help start the motor. Capacitors cause capacitive reactance in a circuit, which is the opposition to the flow of current that causes voltage to lag behind current in an ac cycle. In a motor with both start and run windings, capacitors are added in series with the start winding.

Time Goodheart-Willcox Publisher

Figure 15-10. Phase splitting occurs in a single-phase motor because the run winding has less inductance than the start winding so current flows through the run winding ahead of the start winding.

Centrifugal switch

Start windings

Run windings Photo courtesy of A. 0. Smith

Figure 15-9. Start windings in a single-phase motor are made of smaller gage wire than run windings.

Goodheart-Willcox Publisher

Figure 15-11. A centrifugal switch is used to disconnect power to the start winding. The circuit for the run winding remains complete so power is still provided to the run winding.

Copyright Goodheart-Willcox Co., Inc. 2017

I

332

Modern Refrigeration and Air Conditioning Capacitive Phase Splitting

+

17

Larger phase shift caused by capacitive reactance Current in run winding

Start capacitor

Centrifugal switch

~-------1i(1-----o

~">---+--

Input power

/Current in start winding

.. C

~o~-- - - - -~ - ~~- - - -~ - ~ -

:::, (.)

Run winding

Time Goodheart-Willcox Publisher

Goodheart-Willcox Publisher

Figure 15-12. With a capacitor wired in series with the start

Figure 15-13. A start capacitor is wired in series with the

winding, current leads voltage due to capacitive reactance, while current lags voltage in the run winding due to inductive reactance. This causes a larger phase shift than in motors that do not have capacitors.

start winding and a centrifugal switch. The centrifugal switch disconnects both the start winding and the capacitor after the motor starts running.

capacitors are only used for a fraction of a second. A centrifugal switch or relay drops the start winding and start capacitor out of the motor circuit after the motor starts running, Figure 15-13. A start capacitor is never used in a stator's run winding circuit. Start capacitors are usually dry electrolytic capacitors. A typical start capacitor is shown in Figure 15-14. Caution

Start Capacitor Duty Start capacitors are built to specifications for cyclical duty. They are only meant to operate for short periods of time. If a start capacitor is left in a circuit too long (due to a faulty centrifugal switch or relay), it may damage the motor windings.

A run capacitor operates in the same way as a start capacitor, except it remains in the start winding circuit while the motor is running. It provides a signal that is out of phase for added torque during the motor's entire operation. Run capacitors are filled with oil and are designed to dissipate the heat generated by the high current used to run a motor, Figure 15-15. Caution

Defective Run Capacitors Be sure that any run capacitors you encounter on service calls are operating properly. Defective run capacitors can cause a motor to draw higher than normal current. This could trip the overload protection device or damage the motor.

DiversiTech Corporation

Figure 15-14. Start capacitors are used to increase a motor's

starting torque.

Split-Phase Motors A split-phase motor is a single-phase induction motor used in applications that operate in the fractional horsepower range. Small condensing units and fans that require up to 1/3 hp are often driven by split-phase motors. A split-phase motor uses the different inductance values of its start winding and run winding to produce phase splitting and achieve initial rotation. As a result, the starting torque is lower than motors that use capacitors, which means split-phase motors must be used on systems with an easily starting load. Split-phase motors are very popular in HVACR

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 15 Electric Motors

333

on the end of a motor shaft that disconnects the start windings from the circuit. See Figure 15-16. A centrifugal switch consists of weights that are held close to the motor shaft by the force of a spring. These weights hold a plate against a set of electrical contacts to keep them closed. As the motor shaft approaches its running speed, the weights overcome the spring's force and move away from the shaft due to centrifugal force. This pulls the plate away from the electrical contacts and opens the circuit.

Capacitor-Start, Induction-Run Motors

Goodheart-Willcox Publisher

Figure 15-15. Run capacitors are used to increase a motor's

running torque.

systems that use a capillary tube metering device. In these systems, low-side and high-side pressures balance when the system cycles off. Thus, the split-phase motor is not required to start the compressor under full-load conditions.

A capacitor-start, induction-run (CSIR) motor is a single-phase induction motor that has a start capacitor wired in series with the start winding. The start capacitor puts the current in the start winding out of phase with the current in the run winding. The electromagnetic flux of the two out-of-phase windings provides very high starting torque. The capacitor is usually placed on top of the motor in a metal or plastic cylinder, Figure 15-17. During start-up, current passes through both the start winding and the run winding. At about 75% of the motor's rated speed, a centrifugal switch or a relay in series with the start winding opens. This disconnects the start winding and start capacitor. The motor continues to run as an induction motor using only the run winding. Capacitor-start, induction-run motors

Electrical Weights attached to spring

Spring

Pro Tip

Unloaders Split-phase motors can also be used in systems with an electrical, mechanical, or hydraulic unloader. An unloader is a device that can be used to reduce a compressor's load on start-up or for capacity control. If a system is equipped with an unloader, a split-phase motor can be used regardless of the type of metering device in the system. Unloaders will be covered in greater depth in later chapters.

Split-phase motors are typically built using a squirrel cage rotor and are available in either 120 V or 240 V. A split-phase motor's start windings are disconnected by a centrifugal switch or relay when the motor reaches approximately 75% of its running speed. A centrifugal switch is an electrical device mounted

Motor shaft

Weights move outward as shaft rotates

Plate

Motor frame

Goodheart-Willcox Publisher

Figure 15-16. A centrifugal switch is mounted on the end of a

motor shaft. As the shaft approaches its full rotational speed, the weights on the switch move outward, pulling the spring and disconnecting the start windings from the motor circuit.

Copyright Goodheart-Willcox Co., Inc. 2017

&

334

Modern Refrigeration and Air Conditioning Start capacitor enclosed in cylinder

Run capacitor Start capacitor

Start

j

Centrifugal switch

_ ____.,____, 1-----o

~'.)--- 0.08i-----1;-----i----i----t--,ts;:-----t--1rIA

"''" r-1/4

r-114 6-0 .68

6 -9 .68

"''"

r-1/4 7-0 .70

nm

1

Utilization range "A" in accordance with ARI standard 110. 2 Dual element fuses or HACR circuit breaker. 3 The Unit Charg e Is correct for the outdoor unit, matched Indoor coll and 15 feet of refrigerant tubing. For tubing lengths other than 15 feet, add or subtract the amount of refrigerant calculated, using the difference in length multiplied by the per foot value. York International Corporation, Unitary Products Group

Figure 46-14. Unit label indicating capacity and efficiency must be carried by all air conditioning and refrigeration units. Copyright Goodheart-Willcox Co., Inc. 2017

I

1270

Modern Refrigeration and Air Conditioning

The formula for calculating a SEER is: SEER= QC p where Qc = Sum of all cooling outputs under all test conditions (Btu/hr) P = Sum of all power inputs under all test conditions (W) Pro Tip

SEER System Ratings It is important to remember that all parts of a system contribute to a given SEER rating. If you replace a customer's condensing unit, the system may not be able to perform at the SEER rating given to that condensing unit. The condensing unit was given its SEER rating based on use with an evaporator of a certain size. If an evaporator is not replaced with the one specifically designed for the new condensing unit, the system probably will not be able to perform at its SEER rating. Connecting parts of a system based only on their nominal rating will not guarantee SEER rating performance.

46.2.3 Heat Pumps The efficiency of a heat pump's cooling cycle can be measured using the same efficiency ratings used for an air conditioner (COP, EER, or SEER). However, the efficiency of a heat pump's heating cycle must be measured using a coefficient of performance (COP) or a heating seasonal performance factor (HSPF). A heating seasonal performance factor (HSPF) is a measurement of how efficiently a heat pump works throughout the heating season. Whereas a heat pump's COP is a snapshot of how efficient the heat pump is under a specific set of operating conditions, the HSPF gives the consumer a better idea of how efficient the heat pump is under varying operating conditions. The relationship between a COP and an HSPF is very similar to the relationship between an EER and a SEER. An HSPF is calculated as follows: HSPF = Qh p where

Qh = Sum of all cooling outputs under all test conditions (Btu/hr)

Coefficient of Performance A coefficient of performance (COP) is similar to

P = Sum of all power inputs under all test

an EER in that it is used to express the heating or cooling effect per energy input under specific operating conditions. However, in calculating a COP, the heating or cooling output and the power input are both converted to the same units before the calculation is made. As a result, the units cancel out. Therefore, a COP rating has no units associated with it.

46.3 HVAC Alternatives for Energy Conservation

E

COP=_Q Ei where E0 = Energy output under test conditions (Btu) Ei = Energy input under test conditions (Btu)

A COP is typically a snapshot of the system's efficiency, like an EER. However, a COP can also be used to describe the average efficiency of the system throughout a heating or cooling season. This is commonly referred to as a seasonal average COP. E

Seasonal Average COP = ~ E.SI where E50 = Total energy output for season under all conditions (Btu) Esi = Total energy input for season under all conditions (Btu)

conditions (W)

Because HVAC systems account for such a large portion of energy use and utility costs, various methods of improving their efficiency have emerged. Through the use of new refrigerants, energy-efficient motors, and system balancing, the HVAC industry is continually improving the efficiency of systems and buildings. New, energy-efficient systems are a good alternative for new construction, but facility managers in existing buildings have to consider whether installing an entirely new system is more cost-effective than revamping the existing system. Even if the new system is considerably more efficient to run, they calculate how long it would take to recoup the initial expense of a new system. The first step toward saving energy should therefore be to take a close look at the existing system. A formal energy audit should be performed, as described in Chapter 45, Energy Management. The audit may identify several low-cost opportunities for saving energy, such as improving duct insulation in certain areas and fixing detected air leaks. Resolving these performance

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 46 Energy Conservation issues may result in a surprising increase in energy efficiency. Code Alert

Equipment Performance Requirements Local building codes may specify a minimum efficiency requirement for new and replacement equipment. If the components of a system are designed to work together, the efficiency rating for the system is typically available from the manufacturer. If separate components from different manufacturers are assembled on site, the system designer may need to provide calculations proving their combined efficiency.

46.3.1 Energy-Saving Components After all of the obvious performance issues have been addressed, the next step may be to determine whether updating specific parts of the system (rather than replacing the entire system) would result in a significant reduction of energy use. Some components require only minimal installation cost, while others are more labor-intensive.

Programmable Thermostats Possibly the easiest component to change in an existing system is the thermostat. Both residential and business customers can benefit immediately by replacing older thermostats with programmable thermostats. Programmable thermostats are discussed in greater detail in Chapter 36, Thermostats.

Variable Speed Motors and Variable Frequency Drives Replacing older compressors with compressors that can operate at variable speeds can result in greater equipment efficiency and lower electrical costs. A variable speed motor is one in which the output speed can be changed based on need or other external factors. Variable frequency drives can also be installed in certain applications to help save energy. A variable frequency drive varies the frequency and voltage of a motor to control motor speed and torque. For example, variable frequency drives can be added to pumps to intelligently adjust their motor speeds. Some larger fan motors on older HVAC units can be fitted with variable frequency drives to adjust airflow. Smaller fan motors might be replaced with an ECM (electronically commutated motor).

Heat Exchangers In traditional HVAC systems, a large amount of energy is wasted when conditioned air is simply exhausted to the outside of the building. This energy

1271

can be recaptured and conserved with the proper equipment. Heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs) are air-to-air heat exchangers. The heat of outgoing conditioned air is passed to incoming unconditioned air. This brings unconditioned air closer to indoor temperature, meaning that it will require less conditioning and less energy expended by the HVAC system. Heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs) improve system efficiency by adjusting the temperature of incoming air and conserving the energy already used in conditioning the air. These air-to-air heat exchangers are discussed in Chapter 29, Air Quality. Thinking Green

Outdoor Ventilation and Heat Reclamation Although outdoor air can be brought in to improve indoor air quality, this approach can result in considerable energy waste. The use of heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs) can reduce the waste by 60%-80%.

Another type of heat exchanger is the refrigerantto-water heat exchanger. This type can be used in domestic hot water systems in residential settings to heat the water using waste heat from an air conditioning system or heat pump. If a heat pump is used, this system can provide hot water year-round. If an air conditioner is used, the water is heated only when the air conditioner is running.

Solar Products In residential applications, the addition of a solar water heater or solar pool or spa heater can result in significant energy savings. Also, some air-conditioning equipment is designed to be powered by the energy produced by solar photovoltaic (PV) systems. These different systems are described in more detail in Chapter 44, Solar Power and Thermal Storage.

Building Control Systems As described in Chapter 45, Energy Management, an existing HVAC system can be integrated into an automated building control system. The building control system reduces energy costs by integrating the HVAC system with other building systems so that all of the systems run at top efficiency.

Methods of Subcooling One method of increasing a mechanical HVAC system's capacity and energy efficiency is to increase its subcooling. This is especially beneficial when outdoor ambient temperature is high. Higher ambient

Copyright Goodheart-Willcox Co., Inc. 2017

I

1272

Modern Refrigeration and Air Conditioning

temperature requires a mechanical HVAC system to work harder to displace heat outdoors. The harder a system must work, the more energy it must expend in operation. An air-cooled condenser must raise head pressure to a level that corresponds with a high-side refrigerant temperature that is a certain number of degrees hotter than ambient air. Remember that a temperature difference is necessary for heat displacement. Therefore, high-side refrigerant must be hotter than ambient temperature to displace heat into outdoor air. The hotter ambient temperature is, the higher the corresponding head pressure must be. The higher the head pressure, the more amps the compressor must pull to create the high pressure. The more amps, the more the electrical power consumed during operation. The change of one variable can create a chain reaction affecting other variables, Figure 46-15. There are two primary factors to change when trying to increase subcooling. These are as follows: • Increasing the amount of heat displaced from the refrigerant on the high side. • Decreasing the amount of electrical power used to operate the HVAC system. Often, these two factors can go hand in hand. There are several different methods of increasing subcooling in a mechanical HVAC system. A simple preventive method of doing this is to block the condenser from the rays of the sun. The sun transmits a tremendous amount of heat energy, and sunlight on a condensing unit is adding to the heat load. A source of shade, such as a sun barrier or a tree can help reduce the initial heat load of a system. This can help keep subcooling lower and save energy; however, adequate airflow must still be ensured for proper system operation. Another method of increasing subcooling is to use a suction line-liquid line heat exchanger. There are different builds available for these devices. A common example is a tube-within-a-tube construction. These

allow the cool suction gas of the low side and the warm liquid of the high side to exchange heat. The cooler the liquid going to the evaporator, the greater its cooling effect. Evaporative or water cooling is another effective method of increasing subcooling. Remember that water can absorb more heat than air. In such a system, water vapor is sprayed into the condenser airways, where it evaporates and increases the amount of heat removed from the high-side refrigerant. Home kits are available using flexible tubing, nozzles, and a water pump, Figure 46-16.

Nozzles

Water tubing

Malcolm Prather

Figure 46-16. When this residential condensing unit cycles on, a water pump pressurizes the water line to create an envelope of mist that is drawn in through the sides of the unit. The water droplets increase the amount of heat absorbed from the refrigerant and decrease its temperature, increasing subcooling and system performance.

Chain Reaction of Factors Affecting Efficiency Factors High-Side Temperature

Ambient Temperature Rise

Ambient Temperature Drop

Increases

Decreases

High-Side Pressure

Increases

Decreases

Compressor Current Draw

Increases

Decreases

Electrical Power Usage

Increases

Decreases Goodheart-Willcox Publisher

Figure 46-15. The change of one variable in an HVAC system can cause a chain reaction. Variables associated with subcooling and HVAC system efficiency are shown here. Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 46 Energy Conservation

46.3.2 Total Energy Systems Recently, many large buildings have been constructed using "total energy" or "single energy" systems. In a total energy system, all of the energy-using devices are designed to capture and use all or most of the so-called "waste" energy and by-products of combustion. Heat energy that was formerly considered waste is used to generate power. In other words, the system generates both heat and electricity in the building. Systems that produce both heat and electricity are also known as combined heat and power (CHP) or cogeneration systems. These concepts were first introduced in Chapter 34, Absorption and Evaporative Cooling Systems. For example, instead of depending on the electric utility to provide both electricity and gas to a building, the building is supplied with natural gas only. The combined heat and power unit burns this fuel to produce both heat for the building and to run an electric generator to produce electricity on site. Advantages of a total energy system include the following: • Increased energy efficiency. • Reduced air pollution. • Lower utility costs. • Reduced demand on the utility grid. Electricity is generated using a reciprocating gas engine, gas turbine, or steam turbine. The engines drive electric generators in the building. While running, these engines produce heat that must be displaced. Water is often used to cool the engine and capture this heat, which would otherwise be wasted. Once captured, the heat can be put to another use. Engine exhaust gas is another major heat source. A gas engine or turbine uses about 33% of its fuel energy to generate electricity. Another 30% produces heat that ends up in the water-cooling jacket. About 30% produces heat that is released in the exhaust gases. The remaining 7% produces heat that warms the lubricating oil or is lost by radiation. Heat from the cooling jacket water, exhaust gases, and hot lubricating oil can be converted to useful purposes: • Absorption systems often use the exhaust heat to operate air conditioning (comfort cooling). • Exhaust heat from turbines may be used to raise the temperature of water in a hydronic heating system during the heating season. • Heat from the water jacket can be used to furnish nonpotable hot water or with a heat exchanger to heat potable water. A total energy system may include other aspects as well. For example, heat from the return air ducts of the heating system can be released through channels

1273

in lighting fixtures. As the cool return air passes across the lighted fixtures, the air is heated. The needed heat for that room is then supplied through a duct. The surplus heat from the lights is passed back to the primary duct system, where it can be directed for use wherever it is needed in the building. Total energy systems are gaining in acceptance as companies and utilities strive to lessen their impact on the environment and to reduce cost. The principle has also been applied to more than just wasted heat energy. For example, large industrial plants may perform operations that generate large amounts of waste products, such as waste heat from manufacturing processes or wood chips from paper mills. In the past, these were considered necessary wastes and were disposed of. Total energy systems improve energy efficiency by using these wastes to generate useful energy, such as electricity, comfort heating, or process heating.

46.3.3 Evaporative Cooling Designs If warm, dry air is moved rapidly over water at the same temperature, some of the water evaporates. When the water molecules evaporate, they absorb heat energy from the surrounding molecules, causing the temperature of the remaining liquid water to drop. When this natural cooling process is used to cool a space or objects, it is referred to as evaporative cooling. This cooling method is used primarily in climates with high ambient air temperatures and low relative humidity. In dry climates, evaporative cooling is both effective and practical. Evaporative cooling and roof mist cooling systems are discussed in greater detail in Chapter 34, Absorption and Evaporative Cooling Systems.

Ponded Roofs In some parts of the world, buildings are provided with some summer comfort cooling by ponded roofs. A ponded roof is a flat roof that maintains 2" to 3" (5 cm to 8 cm) of standing water, which absorbs heat from the sun and dissipates that heat through evaporation. This type of cooling is well suited to commercial one-story factory and market buildings that are constructed to be able to handle the additional weight of a ponded roof. To be effective, the roof area should be as large as the floor area. The cooling effect comes from the evaporation of water from the roof. Naturally, ponded systems are most effective in areas that have a high temperature, low relative humidity, and bright sunshine. By ponding, roof temperature may be kept below that of the surrounding atmosphere. Ponded roofs require a means of maintaining a constant level of water on the roof. This is accomplished

Copyright Goodheart-Willcox Co., Inc. 2017

I

1274

Modern Refrigeration and Air Conditioning

by placing the roof gutter drains at the correct height to maintain an adequate water depth on the roof. Once the water reaches this height, excess water drains from the roof. Drains are needed to take away excess water due to rain. If the roof is large, wave breakers are installed to prevent waves from forming under high winds. The waves could cause a large quantity of water to be blown off the roof edge. A ponded roof may reduce the required air conditioning capacity by as much as 30%. However, the added weight on the roof results in higher construction costs. A ponded roof should never be added to an existing building unless the roof structure can support the additional weight of the water. Code Alert

Evaporative Cooling Local building codes regulate the installation of ponded roof, roof misting, and other evaporative cooling systems. The codes specify details such as acceptable roof grade, proper flashing techniques and material, proper drainage, and acceptable methods of potable water backflow prevention.

46.4 The Role of the HVACR Technician The majority of this chapter discusses building and HVAC efficiency and provides ideas for energy conservation in general. Many of the decisions regarding building construction and HVAC specifications are not within the scope of the HVACR technician. However, there are several things technicians can do to help customers achieve better energy efficiency.

46.4.1 Know and Explain the Options As fuel costs continue to rise and as people become more concerned about the effect of fossil-burning fuels on the environment, opportunities to educate customers increase. During routine maintenance and service calls, it is not uncommon for a building owner,

manager, or resident to ask questions about the current HVAC system and to ask for suggestions for improvements in energy efficiency. A technician who keeps up with current technologies and best practices can discuss options for improvement based on the customer's specific system. Many people believe that the cost of switching to a more energy-efficient HVAC system is beyond their financial means. The technician can help in these cases by explaining current financial incentives from the government and other sources. In addition, the technician may be able to suggest small alterations in the customer's current system that will make the system more energy-efficient, as described earlier in the chapter.

46.4.2 Perform Proper Installation and Maintenance Even the most efficient system will not conserve energy if it is improperly installed. Be sure to follow the manufacturer's recommended installation procedure for each system carefully. For customers who cannot afford or do not want to purchase an entirely new system, one of the most effective methods for conserving energy is to perform proper preventive maintenance. Tips for careful system maintenance include: • Check for leaks at every service call. • Check for acid and water in the refrigerant at every service call. • Explain EPA Clean Air Act requirements to equipment owners as necessary to help them comply. • Clean heat exchanger coils (evaporator and condenser). • Maintain clean, unobstructed airways (at the air handler and the condenser). By checking a system routinely, the technician can often find and fix problems before they become major. Not only does this help prevent potential pollution, it may also prevent expensive equipment (or refrigerant) replacement.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter Review Summary •

•

•

•

•

•

Review Questions

The energy efficiency of a building is directly impacted by its construction and materials, including insulation as well as roof and wall construction. A building's fuel or energy consumption per degree-day can be estimated by dividing the fuel or energy consumption by the number of degree-days. This estimated fuel consumption rate can then be used to determine fuel remaining in a tank or to project the heating or cooling cost for the season. Annual fuel utilization efficiency (AFUE) rating is the ratio of fuel burned to heat provided. Various ratings are used to evaluate the efficiency of HVAC equipment. AFUE is used to measure the efficiency of furnaces and boilers. EER, SEER, and COP are used to evaluate air conditioners and the cooling cycles of heat pumps. HSPF measures the efficiency of a heat pump's heating cycle. Energy-saving components such as programmable thermostats, variable speed motors and variable frequency drives, heat exchangers, various solar products, and subcoolers can be added to existing systems to increase their efficiency. Integrating an HVAC system into a building control system to manage operation can also increase efficiency. Total energy systems can be designed for use as combined heat and power (CHP) or cogeneration systems, making use of all the energy created in its different forms. This can increase energy efficiency, reduce air pollution, reduce utility costs, and reduce demand on the electrical grid. Although the HVAC technician usually has no direct control over system specification, the technician can contribute to energy conservation by explaining alternatives to customers when asked, installing and maintaining systems properly, and conserving refrigerant as best as possible.

Answer the following questions using the information in this chapter. 1. The US Department of Energy's _ _ label

may be used to identify buildings, appliances, or HVAC equipment that meets certain efficiency requirements. A. Big E

B. Earth Saver C. Energy Star D. Tree Hugger 2. The degree-days method of determining heating and cooling costs is based on _ _ as a temperature benchmark. A. 45°F B. 65°F C. g5op D. 95°F

3. Which energy efficiency rating is used to measure the efficiency of boilers? A. AFUE. B. COP. C. HSPF. D. SEER. 4. Which of the following efficiency ratios may be used to describe the efficiency of an air conditioner? A. AFUE, COP, EER. B. AFUE, HSPF, SEER. C. COP, EER, SEER. D. COP, HSPF, SEER. 5. Under what conditions is an EER rating calculated? A. 65°F outdoor temperature and 70°F indoor temperature at 80% relative humidity. B. 75°F outdoor temperature and 75°F indoor temperature at 40% relative humidity. C. 85°F outdoor temperature and 80°F indoor temperature at 50% relative humidity. D. 90°F outdoor temperature and 75°F indoor temperature at 60% relative humidity. 6. The difference between the EER and the SEER is that-~· A. SEER can be used to rate furnaces and boilers B. SEER estimates average efficiency throughout the entire season C. SEER is less accurate than EER D. SEER units are all converted to Btu before the calculation is made

Copyright Goodheart-Willcox Co., Inc. 2017

1275

I

1276

Modern Refrigeration and Air Conditioning

7. Which of the following statements is true about the coefficient of performance ratio? A. It can be used to rate the heating efficiency of a heat pump. B. It has no units associated with it because the units cancel out. C. It is calculated by dividing output in Btu/hr by input in watts. D. It reflects how efficiently a furnace operates over an entire season. 8. The first step toward changing a building's HVAC system to increase energy efficiency is to A. add random energy-efficient components if they are already in the truck B. evaluate the existing system C. install a building control system D. install a completely new energy-efficient system

9. The key feature of total energy systems is the A. generating of all of its own power without any energy input B. use of a chemical process to generate electricity C. use of nuclear fusion to produce heat D. use of waste heat to perform a useful function 10. One of the most effective ways a technician can help a customer reduce energy costs is to A. encourage the customer to replace the entire system B. perform proper preventive maintenance C. report the customer's energy needs to a supervisor D. talk trash about their old system

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 46 Energy Conservation

1277

Photo courtesy of Trane, a brand of Ingersoll Rand

Variable airflow, variable refrigerant flow, zoning options, wireless controls, and integrated building management systems work together to maximize a building's energy conservation.

Copyright Goodheart-Willcox Co., Inc. 2017

I

CHAPTER47

OvervievV of Cotntnercial Refrigeration Systetns

Chapter Outline 47.1 Applications 47.1.1 Bakeries 47.1.2 Supermarkets 47.1.3 Other Applications 47.2 Commercial Refrigeration Systems 47.2.1 Commercial Cabinet Construction 47.2.2 Walk-In Cabinets 47.2.3 Florist Cabinets 47.2.4 Hot and Cold Merchandisers 47.2.5 Display Cases 47.2.6 Quick Chillers and Blast Chillers/Freezers 47.2.7 Refrigerated Dispensers 47.2.8 Milk Coolers 47.2.9 Ice Machines 47.3 Industrial Applications 47.3.1 Industrial Processes 47.3.2 Industrial Freezing of Foods

Learning Objectives Information in this chapter will enable you to: • Recognize the various types of commercial refrigeration systems and their applications. •

Explain how and why ice banks are used in certain refrigeration systems.

•

Explain how and why air curtains are used in commercial refrigeration systems.

•

Explain how quick chillers and blast chillers differ from other commercial refrigeration systems.

•

Describe the two types of water coolers.

•

Describe the different types of evaporators found in commercial ice machines.

•

List some of the industrial applications of refrigeration systems.

Chapter 47 Overview of Commercial Refrigeration Systems

1279

Technical Terms

Introduction

blast chiller blast freezer cryogenic food freezing dispensing freezer distributed system hot and cold merchandiser ice bank ice machine locker plant milk cooler parallel compressor rack

Commercial refrigeration systems vary greatly to meet their different applications. Many are high-capacity systems with aluminum and stainless steel cabinets for greater durability and ease of maintenance. Commercial refrigeration systems may also use multiple compressors, condensing units with multiple fans and flow controls, and specialized evaporators. Examples of commercial systems include supermarket refrigeration units, food display cases, refrigerated beverage and ice cream dispensers, and ice machines.

passively chilled beverage dispenser pressure dew point processing plant quick chiller self-contained water cooler tap water cooler ultraviolet lamp walk-in cabinet water cooler

Review of Key Concepts Reviewing these concepts from earlier chapters will help you understand new concepts presented in this chapter: • Refrigerating or freezing food helps preserve freshness and product integrity. (Chapter 23) •

Refrigerating systems for food preservation use not only compressors, condensers, metering devices, and evaporators but also different heat exchangers, dampers, and defrost and condensation controls. (Chapter 24)

•

Gay-Lussac's law states that at constant volume, the absolute pressure of a given quantity of a gas varies directly with its absolute temperature. In other words, when a gas is held at a constant volume, its pressure and temperature will rise together or will fall together. (Chapter 5)

•

Many large food processing plants use cryogenic fluids, such as liquid nitrogen or carbon dioxide, to rapidly freeze foods. These liquid refrigerants range in temperature from -250°F (-157°C) to nearly absolute zero (-460°F or -273°C), which is the cryogenic range. Common cryogenic fluids are R-702 (hydrogen), R-704 (helium), R-720 (neon), R-728 (nitrogen), R-729 (air), R-732 (oxygen), and R-740 (argon). (Chapter 9)

1280

Modern Refrigeration and Air Conditioning

47.1 Applications Commercial refrigeration systems can be found in many types of businesses. Since refrigeration systems can be designed to achieve and maintain a variety of temperature ranges at different rates and efficiencies, they have been incorporated into the day-to-day work of numerous commercial operations.

47.1.1 Bakeries Many raw products used by bakeries must be refrigerated to preserve or improve quality. Bakeries must refrigerate perishable raw materials, perishables during interrupted processing, and finished products, Figure 47-1. Frozen ingredients must be stored. Even water and flour used for bread making require cooling during certain periods of the year. Health codes require

Trau/sen Refrigeration

Figure 47-1. This type of refrigerated cabinet can easily be used in bakery applications. It has a top-mounted condensing unit.

these practices. Creams and custards can last longer when at a cool temperature. Both medium-temperature and low-temperature refrigeration is used. Normal refrigeration is suitable for butter, eggs, coconut, cream, fat, meat, margarine, nuts, and yeast and also for dough retardation (controlling length and strength of fermentation). Lowtemperature systems are needed to freeze baked goods that are sold frozen. For example, bread that is fastfrozen to -1°F (-18°C) will remain fresh for almost a month. Controlled temperature and humidity are important in many baking processes. Therefore, air-conditioning equipment is also used in bakeries.

47.1.2 Supermarkets In the United States, the FDA Food Code and its supplements set requirements for food storage equipment, whether in bars, restaurants, grocery stores, or wholesale distribution warehouses. It is important that the food temperature be at or below 41°F (5°C) at all times. As equipment has become more efficient, operating temperatures for discharge and return air have risen. For example, for reach-in merchandisers manufactured in 2010, the evaporator air temperature was 27°F (-2.8°C), and the discharge air temperature was 32°F (0°C). By comparison, for reachin merchandisers manufactured in 1990, the evaporator air temperature was 23°F (-5°C), and the discharge air temperature was 30°F (-1.1°C). A supermarket will have a mix of reach-in displays with different arrangements and multiple decks. These units will refrigerate dairy, produce, fresh meat, salads, and delicatessen meats and cheeses. A supermarket reach-in case may have multiple doors. Doors and door frames are chosen separately from the case. A door is typically 30" x 67" for standard height or 30" x 75" for tall cases, Figure 47-2. The location of the coolers, freezers, reach-ins, and other types of refrigeration units will vary with each supermarket installation. The layout of the store factors into what type of unit may be used. The condensing unit may be split from the cooling cabinet and located remotely, or the condensing unit may be packaged with the rest of the refrigeration system. When packaged as a single unit, the condensing portion of a system is usually located in the top or the base of the cabinet. A forced-draft evaporator installed inside the cabinet connects to a forced-draft condenser outside the cabinet. Larger installations may use multiple condensing units. These condensing units may be located on top or in the back of the unit, in a separate area of the building, or outside the supermarket on a pad or on top of the roof, Figure 47-3.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 47 Overview of Commercial Refrigeration Systems

REMIS AMERICA, LLC

Figure 47-2. Supermarkets frequently connect several display cases in a row to form aisles.

Condensing unit

A

1281

A parallel compressor rack is an arrangement of compressors piped in parallel with a common suction line, a common liquid line, and a common liquid receiver. It is the central part of a multiple-compressor refrigeration system. Figure 47-4 shows a parallel compressor rack with four compressors piped in parallel. Each display case connected to a parallel compressor rack has its own evaporator and refrigerant metering device. The unit shown uses a microprocessor control system to operate the units for maximum efficiency. The compressors may all run at the same time for high-load conditions or be cycled to provide refrigeration during low-load conditions. Parallel compressor racks are often located away from the display cases and refrigerated spaces to which they connect. Often these compressors and equipment are located in a machine room in the back of the store or on the roof. The machine room may include multiple condensing units that were provided prewired and piped. Machine rooms are designed with their own ventilation, lighting, energy management, and electrical systems. The compressors and condensers are piped and wired to the various food display cases and walk-in coolers located inside the supermarket, Figure 47-5. Another option is to install equipment as close as practical to the refrigerated cabinets. The compressors can be placed near the conditioned spaces, such as at the end of an aisle of display cases in a supermarket. A distributed system is a unit containing multiple compressors that circulate refrigerant through the evaporators in a nearby conditioned space, such as reach-in coolers or display cases. A distributed system typically contains only compressors and their controls. A distributed system is usually connected to a remote condenser located elsewhere. This remote condenser is often an air-cooled unit directly above on the rooftop. In some cases, a distributed system may contain a water-cooled condenser.

B Zero lane, Inc.

Figure 47-3. A-Large display case with a condensing unit mounted on top and out of sight of customers. B-Top-mounted condensing unit.

Hill Phoenix, Inc.

Figure 47-4. Note the use of a liquid line receiver on this commercial mechanical rack of four compressors in parallel.

Copyright Goodheart-Willcox Co., Inc. 2017

1282

Modern Refrigeration and Air Conditioning

A distributed system's compressors operate in a multiplexed manner (common discharge line and common suction line) to refrigerate a row of coolers containing products, such as produce, dairy, or frozen food, Figure 47-6. As the temperature of one or more of the products is satisfactory, the compressor or compressors are cycled off to match the capacity of the refrigeration system to the coolers. The primary benefit of distributed systems is a 65% to 75% reduction in refrigerant charge. This reduced refrigerant charge is cost-effective. Additional benefits include flexibility in installation and controls.

Controls

47.1.3 Other Applications Heating, cooling, preserving, and freezing food, drinks, and other products are a major part of numerous businesses. Many of the refrigeration systems used in bakeries and supermarkets can be found performing in similar situations in restaurants, pharmacies, convenience marts, and other businesses. Refrigeration systems with special temperature and humidity controls

Compressors Zero Zone, Inc.

Figure 47-6. Opening the front panels reveals several multiplexed compressors and their controls. Distributed systems may be used to operate multiple produce, dairy, meat, or frozen food merchandisers.

are also used in florist shops, laboratories, hospitals, and funeral homes. Refrigeration systems used in different applications for different purposes vary in their controls, operating components, and capacity, Figure 47-7.

47.2 Commercial Refrigeration Systems

Hill Phoenix, Inc.

Figure 47-5. Supermarket machine room. Top-Exterior view of a rooftop machine room. The vents are used to release condenser heat. Bottom-Inside view of a machine room. The multiple condensing units are prewired and piped by the manufacturer.

The type of refrigeration system installed depends on the conditions that must be maintained. Owners and installers must consider the amount of product that must be refrigerated, the space required to fit the unit, the temperature and humidity set points, the amount of heat that will be introduced throughout the day, and related factors. Some commercial refrigeration systems are prebuilt for specific applications. Others must be custom-made.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 47 Overview of Commercial Refrigeration Systems

1283

47.2.1 Commercial Cabinet Construction

Refrigerated tray

Commercial cabinet surfaces are either metal or plastic. Finishes are designed for easy cleaning. Structural members are steel, capable of supporting the evaporator and condensing unit. Insulation is usually polystyrene or urethane in slabs or foamed in place. The capacity of the evaporator and condensing unit must be adequate for the required application. Refrigeration should be possible under the most severe service conditions. In some cases, heat leakage may cool the cabinet surfaces enough to cause moisture to condense on them. To avoid this condition, some cabinets have a resistance heating strip. The strips are located around certain outer surfaces to warm them and prevent condensation from forming. Many commercial refrigeration cabinets are designed to be used with a remote condensing unit. These condensing units may be simultaneously connected to several refrigeration cabinets of different temperatures. Most commercial refrigeration condensing units are air-cooled, but some are water-cooled.

--

Temperature Alarm Systems In some installations, such as food freezers, an electrical alarm system will sound if the temperature in the cabinet rises above an upper safe limit. These systems sometimes operate from the electrical circuit powering the compressor. Others are provided with a battery arrangement. If the batteries are in good condition, the alarm will work even during a power failure.

Condensing unit Refrigerated storage

A Condensing units

Ice Banks

B Trau/sen Refrigeration; U.S. Coaler Campany

Figure 47-7. A-A refrigerated preparation table keeps pizza

and sandwich ingredients cool, fresh, and readily available for restaurant orders. B-Refrigerated mortuary body boxes are used to prevent deterioration of the deceased.

Many types of coolers, vending machines, and other medium-temperature commercial refrigeration systems use ice banks to provide reserve cooling capacity. During periods when the system cooling demand is low, an ice bank (solid block of ice) forms around the evaporator. When demand increases, the ice bank is able to absorb much of the heat and prevent the refrigeration system from being overloaded, Figure 47-8. A typical system of this type is found in some drink dispensers. Ice builds up during periods of low activity. This ice is then used to cool beverages during periods of high activity. Another use for this type of refrigeration is in an air-conditioning system used on a periodic basis (such as a church building). A control is needed to limit the amount of ice that is formed. A conventional temperature control cannot distinguish between ice and water, since either can exist at 32°F (0°C). An ice bank control must be used. Older ice bank controls use a sensing bulb with two compartments divided by a membrane. One side contains water. When frozen, the water expands and flexes the membrane. The other side contains a liquid that transmits

Copyright Goodheart-Willcox Co., Inc. 2017

I

1284

Modern Refrigeration and Air Conditioning

Evaporator

To ice bank

Sizes of walk-in cabinets vary. However, two heights are usually considered standard: 7'-6"and 9'-10" (2.3 m and 3 m) outside dimensions. The dimensions of other walk-in cabinet sizes are listed in Figure 47-10. Many cabinets are made with metal linings and exteriors. The usual metal is galvanized steel or aluminum. Vinyl, porcelain, and stainless steel are also used extensively. Cabinet doors are usually of the same construction as the box. They are lined with gaskets to make the box airtight. Door latches must be accessible from inside a cabinet for safety, Figure 47-11. The doors may also be provided with heating wires along the edge. These wires eliminate sweating and freezing. Often these heating wires are called mullion heaters. Some walk-in cabinets are dual-temperature units. They have a compartment to keep food cold (about 40°F) and a compartment to keep food frozen (below 0°F). Cabinets may have additional reach-in doors, usually with two, three, or four panes of glass. Instead

Sensing bulb Invensys Cli ate Controls Americas

Figure 47-8. In this ice bank, a sensing bulb Tonitors temperature and pressure. If the temperature is below 32 °F, the ice bank control turns the compressdr off. If the temperature is at 32 °F, the bulb senses a pressure increase due to ice crystals forming in part of the bulb. Then the control also turns the compressor off.

the membrane movement up to the head of the sensing bulb. This movement operates a control switch that turns off the compressor. With the system off, some of the ice is allowed to melt. The cycle for making and detecting ice can then begin again. These bulb-type ice bank controls are still sold and in use to some extent. A modem ice bank control senses the electrical conductivity between a set of stainless steel electrodes and modifies its output accordingly. The electrodes are used to measure the thickness of the ice and control the level of liquids. Ice bank controls are used in a small refrigeration system to provide a greater than normal refrigeration capacity for set periods of time during peak demand. Ice bank controls are typically used in vending machines, milk coolers, beer coolers, and similar systems.

47.2.2 Walk-In Cabinets Many restaurants and supermarkets store perishable items in walk-in cabinets, which provide a large commercially refrigerated space for various products. These cabinets have large doors and sometimes windows. Walk-in cabinets may be referred to as

butcher boxes, Figure 47-9.

Hill Phoenix, Inc.

Figure 47-9. Walk-in cabinet with a 37°F (3°C) holding temperature with an R-502 air-cooled thermal balanced semiautomatic system.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 47 Overview of Commercial Refrigeration Systems Hook hangers

Walk-In Cabinet Dimensions Length

Height

Width

5'

9'-10"

8'

6'

9'-10"

8'

8'

9'-10"

9'

7'

9'-10"

12'

10'

9'-10"

6'

5'

7'-6"

6'

6'

7'-6"

7'

6'

7'

1285

7'-6" Gaadheart-Willcax Publisher

Figure 47-10. Walk-in cabinets are available in the sizes listed.

Safety release lever Blower evaporator U.S. Coaler Campany

Figure 47-12. This walk-in cabinet meat locker contains hooks

for hanging meat and a forced-draft evaporator to distribute cool air evenly.

Gaadheart-Willcax Publisher

Figure 47-11. An inside safety release lever is connected to

the door latch. The inside lever can be used to open the door in an emergency.

of insulation, these doors have two or three dead-air spaces arranged in such a way that they are airtight. Plate glass is usually used. Special chemicals, such as calcium chloride, keep the spaces between the panes free from moisture. Most walk-in cabinets now use foamed-in-place insulation of rigid polyurethane. Foamed between the inner and outer walls, such insulation produces a strong wall. The insulation eliminates the need for metal framing. Insulation is usually 4" (10 cm) thick. Floors are also insulated to prevent heaving from underlying frost. Walk-in cabinets usually have a lighting system. Some have a wall-mounted evaporator that is separated from the main part of the cabinet interior by a vertical baffle. Forced-draft evaporators are popular, Figure 47-12. The temperature of a walk-in cabinet depends on its use. For meat or fresh produce storage, a temperature between 35°F (2°C) and 40°F (4°C) is needed. Relative humidity should be about 80%. Air movement is necessary. Ultraviolet lamps may also be used to help control bacteria and mold growth, Figure 47-13.

Steril-Aire

Figure 47-13. The blue glow from ultraviolet lamps floods

these evaporators with protection from undesirable biological growth.

Caution

Ultraviolet Light Overexposure to ultraviolet rays is dangerous. Therefore, people working near these lamps must be protected from the rays. Otherwise, the lamps must be turned off when anyone is in the cabinet.

Copyright Goodheart-Willcox Co., Inc. 2017

I

1286

Modern Refrigeration and Air Conditioning

Some type of drain is recommended for walk-in cabinets. Figure 47-14 shows a common drain installation method for a prefabricated walk-in that is located on a permanent floor. In systems where dehydration of foods is not important, colder temperatures may be used. Less attention may be paid to relative humidity. Examples of this are milk storage and beverage cooling. Forced-draft evaporators are commonly used in these installations. Walk-in cabinets (walk-in freezers) are also used for storing frozen foods.

Wall panel

Floor panel

Nipple

47.2.3 Florist Cabinets Florist cabinets form a unique market in the refrigeration industry. Display cabinets are often the focal point of the entire retail operation, both in cabinet design and product presentation. See Figure 47-15. Florist cabinets differ from food service cabinets in the following ways: • Humidity is important in maintaining a long life for the floral items. • Because many floral items are fragile, only lowvelocity air evaporator coil designs are used. Proper equipment sizing allows for warm air defrost. • Floral cabinets use large amounts of glass in their doors and windows. The additional heat loads from these components must be taken into account when sizing the equipment. • The lighter product load of floral equipment should be a factor when determining a final Btu/hr load requirement. The Society of American Florists (SAF) recommends a storage temperature range between 34°F and 38°F (1°C and 3°C) with 80%-90% humidity. Due to the delicate nature of flowers, the upper ranges of this temperature range are recommended to avoid product loss. Humidity levels above 90% can cause condensation, which may create a mold infestation. Cabinet walls and ceiling panels should be well insulated with a minimum of R-22 value insulation. A vapor barrier between the cabinet and outside wall must be designed to prevent condensation on outside walls. Insulated floors, while not required on grade level applications, do provide energy savings. They are required if any cavities exist under the floor on which the cabinet rests. Glass doors should be rated for operation in a refrigerated application. Glass panels should be of thermal-pane construction, and glass heat will be required if ambient store conditions will not be properly maintained. Due to the high humidity present, all internal shelving should be constructed of nonabsorbent, corrosion-proof material. All drain lines must be trapped to allow for free flow of condensate

Finish floor

\ 1x4

treated shims 24" O.C. Goodheart-Willcox Publisher

Note that the connection of this walk-in cabinet drain is part of the prefabricated bottom section. Figure 47-14.

Courtesy of Spar/an Division - Parker Hannifin Corporation

Figure 47-15. A florist's

refrigerated display cabinet with passthrough glass doors, lighting, and low-velocity evaporator coils. water to an approved drain, and to eliminate negative pressurization of the evaporator coil. Ethylene gas is a naturally occurring hormone produced by all plant tissue. It is an odorless gas that accelerates the aging process in flowers and plants. This causes up to 30% of loss, due to shrinkage, in the floral industry. Improper temperature and humidity accelerate the production of ethylene gas. Signs of ethylene damage include discoloration, wilting, spotting, and the dropping of petals and leaves. Ethylene gas filters (EGF) are formulated with a natural mineral compound. These filters trap and neutralize the gases. Replace EGFs every 120 days, Figure 47-16.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 47 Overview of Commercial Refrigeration Systems

1287

Heated section

Refrigerated section Alto-Shaam, Inc.

Figure 47-17. Unit designed to warm items in the upper section while cooling the items in the lower section. SRC Refrigeration

Figure 47-16. Ethylene gas filters (EGF) decrease damage due to ethylene in floral cabinets.

47.2.4 Hot and Cold Merchandisers The combination of both hot food holding and cold food refrigeration in one unit is called a hot and cold merchandiser. The heated upper section uses a gentle, uniform heat source, providing better moisture retention, appearance, and a longer holding life of food. The upper canopy includes a sneeze guard for an extra measure of safety, while overhead heating and lighting illuminates the product display. The lower refrigerated display base may be used for packaged side dishes, soft drinks, and precooked entrees, Figure 47-17.

47.2.5 Display Cases Refrigerated display cases are used by merchants to provide ease of shopping and to promote their products. These cases have a top or side opening through which customers can see and access the items. At the same time, the food is kept safely refrigerated behind glass doors or air curtains. Display cases are often used for fresh produce, frozen foods, fresh meats, and dairy products, Figure 47-18.

I Courtesy of Spar/an Division - Parker Hannifin Corporation

Figure 47-18. An assortment of refrigerated display cases.

Copyright Goodheart-Willcox Co., Inc. 2017

1288

Modern Refrigeration and Air Conditioning

The location of display case evaporators may be overhead, on an end, or at the base. These evaporators must be narrow to allow for maximum conditioned cabinet space. The evaporators are made with fins as small as 11/4" (32 mm) wide. Some of the shelf evaporators are plain tubing. Display case evaporators are usually connected in series. Many display cases are now using forced-draft evaporators for cooling, since these require little space. Because of the circulating air, they provide even refrigeration temperatures throughout the display case. Higher temperature cases are used for fresh meats and dairy products. In order for frozen food display cases to maintain temperatures near 0°F (-l8°C), the evaporators must operate at -10°F to -15°F (-23°C to -26°C). Temperature settings are based on the contents in a display case. Figure 47-19 shows the recommended temperature for some common applications. The need to maintain a low temperature presents an evaporator defrosting problem. The evaporator must be defrosted at least once a day. This must be done quickly to prevent the case from getting too warm. The defrosting is often done automatically with a timer signaling the system to perform hot-gas defrosting or to energize an electric heat defrosting device. See Chapter 21, Heat Exchangers, for details concerning defrosting methods and controls. Display cases vary in design, length, and height. They can be designed as upright cabinets or as chests. Both of these designs can be made as one of the two general types: • Glass-enclosed display case. • Open display case.

Glass-Enclosed Display Cases Glass-enclosed display cases may be designed as a chest or an upright cabinet, Figure 47-20. Some cases have additional refrigerated storage space beneath the display section of the counter. Such cases usually serve as a temporary container for food or produce. Those contents are then transferred to a walk-in cabinet overnight. Therefore, temperatures may be kept at 40°F (4°C) to 45°F (7°C) in both compartments. The low temperatures used in storing and displaying frozen foods in glass-enclosed cabinets may cause moisture to condense on the glass and obstruct the view. In order to stop condensation, heater wires are installed along those parts of display cases where condensation from the air might collect.

Open Display Cases Chest-style open display cases have an opening on top that provides access to the refrigerated products

Display Case Temperatures Temperatures Type of Fixture

Minimum•

Maximumb

Dairy Multi deck

36°F

38°F

Single-Deck

35°F

38°F

Multi deck

35°F

38°F

36°Fb

b

32°F

34°F

Single-Deck

24°F

26°F

Multi deck

24°F

26°F

Single-Deck

C

-13°Fd

Multi deck

C

-10°Fd

Glass Door Reach-in

C

_50Fd

Single-Deck

C

-24°Fd

Multi deck

C

-12°Fd

Produce,Packaged

Meat, Unwrapped (closed display) Display Area Deli, Smoked Meat Multi deck Meat, Wrapped (open display)

Frozen Food

Ice Cream

"These temperatures are air temperatures, with the thermometer in the outlet of the refrigerated airstream and not in contact with the product displayed. bUnwrapped fresh meat should only be displayed in the closed, service-type display case. The meat should be precooled to 36°F internal temperature prior to placing on display. The case air temperature should be adjusted to keep the internal meat temperature at 36°F for minimum dehydration and optimum display life. Display case air temperature varies with manufacturer. cMinimum temperatures for frozen foods and ice cream are not critical (except for energy conservation); maximum temperature is important for proper preservation of product quality. ct Toe differences in display temperatures among the three different styles of frozen food and ice cream display cases are a result of the orientation of the refrigeration air curtain and the size and style of the opening. The single-deck has a horizontal air curtain and opening of 30': The open multishelf has a vertical air curtain and opening of 42'' to 50': The glass door reach-in has a vertical air curtain protected by a multipane insulated glass door.

Reprinted by permission of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, Georgia, from the 1994 ASHRAE Handbook-Refrigeration

Figure 47-19. These temperatures are recommended for the display of certain foods.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 47 Overview of Commercial Refrigeration Systems

A

1289

B REMIS AMERICA, LLC

Figure 47-20. A-Chest-style glass-enclosed display case. B-Upright cabinet-style glass-enclosed display case.

inside. The walls, or the upper part of the walls, may be enclosed in three to four layers of glass, Figure 47-21. Evaporators are usually below the conditioned space with ducts circulating chilled air, Figure 47-22. Upright cabinet-style open display cases use forceddraft evaporators to distribute cold air through ducts. These ducts connect to grilles at the rear of the case that may be at the level of the refrigerated products or above the products. The warm air returns down the front of the case.

Air curtain discharge

Air curtain return

Many supermarkets have upright cabinet-style open display cases for produce. These cases are kept at about 40°F (4°C) and at a high humidity level. If dry air circulates over the contents, some of the moisture will be removed. This dry air spoils the appearance and decreases the weight of the produce. To keep humidity high, misting systems are often installed in open display cases, which include nozzles that spray water droplets over the produce, Figure 47-23. Produce that is sensitive to humidity includes cabbage, celery, carrots, radishes, herbs, and leafy greens, such as kale, chard, cress, and collard greens. To maintain a low-temperature, high-humidity atmosphere, produce is isolated from ambient air using an air curtain. An air curtain is a stream of air that blows between a conditioned space and an unconditioned space to isolate the two spaces. Some cabinets use two or three air curtains. The principle of operation is shown in Figure 47-24. Open display cases must be protected from drafts produced by grilles, unit heaters, and fans. Drafts will interfere with the air curtain of the case. This will cause higher operating costs and defrosting problems. Since several open display cases are usually connected endto-end in supermarkets, the total electrical load must be carefully checked to see if it will provide enough service to avoid any overloads.

47.2.6 Quick Chillers and Blast Chillers/ Freezers

Condenser air discharge Hill Phoenix, Inc.

Figure 47-21. Multilevel open display case using a refrigerated air curtain to maintain low temperatures and separate its contents from ambient air.

Food preparation and food safety are extremely important aspects of health and human safety. Refrigeration continues to play a significant role in reducing health-related problems due to bacteria or

Copyright Goodheart-Willcox Co., Inc. 2017

I

1290

Modern Refrigeration and Air Conditioning Air curtain

Insulation

Coil

Drain

Fan Hill Phoenix, Inc.

Figure 47-22. Notice the air curtain across the top of this open frozen foods case.

mold growth in foods. Cases of food poisoning around the world are often the result of improper food preparation and storage. The US Department of Agriculture (USDA) has issued guidelines on food preparation and storage. The USDA requires hot foods to be cooled to 70°F (21°C) within two hours and then to 41 °F (S C) within the next four hours. It is critical that food not be cooled too slowly or too quickly. Cooling the food too slowly will allow the growth of bacteria. If food is cooled too rapidly, there will be a loss of flavor. Controlled rapid cooling of foods is necessary to meet these requirements. In large kitchens, such as cafeterias and banquet halls, mass quantities of food are cooked in advance and stored for future use. Hot food must be brought from its oven temperature of up to 4S0°F (232°C) down to 70°F (21°C) within two hours. It must then be cooled to 41 °F (S C) within the next four hours. This type of cooling is performed by a quick chiller or blast chiller. Standard refrigerators and freezers are designed to operate as refrigerated storage cabinets. They are not designed to displace the heat load required to safely and quickly lower the temperature of hot, cooked foods. Placing cooked food in a standard refrigerator or freezer could cause its compressor to operate continuously to remove the high heat load. Continuous operation could cause a standard refrigerator-freezer's evaporator to become frosted, which reduces heat transfer efficiency. A standard refrigerator-freezer's defrost function would stop the 0

compressor to defrost the evaporator, which would prolong the cooling of the cooked food. Unlike standard refrigerator-freezers, quick chillers and blast chillers are designed to handle the large heat load required to cool hot, cooked food. A quick chiller is a type of refrigeration system that cools hot food rapidly and uniformly without freezing the product. Bacteria and germs can grow and spread quickly in the temperature range of 40°F to 140°F (4°C to 60°C). Food products are much safer from bacteria and germs at temperatures below 40°F (4°C)

0

Hussmann Corporation

Figure 47-23. An upright cabinet-style open display case with a concealed overhead misting system and a canopy of air that flows over the fruit to the front of the case.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 47 Overview of Commercial Refrigeration Systems

1291

Warm ceiling air powers the third jet (outer jet) Frigid air wraps the product

--- -------~

Main deck __JU.J-~tc=i===::z::;;c:;z:::z;:z==::g;;z::::.c:1 fixed position

Insulation Warm ceiling air (third jet) spills to the floor-also provides additional lamination for guard and freeze jet streams Kysor//Warren

Figure 47-24. Upright frozen foods display case has three air curtains, each flowing at a different speed.

and above 140°F (60°C). To safeguard food products and consumers, the cooling process must be carefully controlled using a quick chiller, Figure 47-25. Like a quick chiller, a blast chiller cools hot food rapidly and uniformly without freezing the product. However, a blast freezer cools and freezes hot food rapidly and uniformly. An alternative name for blast freezer is shock freezer. Some units can be programmed to function as either blast chiller or blast freezer, Figure 47-26. Modern quick chillers and blast chillers/freezers use microprocessor controls for chilling, defrosting, and maintaining food temperature. Many units also have keypads and built-in printers. These controls and

options may be used to record food-cooling operations and provide a printed report of system operation. Some chilling units use temperature probes that are inserted into the food. The temperature probes provide accurate measurement of each tray of food placed in the cabinet.

47.2.7 Refrigerated Dispensers Refrigeration systems are often incorporated into food and drink dispensing machines. Such systems cool and prepare products for convenient and immediate consumption. These machines are extensively used in restaurants and cafeterias.

Copyright Goodheart-Willcox Co., Inc. 2017

I

1292

Modern Refrigeration and Air Conditioning Report printer

Controls

• Traulsen Refrigeration

Figure 47-25. This undercounter quick chiller is capable of rapidly and uniformly cooling hot food in two hours or less. Note the microprocessor control.

Beverage Dispensers Many restaurants, convenience stores, and fastfood establishments use beverage dispensers to dispense both carbonated and noncarbonated soft drinks. These generally fall into two categories, passively chilled and mechanically chilled. Industry standard is for the delivered beverage to be 40°F (4°C) or below. Passively chilled beverage dispensers transport syrup and water through stainless steel tubing that is surrounded by the same ice that is dispensed for individual drinks. The tubing is encased in an aluminum cold plate that is situated at the bottom of an insulated bin filled with ice. The ice provides for both chilling of the product (through heat exchanged from the cold plate to the ice) and for being served with the individual drink. There are two ice options for passively chilled beverage dispensers: • A mechanism that automatically dispenses ice on demand. • Manually scooping ice from the bin. The automatic type is typically equipped with an ice machine installed on top of the unit that dispenses ice directly into the ice bin, to provide for a constant supply of ice, Figure 47-27. The manual type requires ice to be loaded into the ice bin by hand, Figure 47-28. For mechanically chilled beverage dispensing equipment, syrup and water pass through stainless steel tubing located in an insulated water bath. The refrigeration system's evaporator coil is also immersed in the water bath. Heat from the syrup and water is absorbed into the water bath, and the refrigerant in the evaporator coil absorbs heat from the water bath. For

Condensing unit Traulsen Refrigeration

Figure 47-26. This blast chiller's condensing unit is in its base. Controls and a report printer are along its side.

more information on these types of evaporators, see Chapter 21, Heat Exchangers. In operation, the coil becomes covered with 3" to 4" (76 mm to 100 mm) of ice across its entire surface, providing for the cooling of the water bath (32°F to 34°F, 0°C to l C). A motor agitates the water constantly, keeping colder water in contact with the product tubing to allow for heat exchange. The compressor is controlled by a device that senses the size of the ice bank, not the temperature of the water bath. This device 0

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 47 Overview of Commercial Refrigeration Systems

1293

switches the compressor off to prevent the ice bank from coming into contact with the product tubing. It will then switch the compressor on when the ice bank has been depleted from absorbing heat due to product being dispensed. Mechanically chilled equipment is generally sized by compressor capacity, which ranges from 1/4 hp to 3/4 hp (186 to 560 watts). As a rule, these systems use capillary tubes as opposed to expansion valves. Refrigerant charge amounts are usually less than 1 lb (454 grams), making for a delicately balanced system.

Ice bin

Water Coolers Water coolers are refrigeration units designed

Lancer Corporation

Figure 47-27. The ice dispensed from the chute in the center of this passively chilled beverage dispenser also cools the beverages in the unit.

to cool and dispense drinking water. Water coolers chill water using mechanical refrigeration. Evaporator tubing absorbs heat from the dispensing water. This heat is expelled through a condenser into ambient air. The water-cooling section containing the evaporator is insulated. Removing an access panel from a unit reveals a small hermetic compressor, a condenser, and controls, Figure 47-29. Since the cooling demand on water coolers is very irregular, they must have some hold-over capacity. However, they must not overcool the water. Hold-over capacity is provided either by using a large insulated cooled water storage tank or having a large water-cooling surface in the evaporator that quickly chills water. Different water cooler models are used in different applications. In many schools, hospitals, and businesses, plumbed tap water coolers are used and commonly called drinking fountains. In office buildings

Drain

Condenser

Compressor Goodheart-Willcox Publisher Lancer Corporation

Figure 47-28. A passively chilled beverage dispenser with a bin where the ice is stored.

Figure 47-29. A water cooler usually contains a small hermetic compressor and small condenser for the low heat load it handles.

Copyright Goodheart-Willcox Co., Inc. 2017

1294

Modern Refrigeration and Air Conditioning

and homes, self-contained or standalone (with no plumbing) water coolers are used and commonly called water coolers. A tap water cooler is a water cooler that has a plumbed water supply and drain connections. A tap water cooler's supply and drain connections must be installed according to local codes. The plumbing should be concealed. A hand shutoff valve should be installed in the water supply line. A drain pipe, at least 11/4" (3 cm) in diameter, should be provided. Code Alert

Water Cooler Requirements Section 410 of the International Plumbing Code (IPC) covers drinking fountains (water coolers). Section 405 of the IPC covers the installation of fixtures.

D

Tap water

•

Coldwater

D

Low-pressure vapor

•

Low-pressure liquid

D

High-pressure vapor

•

High-pressure liquid

Tap water cooler basins are generally porcelaincoated cast iron, porcelain-coated steel, or stainless steel. The lowest part of the basin is where the water enters the drain piping. Water is dispensed from a valve called a bubbler. The bubbler opening must be above the drain. This eliminates accidental siphoning of the drain water back into the fresh water system. A water pressure regulator determines the water flow. Tap water coolers frequently use heat exchangers to cool tap water before chilling it with refrigeration. Fresh tap water is precooled in a heat exchanger by the chilled waste water going down the drain. Refer to the water cooler diagram in Figure 47-30. A thermostat with a sensing bulb is attached to the water-dispensing tube. It maintains the desired drinking water temperature in the water cooler by communicating with the thermostat motor control, which cycles

Power lines

Water heat

~~:;:::::;::~::::'.~~:;:::::;::~::::'.~~=~=~::::'.=~=~='.:"'motor control

1...r-------,.'--+-----11----+--

Evaporator

f":;7/'717~ - 113~:S-97'---+ ~I-Temperature sensor

Water inlet/ shutoff valve Filter-drier

Liquid line Water pressure regulator • - -----+-Condenser fan

Drain trap

Suction line

t

Compressor

Condenser Gaadheart-Willcax Publisher

Figure 47-30. Diagram illustrating a drinking fountain cooled by a compression refrigeration system. Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 47 Overview of Commercial Refrigeration Systems

the compressor on and off as needed. Water leaving the fountain should be approximately 50°F (10°C). Water cooler condensers generally use a fan to increase heat transfer with ambient air. The condenser fan is connected into the electrical circuit and runs whenever the compressor is running. Article 410.2 of the International Plumbing Code (IPC) and its exception state that where a drinking fountain is required, at least one drinking fountain must comply with requirements for people who use a wheelchair. Figure 47-31 shows a water cooler universally designed for easy access for people who use wheelchairs. This water cooler incorporates a low-profile design with a large touch pad to permit ease of use. A self-contained water cooler is a standalone water cooler that has its own water supply from a tank and does not have a drain. Self-contained water coolers are especially desirable in locations where a building's plumbing cannot be easily tapped. Large plastic containers of water are used for the supply. The refrigeration system used is similar to that found in other water cooler models, Figure 47-32. Multiple water coolers, instead of individual ones, are popular for certain applications. These coolers have

1295

Hot water spigot

Chilled water spigot

Gaadheart-Willcax Publisher

Figure 47-32. Water coolers with self-contained water supplies often have an outlet for chilled water and another for room temperature or heated water.

one large condensing unit supplying refrigeration to many water cooler bubblers. They are often used for large business establishments, office buildings, or factories.

Milk Dispensers Many food service businesses dispense milk from bulk containers. Cans or plastic bags holding 3 to 5 gallons (11 L to 19 L) of milk are installed in dispensers. These units must meet all health and sanitation codes . • Milk is kept at about 36°F (2°C) by a hermetic refrigeration system. Reserve milk containers are kept in a walk-in cooler or in a milk storage refrigerator. Gaadheart-Willcax Publisher

Figure 47-31. Chilled water cooler drinking fountain with a barrier-free design for easy access by people who use wheelchairs.

Dispensing Freezers Special applications of refrigeration systems are used in dispensing freezers. Dispensing freezers are refrigeration systems that cool or fast-freeze and

Copyright Goodheart-Willcox Co., Inc. 2017

1296

Modern Refrigeration and Air Conditioning Mix feed valve

Selector sleeve

Rear product seal

Dasher

Mix tank

Scraper blade

Freezing cylinder

Sweden-Alco Dispensing Systems, a Div. of Alco Foodservice Equipment Co.

Figure 47-33. Components of a dispensing freezer.

dispense liquid mixes into soft serve or batch ice cream, shakes, or frozen beverages. The liquid mix is cooled or frozen under agitation and then dispensed for immediate consumption. The mix is typically purchased from a local dairy or other mix supplier. The liquid mix is poured into a refrigerated mix storage (either a hopper above the freezing cylinder or a lower mix cabinet). The mix is then fed into a freezing cylinder, where it is frozen and dispensed. Inside the freezer cylinder, a beater or dasher is driven by a separate motor. Dispensing freezers typically freeze about one gallon of soft serve in 5-10 minutes. A typical hopper attached to the freezing cylinder and dispensing door is shown in Figure 47-33. Most dispensing freezers can operate virtually continuously, as long as liquid mix to the freezing cylinder is replenished. The freezer is kept within a narrow temperature range, usually within 1°F or 2°F (0.5°C to 1°C) of its set temperature. Either automatic or thermostatic expansion valves are used as the refrigerant metering device. Since temperature requirements vary depending on the mix formulation, many freezers use some type of viscosity control instead of temperature controls, Figure 47-34. Refrigeration systems for dispensing freezers should be rated by Btu/hr, since the compressors

Dispensing Freezer Temperatures Dispenser Freezer Product

Temperature Requirements

Low-fat soft-serve ice cream

17°F to 20°F (-8.3°C to -6.?°C)

Soft-serve ice cream (10% fat)

20°F to 23°F (-6.?°C to -5.0°C)

Milkshakes

24°F to 28°F (-4.4°C to -2.2°C)

Sherbets

14°F to 18°F (-10.0°C to -7.8°C)

Fruit or water ices

14°F to 18°F (-10.0°C to -7.8°C)

Slushes

24°F to 28°F (-4.4°C to -2.2°C)

Frozen carbonated beverages (FCB)

24°F to 28°F (-4.4°C to -2.2°C)

Goodheart-Willcox Publisher

Figure 47-34. Table showing common temperature requirements for products used in dispensing freezers.

usually operate at medium- or low-temperature ranges. Rating dispensing freezers using horsepower or tons of refrigeration is misleading, since those ratings are for high-temperature refrigeration. In a hightemperature application, one ton of refrigeration is

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 47 Overview of Commercial Refrigeration Systems equivalent to 12,000 Btu/hr. A dispensing freezer with a product capacity of 15 gallons (56.8 L) per hour usually has a 9,500 Btu/hr refrigeration system and a dasher motor of 1.5 hp (1119 W). Most dispensing freezers are available either in air- or water-cooled systems. The quality of the mix is very important. Many problems thought to be in the refrigeration system have turned out to be poor mixes. Health codes require keeping the mix in the mix storage and freezing cylinder at safe temperatures, usually by maintaining temperatures below 41 °F (5°C). Daily cleaning may be required, and local health code rules must be followed. Heat treatment soft-serve and shake dispensing freezers are commercially available that go through a timed heating and cooling cycle every 24 hours and only require complete disassembly and cleaning once every 14 to 28 days, depending on local health codes. Another commercial application of the dispensing freezer is the shake maker. This machine is filled with the desired shake mix. While some shake makers are still controlled by temperature, the newer styles are usually viscosity controlled. As the mix is frozen, the torque required to drive the dasher increases. When the amperage reaches a certain set point, the refrigeration system shuts off at servable consistency. Some dispensing freezers are used to make slush drinks. Slush is typically a sugar-water mixture, often containing some flavoring or color. A frozen carbonated beverage is about one part syrup and four parts carbonated water. Most frozen beverages are served at 24°F to 28°F (-4.4°C to -2.2°C).

Vending Machines Refrigerated vending machines for foods or beverages are becoming increasingly popular. They may be used to automatically dispense canned or bottled drinks, packaged ice cream or cold food, frozen desserts, and other foods. Some machines are satellite vending units that are controlled by an accompanying snack vendor. Such vending machines are not coin operated but are connected and housed in the same cabinet as a machine that is coin operated, Figure 47-35. The refrigeration process allows food to maintain health and safety requirements. Cabinets often have an injected foam design and pump R-134a or R-404 through a standard 1/3 horsepower (120 Vac) hermetically sealed compressor to maintain temperatures. Vending machines are equipped with temperature sensors and health safety timers that disable sales of any product if the unit temperature rises above certain set points: • 41°F (5°C) for cold foods. • 0°F (-18°C) for frozen food.

A

B

1297

C WitternGroup

Figure 47-35. A-A satellite unit that maintains 41 °F (5°C). B-A vending machine that is capable of offering a variety of beverages in cans and plastic containers. C-A frozen food vendor using R-404. This unit can maintain temperatures between -12°F (-24°C) and 41 °F (5°C).

Frozen food vending machines typically hold temperatures between -l2°F (-24°C) and 41°F (5°C). To clear condensation, heated glass is used, as is a topmounted evaporator, which provides airflow from top to bottom. Refrigeration systems used in vending machines typically have capillary tube refrigerant controls, hermetic motor compressors, and defrosting devices. The electrical system transfers the materials and operates the coin and currency devices. Some of the vending components of refrigerated vending machines include the following: coin and currency devices (acceptors, rejectors, changers, and steppers and accumulators), cup dispensers, heating systems, transfer systems, and card readers. The automatic operation of vending machines involves motors, magnets, signal lights, and relays. Thus, an elaborate wiring system is necessary, Figure 47-36.

47.2.8 Milk Coolers Raw milk for pasteurization should be cooled to 50°F (10°C) or less within four hours after the completion of milking and down to 45°F (7°C) or less within two hours after the completion of milking. The blended temperature after the first milking and any subsequent milkings should not exceed 50°F (l0°C).

Copyright Goodheart-Willcox Co., Inc. 2017

I

1298

Modern Refrigeration and Air Conditioning Dispenser shown ready to accept nickel, dime, or quarter

10-25 blocking magnet

Refrigerator on Coin tube switch

5¢ blocking magnet

Empty switch Vend relay coil

#1 relay contact N.O.

A

Line #1

Line #2 Goodheart-Willcox Publisher

Figure 47-36. Ladder diagram for a refrigerated bottled beverage vending machine. Top-Vending unit wiring. Bottom-Refrigeration unit wiring.

Milk coolers are refrigeration systems that cool fresh milk to the legally required temperatures in a large tank. Bacterial growth in milk is dramatically affected by temperature. During a 24-hour period, bacteria count will increase as follows: • To 2400 at 32°F (0°C). • To 2500 at 39°F (4°C). • To 3100 at 46°F (8°C). • To 11,600 at 50°F (10°C). • To 180,000 at 60°F (16°C). • To 1,400,000,000 at 86°F (30°C). Some milk cooling systems incorporate precooling heat exchangers and flow controls to reduce the heat load on the bulk cooler, Figure 47-37. Some stainless steel bulk coolers have their evaporator in their base.

Water inlet (outlet on opposite side) and outlet

B BouMatic

Figure 47-37. A-A flow control monitors variables to adjust

the flow of milk through a heat exchanger for optimal energy efficiency. B-Water-cooled plate heat exchangers precool milk before bulk storage and cooling.

Coolers of this type have a 600 to 8000 gallon (2300 L to 30 300 L) capacity. A bulk milk cooler will usually have its condensing unit mounted outside the room where the milk

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 47 Overview of Commercial Refrigeration Systems cooler is located (milk room), Figure 47-38. Since it is usually air-cooled, the condensing unit should not be put in the same room as the vacuum pump. Air flowing over the condenser can be drawn from the milk room. In winter, where permitted, this warm air can be ducted to heat the milk room. All milk coolers and condensing units must be installed by qualified technicians, following applicable plumbing and electrical codes.

47.2.9 Ice Machines Ice machines are refrigeration systems that automatically freeze and form water into ice and dispense it for consumer use. They deposit the ice in storage bins and automatically cycle off when the storage space is full.

1299

The ice formed is clear and sanitary, since only flowing water is used. Cloudy ice cubes are caused by entrapped air or poor water quality. Ice machines are widely used in commercial refrigeration. Different models are available depending on the type and amount of ice required for a given application. Ice production capacity can vary among units from a few pounds up to many tons per day. Daily capacity decreases as water temperature or ambient air temperature increases. Once ice is formed, it is removed from the freezing surfaces in various ways. These include use of electrical heating elements, hot water, hot-gas defrosting, or mechanical devices. A quick inspection of an ice machine should reveal its ice removal method, Figure 47-39. Urethane foam, polystyrene, or fiberglass may be used for cabinet insulation. The freezing surface is usually made from stainless steel or nickel-plated copper. An ice machine's water circuits and ice-freezing parts should be cleaned regularly. The storage bin can

Hot-gas defrost solenoid valve

TXV

Discharge line

A

- - -1

B BouMatic; Danfoss

Figure 47-38. A-This bulk milk cooler has a direct expansion evaporator located in its base. B-Air-cooled condensing unit draws low-pressure refrigerant through the evaporator coils in a bulk milk cooler at a dairy farm.

TXV's sensing bulb

Suction line

Liquid line

Scotsman Ice Systems

Figure 47-39. Ice machine using hot-gas defrosting to remove ice from its mold.

Copyright Goodheart-Willcox Co., Inc. 2017

1300

Modern Refrigeration and Air Conditioning

be made of stainless steel or plastic. Ice machines are often self-contained, having all parts of the system in a single cabinet, Figure 47-40. Other ice machines use remote condensing units located outside. All ice machines require a water supply and drain plumbing.

Ice Machine Controls Ice machines have refrigerant and motor controls located throughout the cabinet to operate the system safely and efficiently. In addition to regular refrigeration system controls, ice machines have controls for icemaking and harvesting. Floats and solenoids control water flow. Different types of sensors and switches monitor the amount of ice in a machine's storage bin. A few of the control devices used for bin capacity include mechanical levers, temperature-sensing bulbs, and photo sensors. For bin capacity using mechanical controls, as the ice bin fills, the accumulated ice pushes against a diaphragm or actuates a lever. This opens a switch that stops the ice machine. Temperature controls for bin capacity shut off the ice machine when the ice comes into direct contact with a temperature-sensing bulb in the storage bin. Both mechanical and temperature controls are located at the top of the ice storage bin. Bin capacity photo sensors detect a light beam that is sent out across the top of the bin. When the ice reaches the top of the bin, the light beam is broken, and the ice machine stops harvesting ice.

Compressor

Some ice machines have a variety of additional controls. These include sensors that measure compressor discharge temperature, water temperature, and cube size. Figure 47-41 shows a microprocessor-based controller used to monitor all processes of operation. An ice machine controller receives inputs from the machine's sensors and controls the reservoir fill time, starting time, harvest cycle time, and unit shut-off time. Figure 47-42 shows a schematic wiring diagram of an ice machine with a hot-gas defrost harvesting function.

Ice Machine Evaporators Ice machines produce ice in different shapes and forms for different purposes. The two main forms are cube and flake. While flakes are relatively consistent, cubes can vary greatly in size and shape depending on the type of evaporator an ice machine has.

Vertical Cube Evaporators An ice machine with a vertical evaporator is shown in Figure 47-43. In this ice machine, evaporator tubing is located just behind cube molds. A water distribution

Condenser

Scotsman Ice Systems

Figure 47-40. Beneath the storage bin, evaporator, and water circuit are the compressor and condenser of this selfcontained ice machine.

Scotsman Ice Systems

Figure 47-41. This ice machine microprocessor-based controller controls the compressor, regulates ice harvest operations, and performs diagnostic functions.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 47 Overview of Commercial Refrigeration Systems

1301

Bin control switch High-temp. safety switch

115 ____.

~h

volt

High-pressure safety switch (water-cooled units only) I

Bk

I Main : contactor

Ice Compressor switch

Wash Hot gas solenoid

Limit switch (cam)

W

I I I

H ____1::_1 J

Harvest motor

Defrost_/ control

Compressor

Pump

Probe motor N.C.

Fan

C. N.O.

"A" Control relay

~-ti-----,

Probe switch

I I

o--J

I I I I

"B" I I________

N.C.

C. N.O.

"C"

I

Water purge valve

Purge switch lce-0-Matic

Figure 47-42. Note the number of devices controlled in this ladder diagram: hot-gas defrost, probe motor, harvest motor, fan, water purge valve, and water pump.

manifold continuously streams water down across the molds where ice gradually forms and builds up. Eventually, ice fills each cell in the mold and connects with the other cubes along their edges. A sensor monitors ice thickness and informs the control unit when to switch from the refrigeration cycle to the hot-gas defrost cycle for harvesting. Hotgas from the compressor's discharge line is pumped

through the evaporator. Once the sheet of ice cubes is loosened, it falls and breaks apart into individual cubes in the storage bin. A large portion of ice is used for beverage cooling. Cubed ice is usually created for serving in drinks or may be bagged for bulk sale. These hard cubes are long lasting. However, ice in other shapes and sizes is frequently desirable.

Copyright Goodheart-Willcox Co., Inc. 2017

I

1302

Modern Refrigeration and Air Conditioning Water distribution manifold Water inlet solenoid valve

Cube evaporator

Light curtain cube sensor

Compressor Deflector tray

Water pump and motor

Ice storage bin

reservoir Float switch

Cabinet insulation Scotsman Ice Systems

Figure 47-43. In this vertical cube evaporator ice machine, water flows into the top of the evaporator and over cube molds. Finished cubes fall into the ice storage bin below.

Inverted Cube Evaporators

Ice cube mold cells

Another method of producing cubed ice uses an evaporator above an inverted mold. Cold water is sprayed upward into the inverted molds. The temperature of the molds is cooled very low by the nearby evaporator coils. Water strikes the mold surface and freezes there, Figure 47-44. Frozen water gradually builds up until complete ice cubes are formed. Next, the ice cube molds are warmed until the cubes fall out. The warming is done by electric heating elements or hot-gas defrosting. The cubes drop down onto a deflector trap, slide through a chute, and arrive in the ice cube storage bin. This ends the ice production cycle. Review the system diagram in Figure 47-45.

Evaporator tubing Overhead View of Ice Machine Scotsman Ice Systems

Figure 47-44. Evaporator tubing runs on top of and alongside ice mold cells in this cube ice machine. Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 47 Overview of Commercial Refrigeration Systems

1303

Capillary tube

Electric defroster Evaporator

Inverted ice cube mold

Deflector tray

-----------ff -

Water supply

Condenser

Controller Goodheart-Willcox Publisher

Figure 47-45. In an ice machine with an inverted mold evaporator, water is sprayed upward into ice cube molds to produce clear ice cubes.

Flaked Ice Evaporators

Evaporator insulation

Flaked ice is preferred for cooling produce, fish, and poultry. It can be packed tightly around an item and will maintain the freshness of the product even as the ice melts. Ice machines that produce ice flakes have evaporators unlike those of cube ice machines. In a flake ice machine, water fills a cylindrical evaporator at 0°F (-18°C) that freezes the water very rapidly. A heavy steel auger driven by an electric motor cuts and scrapes ice from the surface, Figure 47-46. Figure 47-47 shows a diagram and a photo of a flake ice machine. Water from a reservoir enters the evaporator at the water inlet and fills to the level control in the water reservoir. Water freezes, and the auger rotates to cut and move the flaked ice upward. Above the auger, the flaked ice is directed into a chute where it drops into a storage bin. When the bin is full, a sensor or switch shuts off the machine until needed again. Figure 47-48 is a ladder diagram for a flake ice machine.

Water inlet

Refrigerant inlet

Gearbox

Drip pan Auger motor Scotsman Ice Systems

Figure 47-46. The bottom of a flaked ice machine's evaporator with some insulation removed. Water flows in through the inlet to fill the evaporator. This evaporator's auger connects to its motor from below through a gearbox.

Copyright Goodheart-Willcox Co., Inc. 2017

I

1304

Modern Refrigeration and Air Conditioning

Level control

Evaporator insulation

t

Water reservoir

Evaporator--- - tubing _____ Auger Water inlet

motor

DDDDDDDDDD Gearbox

A

Water-cooled condenser

B Scotsman Ice Systems

Figure 47-47. A-Diagram showing the working parts of a flake ice machine. 8-With panels removed, a flake ice machine's components are accessible.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 47 Overview of Commercial Refrigeration Systems

!------

L1

1305

L2

115-120 V/6O Hz/10 - - - - ~....1

S1

X4

AT

81 82 83 84 85 86 PS F1 L

Power switch Ice & water switch Ice switch Water switch Bin control switch Ice making switch Pressure switch Fuse Power lamp X3 Water control relay X4 Ice dispensing relay XS Gear motor protect relay sv Shutter-solenoid valve WV1 Control water valve WV2 Dispense water valve GM Gear motor CM Compressor SR Starter Start capacitor Fan motor Transformer Float switch Control timer Capacitor-GM Thermal protector-CM Motor protector-GM Thermal protector-GM Drain timer Agitation timer WV3 Drain water valve X6 Drain control relay

SC FM TR FS TB C1 OL1 OL2 OL3 DT AT

ss

WV1

X3

WV3 NO

u

S2

WV2

S4

X3t---+o

xs>-----+-u

TB

0 0

"

HMheaMAme,Jca, m,.

Figure 47-48. The identification chart on the right helps the service technician identify specific components in this ladder diagram of an ice machine.

Copyright Goodheart-Willcox Co., Inc. 2017

I

1306

Modern Refrigeration and Air Conditioning

Like flake ice machines, extruded cubelet ice machines use an auger in their evaporators. Cubelet ice machines are frequently used in fast-food restaurants. These small commercial units produce an extruded cubelet of ice rather than a solid cube. Cubelet ice machines are very similar to flake ice machines, but these machines compress the ice flakes into small cubelets before dispensing them.

47.3 Industrial Applications Refrigeration has a variety of applications in industrial and manufacturing processes. The production of high concentrations of heat is often the result of many industrial processes. To minimize or eliminate the negative consequences that could result from high heat, refrigeration is often incorporated into the process.

47.3.1 Industrial Processes While refrigeration is often concerned with food preservation, it can also be used in manufacturing and industrial processing. A few common uses include the following: • Cooling of water, which, in turn, cools electrodes on resistance welders. • Cooling of quenching liquids used to cool metals in heat-treating applications. • Cooling compressed air. Since moisture may rust and corrode air tools or spoil paint spraying, compressed air must be dry. Therefore, air is cooled to keep the coldest spot in the air system below air's dew point. Remember that a gas's dew point is the temperature at atmospheric pressure at which water begins condensing. Cooling the air below that temperature prevents moisture from forming in the lines. Remember that Gay-Lussac's law states that in a fixed volume, as pressure rises or falls, temperature correspondingly rises or falls. Therefore, gas in a fixed volume at a higher pressure will have a higher temperature dew point. Pressure dew point is the temperature at which moisture condenses in pressurized air (air that is not at atmospheric pressure). In industrial compressed air systems, a refrigeration system cools the compressed air below its pressure dew point. Then the air is reheated. Pressure dew points are about 50°F (28°C) above atmospheric dew points at 100 psi (700 kPa) air pressure. Generally, the compressed air is cooled to about 35°F to 50°F (2°C to 10°C).

A 3000 cfm (1.42 m 3 /s) air compressor will need a refrigeration system with about 20 tons (55.2 kW) of cooling capacity. Caution

Explosion-Proof Systems Safety refrigerators and explosion-proof refrigerators are used for flammable liquid and substance storage. Ordinary household refrigerators are not appropriate. Standard refrigerator components (thermostat, relays, and switches) can cause a spark capable of igniting vapors from flammable liquids. Do not locate flammable storage refrigerators in areas containing explosive vapors. However, chemicals that produce explosive vapors can be stored inside these systems.

Refrigerators and freezers of all types are used in research. They are used to maintain constant temperature, constant humidity, and low-temperature control. Low-temperature units are capable of maintaining -140°F (-96°C). These systems usually use 5" (13 cm) of insulation and a cascade refrigeration system. A cascade refrigeration system is two refrigeration systems connected in series with one system's evaporator absorbing heat expelled by the other system's condenser. For more information on cascade refrigeration systems, see Chapter 49,

Commercial Refrigeration System Configurations.

47.3.2 Industrial Freezing of Foods Refrigeration of food is commonly seen in home kitchens and grocery stores. However, the refrigeration of food occurs before such food products even hit the shelves. Industrial freezing of food is performed in two principal types of establishments: • Processing plants. • Locker plants. Processors of frozen foods have freezing centers in many large food-producing areas. For example, fish is packed and frozen along a seacoast and then shipped to all parts of the country. A locker plant is smaller than a processing plant and is designed to prepare, freeze, and store various food products. Refrigeration equipment in processing and locker plants varies considerably. However, the plan for freezing the food is similar. Figure 47-49 shows the flow of food products through a typical plant. The food is weighed and checked for purity and suitability for freezing. Then, it moves to the processing room. There, meat is cut, fowl cleaned and dressed, vegetables blanched, and

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 47 Overview of Commercial Refrigeration Systems

1307

Prepare fruits and vegetables Incoming

Weigh,

""""""----1 check.and foods list

Chill and age beef

Chill other meats, fowl, fish, game

Grind, slice, mix, and cut to family size

Box, wrap, label with name, date, weight, etc.

Quick freeze

Place in patron's locker

Cure, smoke, and salt bacon, hams, sausage, etc. Gaadheart-Willcax Publisher

Figure 47-49. Flow chart showing each step for food moving through its preparation process in a freezing plant.

the various items packaged. The packed foods are next sent to the freezing section, where they are completely frozen and readied for storage. High humidity is very important in rooms where food is cured and stored. Meat tastes better and keeps its weight if relative humidity is kept close to 100%. The temperature should be near 39°F (4°C). This gives the best humidity results with a non-frosting evaporator. A processing plant freezes food rapidly by exposing as much of the food as possible to the lowest possible

temperature using a fast-freezing system. This is usually done by moving produce along a track through an ultra-low-temperature chamber. Some fast-freezing systems use liquid nitrogen or carbon dioxide. This turns perishable fresh food into long-lasting frozen food. This process is commonly referred to as cryogenic food freezing. Temperatures of -320°F (-196°C) are obtained, causing freezing to be instantaneous. This method of quick freezing causes little or no damage to the food.

I Copyright Goodheart-Willcox Co., Inc. 2017

Chapter Review Summary •

•

•

•

•

•

• •

•

1308

Commercial refrigeration systems may use multiple compressors, condensing units with multiple fans and flow controls, and specialized evaporators based on the build of the cabinet. These systems are found in supermarkets, convenience stores, bakeries, restaurants, and other businesses. During periods of low cooling demand, some commercial refrigeration systems form an ice bank. It absorbs heat and prevents system overload during periods of high demand. Generally, ice banks are used in drink dispensers and other liquid-cooling systems. Supermarkets, restaurants, and other businesses often have walk-in cabinets, which are large refrigerated spaces with specially insulated walls, doors, and floors. These units can be used to cool or freeze a variety of products. Walk-in cabinets are specially designed and must be installed to meet various building codes. Florist cabinets use low-velocity evaporators and refrigeration systems that maintain specific temperature and humidity requirements to prolong product quality. Special filters (EGF) are used to remove ethylene gas that would otherwise accelerate plant aging. Hot and cold merchandisers are commercial units built with two separate temperature compartments. One section keeps products cool, and the other section keeps products warm. Display cases can be used to refrigerate or freeze products and can be open or closed. Open display cases may use one or more air curtains to thermally isolate products from ambient air. An air curtain is a stream of air that blows between a conditioned space and an unconditioned space to isolate the two areas. Quick chillers, blast chillers, and blast freezers cool cooked food quickly for storage. Beverage dispensers chill products passively with ice or actively using mechanical refrigeration. Water coolers chill drinking water using a compact mechanical refrigeration system. Milk coolers are large refrigerated tanks used to cool milk after being taken from a cow. Flow controls and precooling heat exchangers reduce the heat load prior to storage. Ice machines may form small ice nuggets, ice flakes, or ice cubes in a variety of sizes. The

•

•

design of the evaporator determines the shape of the ice produced. Common ice machine evaporators include vertical cube, inverted cube, and flaked ice (auger). Other than food processing, industrial applications of refrigeration include cooling water used to cool welding electrodes, cooling of quenching liquids used for heat-treating, and cooling of compressed air. Processing plants and locker plants prepare, freeze, and store various food products in bulk. Cryogenic food freezing is the use of liquid nitrogen or carbon dioxide to instantly fast-freeze and preserve perishable food at temperatures of-320°F (-196°C).

Review Questions Answer the following questions using the information in this chapter. 1. A parallel compressor rack is an arrangement

of compressors piped in parallel sharing the following devices in common, except a(n) A. B. C. D.

air curtain liquid line liquid receiver suction line

2. A distributed system is a commercial refrigeration unit that circulates refrigerant through nearby coolers and contains only A. B. C. D.

compressors condensers evaporators refrigerant metering devices

3. During periods of low cooling demand, some liquid-cooling refrigeration systems form a solid block of ice around the evaporator called an A. ice advantage B. ice bank C. ice block D. ice cube

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 47 Overview of Commercial Refrigeration Systems

1309

4. The rapid controlled cooling of cooked food to inhibit bacteria and germ growth is performed by _ _. A. blast chillers and quick chillers B. blast dispensers and quick merchandisers C. distributed systems and ice machines D. open and glass-enclosed display cases

11. Refrigeration systems that cool fresh milk to the legally required temperatures in a large tank are A. blast chillers B. milk coolers C. milk dispensers D. ultraviolet coolers

5. Floral refrigeration systems typically use a(n) _ _ evaporator to account for floral item fragility. A. flooded B. high-velocity C. immersed D. low-velocity

12. An ice machine that produces flaked ice has an evaporator that uses a(n) _ _. A. auger B. extruding process C. inverted mold D. vertical evaporator and mold

6. Open display cases and some glass-enclosed display cases use a(n) which is a stream of air that blows between a conditioned space and an unconditioned space to isolate the two areas. A. air barrier B. air curtain C. blast chiller D. sneeze guard ___J

7. A beverage dispenser that cools its dispensing liquid using the same ice that is dispensed for drinks is _ _ chilled. A. actively B. cryogenically C. passively D. subcooler 8. A water cooler that has a plumbed water supply and drain connections is a(n) _ _ water cooler. A. ice B. self-contained C. tap D. walk-in

13. An ice machine that distributes flowing water using a manifold also uses a(n) _ _. A. auger B. extruding process C. inverted mold D. vertical evaporator and mold 14. Ice machine controls used for bin capacity include the following, except a _ _. A. diaphragm or mechanical lever switch B. float control and solenoid valve C. photo sensor D. temperature-sensing bulb 15. The process of fast freezing food using liquid nitrogen or carbon dioxide is called _ _. A blast cooling B. cascade preservation C. cryogenic food freezing D. ice bank freezing

9. A standalone water cooler that has its own water supply from a tank and does not have a drain is a(n) _ _ water cooler. A. ice B. self-contained C. tap D. walk-in 10. Refrigeration systems that cool or fast-freeze and then dispense a mix for consumption are A. B. C. D.

dispensing freezers ice machines milk dispensers processing plants

Copyright Goodheart-Willcox Co., Inc. 2017

I

CHAPTER48

Special Refrigeration Systetns and Applications

Chapter Outline 48.1 Transportation Refrigeration 48.1.1 Truck and Trailer Refrigeration 48.1.2 Railcar Refrigeration 48.1.3 lntermodal Shipping Container Refrigeration 48.1.4 Marine Refrigeration 48.2 Alternative Refrigeration Methods 48.2.1 Expendable Refrigeration Systems 48.2.2 Dry Ice Refrigeration 48.2.3 Thermoelectric Refrigeration 48.2.4 Vortex Tubes 48.2.5 Jet Cooling Systems 48.2.6 Stirling Refrigeration Cycle

Learning Objectives Information in this chapter will enable you to: • Identify the different types of refrigerant metering devices, evaporators, compressors, and condensers used in transportation refrigeration systems. •

Summarize the operation of various expendable refrigeration systems.

•

Explain how thermoelectric couples produce heating and cooling using electricity.

•

Summarize the operation of vortex tubes, steam jet systems, and refrigerant jet systems.

•

Describe the operation of a basic Stirling refrigeration system.

Chapter 48 Special Refrigeration Systems and Applications

Technical Terms dry ice eutectic plate keel cooler Peltier effect quench valve refrigerant jet system standby power steam jet system

Stirling cycle sublimation thermoelectric couple thermoelectric module thermoelectric refrigeration vortex tube

Review of Key Concepts Reviewing these concepts from earlier chapters will help you understand new concepts presented in this chapter: • A hot-gas defrost system redirects hot, compressed vapor from the compressor through the evaporator to defrost it. A bypass line equipped with a solenoid valve connects the compressor discharge line to the evaporator inlet. (Chapter 21) •

An expendable refrigerant is used only once in a system and then released into the atmosphere. It is not collected and recondensed for additional refrigeration cycles, as is the case with most compression refrigeration systems. (Chapter 9)

•

Rupture discs are protective devices for refrigerant cylinders that are designed to open under excessive pressure, but they do not close again. They allow a cylinder's entire refrigerant charge to vent before the cylinder bursts. (Chapter 10)

•

When impurities are added to a pure semiconductor, such as silicon, it is called doping. Doping produces either N-type or P-type material depending on whether the impurity causes an excess or a shortage of electrons in the material. (Chapter 14)

•

Electronic circuits include semiconductor devices. These devices can also be called solid-state devices, because there are no moving parts in a semiconductor. They switch roles from acting as an insulator to acting as a conductor on the atomic level, rather than using moving contacts. (Chapter 14)

1311

Introduction The refrigeration systems that have been described up to this point have mainly been mechanical compression systems used in stationary locations, such as supermarkets. This chapter covers systems that are designed to be mobile, systems used in spaces with limited access, and systems that achieve extremely cold temperatures. Some of the specialized systems covered are adaptations of compression refrigeration systems. Others use methods other than mechanical compression to produce refrigeration temperatures.

1312

Modern Refrigeration and Air Conditioning

48.1 Transportation Refrigeration

thermostatic expansion valve (TXV), and direct-expansion evaporator. These systems typically use 1 1/2 hp to 2 hp (1120 W to 1490 W) compressors and R-134a or R-404A as a refrigerant. Whereas R-134a is used for mediumand high-temperature refrigeration, the properties of R-404A make it suitable for use in medium- and lowtemperature applications. Both refrigerants have low toxicity and low flammability, making them safe for transportation refrigeration. Components specific to truck and trailer refrigeration include the following: a quench valve, a subcooler, hot-gas solenoid valves, and vibration absorbers. See Figure 48-1. The quench valve is a thermostatic expansion valve (TXV) that acts as a liquid injection valve from the liquid line to the suction line. During periods of low load, not much refrigerant is required for cooling, so less refrigerant is passed into the evaporator from the high side.

There is minimal difference between transportation refrigeration systems and other types of commercial refrigeration systems. Transportation refrigeration systems are designed for various ambient temperatures and operating temperatures. The systems must also be designed to withstand the stress and vibrations that occur during transport. Depending on the type, size, and purpose of the primary refrigeration system, a backup system may be installed to prevent loss of cooling in case the primary refrigeration system fails.

48.1.1 Truck and Trailer Refrigeration The main components of truck and trailer refrigeration systems are the compressor, air-cooled condenser,

Hot-gas line

External equalizer

Thermostatic expansion valve

Liquid receiver

BypaSS/:==:::d check valve

r-----1

Sensing I bulb I

I Liquid solenoid valve

Evaporator

Liquid

Shutoff valve

line~===:::: Quench valve Vibration absorber

Hot-gas bypass line

Quench valve bulb

Hot-gas solenoid valves Discharge check valve

I~======-Condenser I . - - - - ===~ pressure I control 1'-a _ _ _ _.__ _ ____ .------~ solenoid

L _____ _ Condenser

Cooling Cycle Carrier Transicald Division, Carrier Carp.

Figure 48-1. Diagrams illustrating the cooling cycle and the heat-defrost cycle of a trailer refrigeration system. (continued) Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 48 Special Refrigeration Systems and Applications

1313

t

I I I I

L _____ _

D

High-pressure vapor

D

Low-pressure vapor

•

High-pressure liquid

•

Low-pressure liquid

Heat-Defrost Cycle Carrier Transicold Division, Carrier Corp.

Figure 48-1. Continued.

Remember that suction vapor is often used to cool the compressor. When less refrigerant is available, compressor temperature can rise, and it could begin to overheat. The quench valve monitors this with a sensing bulb on the discharge line. When necessary, the quench valve opens to inject a small amount of liquid refrigerant into the suction line. This refrigerant quickly evaporates and flows into the compressor to cool it. The subcooler, which occupies a portion of the condenser, removes additional heat from refrigerant leaving the liquid receiver to help ensure that only liquid refrigerant enters the thermostatic expansion valve. Vibration absorbers are placed in the suction and discharge lines to decrease vibration. When energized, hot-gas solenoid valves open to allow heated vapor refrigerant discharged by the compressor to enter the evaporator. This is done to initiate the heating cycle or defrost cycle. The main difference between heating and defrosting is that the evaporator fans continue to run during the heating cycle to blow air over the evaporator coil. During the defrost cycle,

the evaporator fans stop to allow the heated refrigerant vapor to defrost any ice buildup on the evaporator.

Refrigerated Trailers Refrigerated trailers, also called reefers, require special trailer bodies that are typically 28' to 53' (8.5 m to 16 m) long. The trailer bodies should be light and well insulated. The boxes on most refrigerated trailers have fiberglass, composite, or metal walls. Various thicknesses and types of insulation fill the space between the inner and outer walls, depending on the application. Spray foam insulation is most often used. The insulation limits heat transfer through the trailer walls and also gives the trailer body added rigidity. Constant vibration and rough handling may reduce the insulating value of the trailer walls if they are not constructed soundly. Because there are numerous applications for trailer refrigeration, the trailer must be properly insulated for its intended use. A trailer carrying frozen goods must be insulated for -15°F (-26°C). Fresh foods require insulation for temperatures in the range of 32°F

Copyright Goodheart-Willcox Co., Inc. 2017

I

1314

Modern Refrigeration and Air Conditioning

to 35°F (0°C to 2°C). Fresh produce, flowers, and fruits also need accurate control of humidity and ventilation because these products have a tendency to lose fluids to the surrounding air. The refrigeration systems used in most refrigerated trailers are similar to other compression refrigeration systems. The major difference is the method used to drive the compressor. There are two common ways of driving the compressor: • A diesel-powered generator on a trailer supplies electrical power to the compressor motor. See Figure 48-2. • The refrigeration system is plugged into the electrical grid while the trailer is idle. This is referred to as standby power.

Oil

Electrical line to clutch

Refrigerated Trucks Refrigerated trucks are similar to refrigerated trailers. A cube-style or flat-plate evaporator is mounted on the wall inside the box of the truck. The condenser and condenser fans are mounted in a case on the roof or on the front of the box. In small-capacity trucks, the compressor is typically mounted in the engine compartment and driven by the vehicle engine, Figure 48-3. In some larger-capacity trucks, an externally mounted generator provides the power for the compressor, condenser fans, and evaporator fans. Some trucks include both a compressor driven off the vehicle's engine and a separate engine, generator, and standby compressor. Having an external generator allows the refrigeration system to continue running

Diesel-powered trailer refrigeration system

Carrier Transicold Division, Carrier Corp.

Figure 48-3. A refrigerated truck that uses the vehicle's engine to drive the compressor.

when the vehicle's engine is shut down. Figure 48-4 shows a finished installation mounted over a truck cab. A remote control module inside a refrigerated truck allows the driver to control the refrigeration system. The remote control module includes an electronic temperature controller, temperature selector, and defrost controls. Most truck refrigeration systems use hot gas for the defrost process.

Nose-mount unit

Truck box

Refrigeration system controls

Truck cab Thermo King Corporation

Figure 48-2. Trailer refrigeration system containing a dieselpowered generator, a compressor, and a condenser. The evaporator extends from the back of the unit into the trailer.

Thermo King Corporation

Figure 48-4. A condensing unit mounted above a truck cab is called a nose-mount or front-mount unit. The unit contains a small engine, a generator, a compressor, and a condenser.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 48 Special Refrigeration Systems and Applications

1315

Eutectic Plates Another type of truck and trailer refrigeration system uses eutectic plates to provide passive refrigeration. A eutectic plate is a thin, rectangular tank containing an evaporator surrounded by a solution that freezes at a desired temperature. Refrigerant flow through the evaporator freezes the solution, called a eutectic solution. Once the eutectic solution is frozen, the refrigeration system can be shut off, and the eutectic plates alone provide many hours of passive cooling, Figure 48-5. Eutectic plates may be housed inside a shroud with blower fans at the top. The fans draw in warm box air and force it over the surface of the eutectic plate. The air is cooled as it passes over the plate. The chilled air then exits through the bottom of the housing and circulates throughout the box. The condensing unit for a refrigeration system with eutectic plates may be permanently mounted on a truck, like the nose-mount units already described. However, for smaller delivery trucks that return to the shop at the end of the day, the condensing unit is often a small, portable unit that is separate from the truck. When the truck has completed its deliveries for the day and returns to the shop, the portable condensing unit is connected to the eutectic plate. The condensing unit contains a compressor, condenser, filter-drier, and metering

Thermostatic expansion valves

Eutectic plates Transcald Distribution, Ltd.

Figure 48-5. A trailer equipped with multiple eutectic plates. Note the thermostatic expansion valves on each plate, which meter the refrigerant flow to the evaporators contained inside the plates.

device. The ends of the evaporator coil protrude from the eutectic plate and are equipped with quick-connect fittings. The quick-connect fittings allow the condensing unit to be connected and disconnected from the evaporator without refrigerant loss, Figure 48-6.

Eutectic plate filled with eutectic solution

I

Condensing unit

D Low-pressure vapor

•

Low-pressure liquid Gaadheart-Willcax Publisher

Figure 48-6. Eutectic plate installed in a refrigerated truck. At night, a condensing unit is connected to the eutectic plate and circulates refrigerant to freeze the eutectic solution inside the plate. In the morning, the condensing unit is disconnected. Copyright Goodheart-Willcox Co., Inc. 2017

1316

Modern Refrigeration and Air Conditioning

The condensing unit is allowed to operate through the night, circulating refrigerant through the evaporator to freeze the eutectic solution. As refrigerant vaporizes inside the evaporator coil, heat is absorbed from the eutectic solution. As sensible heat is removed from the eutectic solution, the temperature of the solution falls. When the freezing point of the solution is reached, additional latent heat is absorbed from the solution as it freezes. In the morning, when the solution in the eutectic plate is fully frozen, the plate is disconnected from the condensing unit. Throughout the rest of the day, the eutectic plate absorbs heat from inside the truck box as the eutectic solution slowly melts. At night, the truck returns to the shop to repeat the process.

Pro Tip Eutectic Plate Terminology Eutectic plates are also commonly referred to as cold plates, holding plates, and hold plates.

to keep the cargo at the desired temperature for the short time the shipment is in transit.

Pro Tip lntermodal Shipping Container Dimensions The International Standards Organization (ISO) has issued standards for the dimensions of intermodal shipping containers. The most common lengths are 20', 40', and 45'. The height may be 8'-6" or 9'-6", and the width is typically 8'.

For longer trips, intermodal shipping containers require a compression refrigeration system to keep the cargo at the desired temperature. The condensers in most shipping container refrigeration systems are water cooled because there may be inadequate airflow for an air-cooled condenser. The compressor is usually driven by a diesel-powered generator on the container. On a ship, the container can be plugged into the ship's power. For warehouse or dock storage, the local power grid may be used to power the container's refrigeration system, Figure 48-7.

48.1.2 Railcar Refrigeration Railcar refrigeration is used for two general purposes: to refrigerate cargo and to provide comfort cooling to passengers. Railcar construction and insulation is similar to that of a refrigerated trailer. While most railcar refrigeration is provided by compression refrigeration systems, some railcar refrigeration systems are absorption systems. Others use steam jet systems, which are discussed later in this chapter. Compressors are usually driven from the railcar axle while the train is in motion. An electric motor is used when the railcar is stopped. Some trailer refrigeration systems with a diesel-powered generator and motor-driven compressor have been modified to be used in railcars. These systems are very similar to the one shown in Figure 48-2. The main differences are additional structural elements to protect the refrigeration system and the use of remote communications (by satellite, cell phone, or radio frequency) to monitor and control the system.

48.1.4 Marine Refrigeration Marine refrigeration equipment is basically the same as land-based refrigeration equipment, but designed for the marine environment. System components must also be designed to withstand exposure to saltwater and high humidity. The demands of the marine environment have led to increasingly

Shipping container

Diesel-powered refrigeration system

48.1.3 lntermodal Shipping Container Refrigeration Intermodal shipping containers are used aboard ships, trucks, railcars, and airplanes to transport perishable goods. For short distances, a container can be cooled by a eutectic plate or dry ice. Before the goods are put into the container, the eutectic solution is frozen using a detachable condensing unit. Once frozen, the eutectic solution provides enough cooling capacity

Refrigeration system controls Carrier Transicold Division, Carrier Corp.

Figure 48-7. An intermodal shipping container equipped with

a compression refrigeration system.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 48 Special Refrigeration Systems and Applications specialized designs, especially for smaller systems used on yachts. Many yachts, especially sailboats, are constrained by the amount of available electrical power. The most efficient systems use hermetic compressors that can operate on the power supplied by a solar panel. An important aspect to consider in all marine applications is that the available space on a ship or boat for refrigeration equipment is limited, Figure 48-8. Refrigerated storage spaces aboard yachts, commonly called cold boxes, are frequently exposed to tropical conditions. The cold box insulation and refrigeration system are often sized for 90°F (32°C) ambient conditions. As a result, the recommended insulation values are R-20 for refrigerator space and R-30 for freezer space. This can be achieved with 4" (10 cm) of conventional foam insulation around the refrigerator or 6" (15 cm) around the freezer. However, many insulating foams, such as polyurethane and polyisocyanurate foams, absorb water easily. If these materials become wet, they lose much of their insulating ability. Extruded polystyrene is preferred as an insulating material for marine applications because it does not absorb water. The required thicknesses of insulating foams for good thermal performance take up valuable space in a

1317

space that is already limited. In some cases, adequate space is unavailable. To solve this dilemma, a technician can use vacuum insulation panels for cold box insulation because of their ability to save space while still maintaining R-values ranging up to R-50. Vacuum insulation panels allow for much thinner box walls, which can double or triple the usable space in a given box. The material is also stable and can provide good thermal performance for the life of the boat. Cold boxes should be designed to minimize the damage that can occur to them during use. Any damage to the cold box's exterior can lead to unwanted condensation, especially around hatches and behind cushions. Access lids and doors should have latches to prevent spillage of the box contents when rough weather is encountered. Marine refrigeration systems are typically assembled from converted industrial or automotive refrigeration or air-conditioning equipment. Although there is some mixing of types, larger systems tend to use opendrive compressors and eutectic plates while smaller systems are more likely to use flat-plate evaporators and hermetic compressors, similar to the type used in domestic refrigerators. See Figure 48-9. There are three common methods for cooling a system's condenser. An air-cooled condenser is the

I Transducers Direct, LLC

Figure 48-8. Refrigeration and air-conditioning equipment on marine vessels must be sized to fit into the limited available space. Copyright Goodheart-Willcox Co., Inc. 2017

1318

Modern Refrigeration and Air Conditioning Hermetic compressor

Evaporator

Refrigerant lines

Keel cooler (condenser)

Evaporator

Thermostat

Compressor

Thermostat

Veco NA - Coastal Climate Control, Inc.

Figure 48-9. An evaporator, keel cooler, and small hermetic compressor used to refrigerate marine cold boxes. Keel cooler

least expensive. However, as the ambient air temperature increases, the amount of energy required by the compressor to maintain high enough head pressure for proper cooling becomes excessive. Water is about 20 times more thermally conductive than air, and an endless supply is available on the other side of the boat's hull. For these reasons, many boats use keel coolers or heat exchangers cooled by pumped-in seawater. Keel coolers run the condenser tubing outside the hull in the vicinity of the keel to take advantage of seawater's ability to absorb heat. Keel coolers must be properly sized for their application. If a keel cooler is oversized, it may operate efficiently in warm water but provide too much subcooling when the boat is in cold water. If a keel cooler is undersized, the head pressure can become excessively high in warm water, leading to high energy use, Figure 48-10. Unlike standard air-cooled condensers at a residence or on top of a building, keel coolers are submerged in water. This subjects them to a wide range of temperature and any chemicals or substances that may be in the water. Many keel coolers are equipped with zinc anodes to minimize corrosion of the condensing coil's housing. See Figure 48-11. Heat exchangers provide economical operation across a wide range of operating temperatures. A pump draws in water through an inlet in the hull of the boat. The water is passed through a heat exchanger where it absorbs heat from high-pressure refrigerant inside the compressor's discharge line. The heated water is then released back outside the boat. See Figure 48-12.

Veco NA - Coastal Climate Control, Inc.

Figure 48-10. A keel cooler houses the condensing coil and releases heat from high-side refrigerant into the surrounding water.

Refrigerant lines

Sintered brass housing containing coil

Veco NA - Coastal Climate Control, Inc.

Figure 48-11. A keel cooler often encloses its condensing coil in a protective housing.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 48 Special Refrigeration Systems and Applications Heat exchanger

Hermetic compressor

1319

refrigeration methods have been developed to meet specific commercial and industrial refrigeration needs. The following sections address some of these alternative refrigeration methods.

48.2.1 Expendable Refrigeration Systems

..

Water-Cooled Heat Exchanger

Cold seawater in

Warmed seawater out

---------7

Hot vapor refrigerant in Top View of Heat Exchanger Veco NA - Coastal Climate Control, Inc.; Goodheart-Willcox Publisher

Figure 48-12. Some marine refrigeration systems pump seawater through a heat exchanger tube on the compressor. The compressor's discharge line is enclosed within the heat exchanger tube to allow heat transfer over the entire surface of the discharge line.

48.2 Alternative Refrigeration Methods To this point, the focus of this book has been largely on compression and some absorption refrigeration systems. However, a number of different

An expendable refrigeration system, sometimes called an open-cycle refrigeration system, is one in which the refrigerant is discarded after it has evaporated. In one variation of an expendable refrigeration system, liquid nitrogen is sprayed directly into a conditioned space. As the nitrogen evaporates, it absorbs heat from the space and is then vented to the atmosphere. This simple system is used in the storage of refrigerated or frozen foods and in trucks and other vehicles in the transportation industry, Figure 48-13. The liquid nitrogen is kept under high pressure in an insulated cylinder inside the conditioned space. A control box is connected to a temperature-sensing element and to a liquid control valve. When the temperature in the conditioned space rises above the cut-in temperature, the control box operates the liquid control valve, allowing some of the liquid nitrogen out of the cylinder. The pressure of the liquid nitrogen drops as it passes through the restriction in the valve. As the low-pressure liquid nitrogen is forced out through the spray nozzles, it rapidly evaporates into low-pressure nitrogen vapor, absorbing heat from the air inside the conditioned space. The temperature-sensing element constantly monitors the temperature in the conditioned space and relays information to the control box. With this information, the control box can regulate the liquid control valve to increase or decrease the flow of liquid nitrogen to the spray nozzles as needed. In this way, these devices work together to maintain the desired temperature in the conditioned space. Because liquid nitrogen evaporates at -320°F (-196°C) at atmospheric pressure, it is well suited for shipping frozen foods. The temperature may be kept as low as desired, usually about -20°F (-29°C). An advantage of expendable refrigeration systems that use liquid nitrogen is their ability to operate without a power source. Also, because of their simple design, these types of expendable refrigeration systems require very little maintenance. However, the liquid nitrogen cylinder must be replaced or recharged periodically. Containers or spaces refrigerated by liquid nitrogen must be equipped with safety devices to shut off the flow of nitrogen when a person opens a door to the space. Heat surrounding the cylinder may cause the liquid nitrogen inside to evaporate rapidly, increasing

Copyright Goodheart-Willcox Co., Inc. 2017

I

1320

Modern Refrigeration and Air Conditioning

Control box

Conditioned space

High-pressure nitrogen vapor _ _,._ ·----·11

Liquid control valve

D High-pressure vapor • High-pressure liquid D Low-pressure vapor • Low-pressure liquid Goodheart-Willcox Publisher

Figu~~ 48-13. One type of expendable (open-cycle) refrigeration system releases liquid nitrogen directly into a well-insulated cond1t1oned space.

the cylinder's internal pressure. If the cylinder pressure rises above a safe limit, an automatic pressure-relief valve on the cylinder opens to allow nitrogen vapor to escape until the cylinder pressure is back below a safe level. Liquid nitrogen cylinders are also equipped with rupture discs. If cylinder pressure becomes excessively high, the rupture disc bursts to vent nitrogen vapor and prevent the cylinder from exploding. Safety Note

Liquid Nitrogen Safety Exposure to liquid nitrogen can result in severe burns and frostbite. Always wear a long-sleeve shirt, insulated gloves, and safety goggles or a face shield when handling or operating a liquid nitrogen cylinder. Use a cart, crane, or lift to move the cylinder. Do not roll a liquid nitrogen cylinder. Keep it in a vertical position at all times.

48.2.2 Dry Ice Refrigeration Dry ice is carbon dioxide (COz) frozen solid. It is pressed into various sizes and shapes, typically blocks or slabs. As dry ice absorbs heat, it changes directly from a solid to a vapor. It does not go through the liquid state. The process of a solid changing directly into a vapor is called sublimation. At atmospheric pressure, solid carbon dioxide sublimates at-109°F (-78°C), Figure 48-14. Figure 48-15 shows a common method of using dry ice to refrigerate frozen food. Dry ice is packed either beside or on top of the food packages. As the dry ice changes to carbon dioxide vapor, it keeps the food frozen by absorbing ambient heat. The carbon dioxide vapor replaces the air in the container or cabinet as the dry ice sublimates. This also helps to preserve the food. One type of expendable refrigeration system, often used on aircraft, uses dry ice as an expendable

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 48 Special Refrigeration Systems and Applications

1321

This heavily insulated bin holds the condenser and dry ice pellets, Figure 48-16. Since dry ice sublimates at a very low temperature (-109°F or -78°C), it causes refrigerant vapor entering the condenser to condense quickly to a liquid. The liquid refrigerant flows by gravity through the condenser and into the evaporator. A thermostatically operated metering device located in the liquid line controls the flow of refrigerant into the evaporator. In the evaporator, the liquid refrigerant absorbs heat as it vaporizes and flows upward into the condenser. At that point, the cycle repeats.

Dry ice

Safety Note

Dry Ice Always wear heavy, insulated gloves when handling dry ice. Touching dry ice with bare skin can cause severe burns and instant frostbite.

48.2.3 Thermoelectric Refrigeration

Carbon dioxide (CO2) vapor Reika/Shutterstock.com

Figure 48-14. A block of dry ice sublimating. Note that there is no moisture produced as the dry ice sublimates.

Frozen food

dry ice Frozen food package

Insulation Goodheart-Willcox Publisher

Figure 48-15. As the slabs of dry ice sublimate, they absorb heat and keep the food in the frozen food container cold.

secondary refrigerant in conjunction with a primary refrigerant that is not expended. A closed refrigeration circuit is connected to an evaporator in the conditioned space and to a condenser located in an insulated bin.

Thermoelectric refrigeration is the process of transferring heat energy from one place to another using the movement of electrons. Heat is transferred on a subatomic level using semiconductor devices. In 1834, French physicist Jean-Charles Peltier discovered that when current was passed through the junction of two dissimilar metals, heat was absorbed in one part of the junction and moved to another part of the junction. This phenomenon, called the Peltier effect, is the basis of modern thermoelectric refrigeration. The thermoelectric refrigeration process removes heat from one area and puts it in another area. Electrical energy, rather than a refrigerant, serves as the heat-transfer medium. Thermoelectric refrigeration requires none of the conventional equipment necessary in a mechanical compression system. There is no compressor, evaporator, condenser, or refrigerant metering device. In fact, there are no moving parts because the cooling process is performed by semiconductors (solid-state components). Semiconductors are processed into either N-type or P-type materials. N-type materials have a surplus of electrons and a negative (-) charge. P-type materials carry a positive(+) charge because they have electron holes, which are positively charged gaps that are ready to receive electrons. A thermoelectric couple is formed by connecting one N-type material and one P-type material. See Figure 48-17. When current flows from the P-type material toward the N-type material, the junction where the materials are connected absorbs heat. The opposite end of each segment becomes hot and gives off heat.

Copyright Goodheart-Willcox Co., Inc. 2017

I

1322

Modern Refrigeration and Air Conditioning Temperature-sensing element

Insulated bin

Insulation

Evaporator

+ Conditioned space

---~-

+

+

Dry ice pellets

Condenser

D High-pressure vapor •

High-pressure liquid

D Low-pressure vapor

Metering device

•

Low-pressure liquid Goodheart-Willcox Publisher

Figure 48-16. One type of expendable refrigeration system uses dry ice in an insulated bin to absorb heat from the condenser. The primary refrigerant flows by gravity and does not require a compressor.

t

Heat

+ +

t

Heat

Cooling surface

N-type material

11 ! 11

!

t ~

.... ,,

Heated surface

,, t

Heat

P-type material

t

t

+ 11 -

~

P-type material

N-type material

t

+ +

!

Heat

..,_

-

11 +

..,_

DC power source

DC power source

A

B Goodheart-Willcox Publisher

Figure 48-17. Diagram showing how a thermoelectric couple absorbs and rejects heat when current is applied. A-When current from the power source flows from the P-type material to the N-type material, the surface at their junction absorbs heat. B-When the flow of current is reversed, the cooling surface and heated surface are switched.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 48 Special Refrigeration Systems and Applications

Reversing the direction of current through a thermoelectric couple switches the hot and cold surfaces. Thus, the same device can be used for either heating or cooling a conditioned space, depending on the direction of current flow through the device.

Rectifier AC power

DC current

Pro Tip Thermoelectric Couples and Thermocouples

+

! t

Although both thermoelectric couples and thermocouples are similar in construction, they are not interchangeable. While thermoelectric couples are engineered to move heat when electric current is applied, thermocouples are designed to produce an electric current when heat is applied. Thermoelectric couples operate under the principle of the Peltier effect, and thermocouples operate under the principle of the Seebeck effect.

Since a single thermoelectric couple produces a minimal cooling effect, several thermoelectric couples are connected in series to form a thermoelectric module, which produces significant cooling. Groups of modules can be connected together in parallel to further increase the cooling capacity. Fins on the cooling surface increase its heat flow. Fins on the outside of the heated surface help reject heat into the surrounding air more quickly. A rectifier supplies a controlled de current to the module, and a thermostat inside the conditioned space controls the current flow through the rectifier. In this manner, the temperature inside the conditioned space is regulated, Figure 48-18.

1323

!• cooling surface

• Heated surface

I

Goodheart-Willcox Publisher

Figure 48-18. A thermoelectric module with three

thermoelectric couples connected in series to increase their cooling effect in the conditioned space of a small refrigerator.

Pro Tip Thermoelectric Module Terminology The following are some of the terms that can refer to thermoelectric or Peltier modules: Peltier device, Peltier cooler, thermoelectric cooler, thermoelectric heat pump, and semiconductor heat pump.

Benefits of using thermoelectric modules include silent operation, compact size, and low maintenance. Although the operation of thermoelectric modules is simple, their thermal efficiency is low. The amount of refrigeration obtained for the electrical energy spent is less than with a conventional compression refrigeration system. Thermoelectric modules are used for the cooling and heating of nuclear submarines and for controlling temperatures in electronic equipment, such as computers and aerospace devices. Other applications for thermoelectric modules include water coolers, portable refrigerators, and medical applications, such as blood analyzers. See Figure 48-19.

I The Coleman Company, Inc.

Figure 48-19. A portable cooler that uses thermoelectric

modules powered by a 12 Vdc supply.

Copyright Goodheart-Willcox Co., Inc. 2017

1324

Modern Refrigeration and Air Conditioning

48.2.4 Vortex Tubes A vortex tube is a simple device that can provide cold air by separating it from hot air. It has no moving parts and does not use refrigerants. When compressed air is released into a vortex tube, a large volume of hot air comes out one end of the tube, and a smaller volume of cold air comes out the other end of the tube. A vortex tube assembly consists of a compressedair supply line, a jet, a swirl chamber, a tube, and a deflector cone (control valve). See Figure 48-20. Compressed air from the supply line passes through the jet and into the swirl chamber. The jet acts as an internal venturi. It restricts the flow of compressed air, causing the air to accelerate and creating a pressure drop. The air passes into the swirl chamber at an acute angle to the chamber wall. This angle causes the airflow to hug the wall of the swirl chamber as it moves through the tube, creating a vortex. The swirling motion continues and intensifies as the air flows through the tube. Due to centrifugal force, the air hugging the wall of the tube compresses slightly. This decreases the pressure of the air closer to the center of the tube, allowing it to expand and cool. When the air reaches the end of the tube, the hot air hugging the tube wall flows past the deflector cone. The cold air in the center of the tube hits the cone and deflects. High pressure forces the deflected cold air to reverse direction and flow back through the tube in the opposite direction. This cold air forms a column in the center of the tube as it flows backward through the swirl chamber and out the other end of the tube.

The deflector cone essentially acts as a control valve at the hot air outlet and determines how much air is allowed to escape from that end of the tube. When the cone is fully open, most of the air in the tube flows out through the hot air outlet. However, the small amount of air that does reverse direction and flows out the other end of the tube is cooled as much as possible. On the other hand, when the deflector cone is nearly closed, much more air reverses direction and flows out the other end of the tube. Since very little hot air is removed, the cooling effect is greatly reduced. Vortex tubes are designed to operate on a continuous basis. Therefore, a large quantity of compressed air is required. No thermostatic control of any type is used to regulate the output temperature of a vortex tube. However, the output of the vortex tube can be changed manually by adjusting the deflector cone. This allows the vortex tube operator to adjust outlet air temperature as needed. When supplied with compressed air at 100 psi (700 kPa) and 70°F (21 °C), vortex tubes can be easily adjusted to deliver cold air with temperatures as low as -50°F (-46°C) or hot air with temperatures as high as 250°F (121 °C). These temperatures, however, cannot be arrived at simultaneously. As the temperature at the cold air outlet is reduced so is the temperature at the hot air outlet. Similarly, when the temperature at the hot air outlet is increased so is the temperature at the cold air outlet. Different types of vortex tubes are used for different cooling applications. In scientific work, vortex tubes can be used to dehumidify gas samples. Vortex tubes can also be used to cool environmental chambers and

Compressed air in

Hot air Jet

Cold air out

.!.

~:::=;::!!~

~

Swirl chamber

~

~~~

Tube

Deflector cone (control valve) ITW Vartec

Figure 48-20. Cross section of a vortex tube. Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 48 Special Refrigeration Systems and Applications electronic cabinets. Vortex tubes are commonly used in various machining and manufacturing processes to spot cool materials, Figure 48-21. A common application of vortex tubes is personal air conditioning. Suits cooled by vortex tubes can be worn by people working in very hot environments or in environments that require heavy, protective clothing. The vortex tube is attached to a diffuse-air vest. Cold air is distributed over the upper body through tiny holes in the vest, cooling the person, Figure 48-22.

1325

Diffuse-air vest

\

48.2.5 Jet Cooling Systems Vortex tube

There are two types of jet cooling systems: steam jet systems and refrigerant jet systems. These systems use a jet pump instead of a compressor to provide air conditioning or refrigeration. A jet pump consists of a centrifugal pump and ejector. A steam jet system uses water as the working fluid whereas a refrigerant jet system uses a refrigerant, such as R-134a. Older units may use R-11 or R-12 as the working fluid.

Steam Jet Systems A steam jet system provides cooling by using high-pressure steam to induce low pressure in an evaporator. The low pressure causes water in the evaporator to evaporate, cooling the remaining water.

Compressed-air supply line

Control valve (deflector cone) ITW Vartec

Figure 48-22. Vortex tubes can be used to provide personal air conditioning in hot environments. Hot air outlet

Control valve (deflector cone)

Vortex tube

Flexible cold air nozzles

Compressed air inlet ITWVortec

Figure 48-21. A vortex tube being used to cool a large saw blade in a paper mill.

The cooled water is then used to absorb heat as it circulates through tubing in a conditioned space. See Figure 48-23. In a steam jet system, the bottom part of the evaporator is filled with water, and the rest is filled with water vapor. A steam-fed booster ejector creates suction that draws water vapor out of the evaporator, causing a pressure drop in the evaporator. Since the pressure decreases, the temperature at which the remaining water evaporates also decreases. As the water in the evaporator begins to evaporate, it absorbs heat from the remaining water. The heat being absorbed by the evaporating water lowers the temperature of the remaining water. This process lowers the temperature of the remaining water to between 40°F and 70°F (4°C and 21°C). Temperatures below 40°F (4°C) are impractical due to the risk of water freezing in the system. A pump circulates the cooled water from the evaporator through tubing in the conditioned space.

Copyright Goodheart-Willcox Co., Inc. 2017

I

1326

Modern Refrigeration and Air Conditioning Steam line To steam condenser

Water return

Steam nozzle

....,_

Spray nozzle

Conditioned spac

Circulating pump •

Hot water

•

Cool water

D Low-pressure water vapor D High-pressure steam Goodheart-Willcox Publisher

Figure 48-23. A steam jet system uses steam to create low pressure in the evaporator. The low pressure causes water to evaporate and absorb heat.

As the cooled water passes through the tubing, it absorbs heat from the conditioned space. The water then returns to the evaporator where it is forced through spray nozzles. After the steam passes through the booster ejector, it is condensed at another location for the following reasons: • To recover some heat from the steam. • To recover the water in the steam. • To reduce the pressure so that it will not back up into the evaporator. Steam pressure at the steam nozzle should be about 150 psia (1030 kPa). Although the steam condenser is not shown in the illustration, the steam pressure inside the condenser is about 3 psia (21 kPa). Knowing this, a technician can review a pressuretemperature (P/T) chart to find the corresponding condensing temperature for water at 3 psia, which happens to be 141°F (61°C). Steam jet systems usually have a capacity of 100 tons or more. A capacity of this size requires a large supply of steam under a fairly high pressure. Exhaust steam from a high-pressure, steam-operated machine is often used to supply a steam jet system. Steam jet systems also require a large supply of water to keep the evaporator filled and to cool the steam condenser. Steam jet systems are typically used to condition air. They are also used to cool water in certain chemical plants. Another application of steam jet systems is

removing water from diluted solutions that contain juices. Orange juice can be concentrated in this way. Steam jet systems accomplish this by evaporating the water in the orange juice at a relatively low temperature. Since the juice remains cool as the water is boiled out, the vitamins in the juice are kept at full strength.

Refrigerant Jet System A refrigerant jet system uses waste heat to help pressurize refrigerant and drive it from the evaporator to the condenser. Like a steam jet system, a refrigerant jet system uses a jet pump instead of a compressor. After refrigerant has released heat in the condenser, a circulating pump moves it through the metering device. The metering device reduces both the pressure and temperature of the refrigerant. At this point, the refrigerant splits and follows two different paths. Some refrigerant flows to the evaporator, and the rest flows through a heat exchanger where it absorbs waste heat, Figure 48-24. The refrigerant passing through the evaporator cools the conditioned space by absorbing heat. The refrigerant path to the heat exchanger runs parallel to the evaporator. As the refrigerant in the heat exchanger absorbs high-temperature waste heat, it vaporizes and increases in temperature significantly. The temperature of the refrigerant leaving the heat exchanger is much higher than the temperature of the refrigerant leaving the evaporator. As a result, the speed at which the refrigerant from the heat exchanger flows through

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 48 Special Refrigeration Systems and Applications the jet pump creates suction. This pulls refrigerant from the evaporator into the jet pump and then on to the condenser. Compared to conventional compression refrigeration systems, refrigerant jet systems have a lower coefficient of performance and require a large condenser to remove heat. However, because the energy input into the system comes from waste heat, this type of system can be very economical in some applications. Refrigerant jet systems are often found in commercial installations where a large amount of waste heat produced by other commercial processes is readily available. Both steam jet and refrigerant jet systems demonstrate how energy conservation can be incorporated into high-waste commercial processes.

48.2.6 Stirling Refrigeration Cycle The Stirling cycle is a closed thermodynamic cycle that can convert thermal energy into mechanical energy and vice versa. It was originally developed in 1816 by Robert Stirling who hoped to create an engine that would be a safer alternative to the steam engine. When heat is applied to a Stirling engine, the heat energy is converted into motion. Conversely, if mechanical energy is applied to a Stirling engine, heat is transferred from one area of the engine to another.

1327

This causes one area of the engine to heat up and the other to cool off. The Stirling cycle was adapted for refrigeration by John Herschel in 1834. It is now used in some refrigeration installations designed to reach very low temperatures. This cycle, when used in a three-stage system, can produce temperatures as low as -450°F (-268°C). In theory, an ideal Stirling refrigeration cycle picks up heat only at the lowest temperature and discards heat only at the highest temperature. There would be no heat gain or heat loss between these two temperatures. In actuality, a practical Stirling refrigeration cycle is almost as good. It conserves the heat energy produced during one part of the cycle and uses it in another part of the cycle. A simple Stirling refrigeration system consists of one cylinder and two pistons with a stationary regenerator between them. The entire space between the pistons is charged with a gas, usually helium. The cylinder walls are insulated to prevent heat transfer. The heat exchangers are uninsulated and are the only places where heat can be added or removed from the system. The cylinder has two heat exchangers: one for rejecting heat and the other for absorbing heat. The regenerator is filled with a porous material that allows the gas in the cylinder to pass freely through it. The material must also

Jet pump

Heat exchanger Boiler (waste heat source)

"f

~aporator

Metering device

I

heat out Pumps Gaadheart-Willcax Publisher

Figure 48-24. The flow of refrigerant vapor from the heat exchanger through the jet pump creates a siphoning effect that draws refrigerant vapor from the evaporator into the jet pump. Copyright Goodheart-Willcox Co., Inc. 2017

1328

Modern Refrigeration and Air Conditioning

have good thermal storage capacity and be able to transfer heat effectively, Figure 48-25. The four steps of the Stirling refrigeration cycle are shown in Figure 48-26. In Step A, the refrigeration system is at its starting point. Both the hot piston and the cold piston are as far to the left as they can go. The hot piston begins moving to the right while the cold piston remains stationary. This compresses the gas. Some of the heat generated by the compression is rejected through the heat exchanger in the compression area. In Step B, the cold piston begins moving to the right at the same speed as the hot piston. During this time, the volume of gas in the cylinder does not change. The gas from the compression area of the cylinder is forced through the regenerator, which absorbs heat from the gas. In Step C, the hot piston is at the end of its stroke and remains stationary. The cold piston continues

Insulated cylinder wall

Hot piston

D Helium gas

Heat-rejection heat exchanger

moving to the right, increasing the volume of gas in the expansion area. As the gas expands, it begins to cool off. Heat is absorbed through the heat exchanger in the expansion area to maintain a constant gas temperature. In Step D, both the hot piston and the cold piston begin moving to the left at the same speed. The volume of gas does not change. As the gas is forced back through the regenerator, it reabsorbs the heat that it had given up to the regenerator in Step B. At the end of Step D, the system is in the same state it was at the beginning of Step A. The cycle is ready to begin again. In practice, the left end of the cylinder is water cooled. The right end of the cylinder is the cooling unit. There are many different designs used to apply the Stirling refrigeration cycle. Although the systems may vary widely in their mechanics, they all operate on the same basic principles described here.

Heat-absorption heat exchanger

Expansion area

Compression area

Cold piston

Regenerator Gaadheart-Willcax Publisher

Figure 48-25. The basic parts of a simplified Stirling refrigeration system.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 48 Special Refrigeration Systems and Applications

1329

Heat exchangers Hot piston

Cold piston

'

A

B 1

~

" " c:::::=

...

"'

..

~

'~ C

D

ID Heated gas

D Cooled gas I Gaadheart-Willcax Publisher

Figure 48-26. Basic operation of a Stirling refrigeration system.

I Copyright Goodheart-Willcox Co., Inc. 2017

Chapter Review •

Summary •

•

•

•

•

•

Truck and trailer refrigeration systems have many of the same components as other mechanical refrigeration systems: compressor, condenser, refrigerant metering device, and direct-expansion evaporator. Components specific to truck and trailer refrigeration include a quench valve, a subcooler, solenoid valves, and vibration absorbers. A diesel-powered generator and plug-in standby power are the two common methods used to drive the compressor in a refrigerated trailer. In refrigerated trucks, the compressor may be driven by the vehicle engine or an external generator. A eutectic plate is a thin, rectangular tank containing an evaporator surrounded by a solution that freezes at a desired temperature. For smaller trucks with eutectic plates, a portable condensing unit is plugged into the plates at night to freeze the solution inside them. During the day, the condensing unit is removed, and the eutectic plates provide passive cooling inside the truck box. The compressors in railcar refrigeration systems are typically driven off the railcar's axle while the train is in motion. Intermodal shipping containers can be cooled by eutectic plates or dry ice for short trips. For longer trips, a generator powers the refrigeration system. Due to the climates in which many boats travel, the cold boxes used in marine refrigeration systems require water-resistant insulation with a high R-value. Larger systems may use an open-drive compressor, while smaller systems typically use hermetic compressors. The three main methods for cooling the condenser are air, pumped-in seawater, and keel coolers. An expendable refrigeration system vents its refrigerant after it has evaporated. One type of expendable refrigeration sprays liquid nitrogen into a conditioned space. As the liquid nitrogen evaporates, it cools the space and is then vented.

•

•

•

•

Dry ice refrigeration relies on the sublimation of solid carbon dioxide to absorb heat from a conditioned space. The dry ice may be used alone, or it may be used as a secondary refrigerant to condense another refrigerant in an adjacent but physically isolated circuit. A thermoelectric couple is made of N-type and P-type semiconductor materials. When direct current flows through a thermoelectric couple, one side of the thermoelectric couple transfers heat to the other side. The hot and cold sides can be switched by reversing the direction of the current. A vortex tube can provide cold air without any refrigerant or moving parts. Compressed air is fed into the vortex tube, which direct airflow in a certain manner. Hot air leaves one outlet in the tube, and cold air leaves the other outlet. Refrigerant jet systems use waste heat to help drive refrigerant from the evaporator to the condenser. Steam jet systems use highpressure steam to siphon off water vapor inside an evaporator. The resulting drop in pressure increases evaporation, which cools the remaining water in the evaporator. The Stirling refrigeration cycle converts mechanical energy into heat energy. A simple Stirling refrigeration system uses a cylinder with two pistons, a stationary regenerator, and heat exchangers. As the pistons move, they transfer heat by compressing and expanding a gas, usually helium.

Review Questions Answer the following questions using the information in this chapter. 1. In truck and trailer refrigeration systems, hot-

gas solenoid valves open to initiate the _ _ cycle. A defrost B. passive cooling C. quenching D. transportation 2. A quench valve is a TXV that opens based on _ _ temperature. A discharge line B. liquid line C. subcooler D. suction line

1330

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 48 Special Refrigeration Systems and Applications

3. Which of the following statements regarding compressors in transportation refrigeration systems is not true? A. The compressor in railcar refrigeration is usually driven by the railcar axle. B. For longer trips, intermodal shipping containers use dry ice as the sole means of refrigeration. C. In small-capacity trucks, the compressor may be driven by the vehicle's engine. D. Some refrigerated trucks use a portable condensing unit that is removed before the truck is put into service for the day. 4. Which of the following types of insulation would be best suited for use in a marine cold box where space is severely limited? A. Extruded polystyrene B. Loose cellulose insulation C. Polyurethane foam D. Vacuum insulation panels 5. Which of the following statements regarding keel coolers is not true? A. Air-cooled condensers are less efficient than keel coolers. B. Keel coolers run the condenser tubing outside the hull. C. Pumps must be used to draw in and discharge the cooling water. D. Zinc anodes are used on keel coolers to minimize corrosion of their housings. 6. Which of the following refrigerants is commonly used in expendable refrigeration systems? A. Liquid ammonia B. Liquid argon C. Liquid carbon dioxide D. Liquid nitrogen 7. The process of dry ice changing from a solid directly into a vapor is called _ _. A. condensation B. desiccation C. evaporation D. sublimation 8. Which of the following phenomena is the basis of thermoelectric refrigeration systems? A. The Electric effect B. The Peltier effect C. The Seebeck effect D. The Watt effect

1331

9. Several thermoelectric couples connected in series form a _ _. A. rectifier B. thermocouple C. thermoelectric module D. vortex tube 10. Eutectic plates designed for smaller delivery truck refrigeration on shorter trips provide -~cooling. A. active B. expendable C. keel D. passive

11. In a vortex tube, the air at the center of the tube is cooled due to _ _. A. change of polarity B. compression C. eutecticity D. expansion 12. In a vortex tube, the jet acts as an internal venturi, which restricts the flow of compressed air. This causes the air to accelerate and creates a(n) _ _. A. electric spark B. N-type material C. pressure drop D. pressure increase 13. In a steam jet system, the steam jet _ _. A. carries heat away from the condenser B. forces refrigerant into the condenser C. lowers the pressure in the evaporator D. spins a compressor turbine 14. Which of the following components is not part of a refrigerant jet system? A. Compressor B. Condenser C. Evaporator D. Heat exchanger 15. A Stirling refrigeration system uses mechanical energy to transfer _ _ energy. A. electrical B. magnetic C. sublimation D. thermal

Copyright Goodheart-Willcox Co., Inc. 2017

I

CHAPTER49

Cotntnercial Refrigeration Systetn Configurations

Learning Objectives Chapter Outline 49.1 Commercial Systems Configuration Overview 49.2 Multiple-Evaporator Systems 49.3 Modulating Refrigeration Systems 49.3.1 Multiple-Compressor Systems 49.3.2 Variable-Capacity, Single-Compressor Systems 49.3.3 Hot-Gas Bypass Capacity Control 49.4 Multistage Systems 49.4.1 Compound Refrigeration Systems 49.4.2 Cascade Refrigeration Systems 49.5 Secondary Loop Refrigeration Systems

Information in this chapter will enable you to: • Explain the difference between packaged and split commercial refrigeration systems. •

Understand the operation and purpose of various components used in sophisticated commercial refrigeration systems.

•

Identify applications for multiple-evaporator systems.

•

Summarize the different methods of achieving variable capacity in modulating refrigeration systems.

•

Explain the refrigeration cycle in compound and cascade systems.

•

Describe the purpose and operation of secondary loop refrigeration systems.

Chapter 49 Commercial Refrigeration System Configurations

Technical Terms cascade refrigeration system compound refrigeration system high-stage compressor intercooler low-stage compressor modulating refrigeration system

multiple-compressor system multi pie-evaporator system multistage system packaged systems secondary loop refrigeration system split system

•

Head pressure control valves operate to maintain a minimum head pressure during periods of low ambient temperature, when head pressure can drop too low. (Chapter 22)

•

Large commercial refrigeration systems often use parallel compressor racks or distributed systems, which have more than one compressor, to serve display cases and other cooling units. (Chapter 47)

•

Refrigeration systems with multiple evaporators require additional controls and components, such as manifold valves, evaporator pressure regulators (EPRs), check valves, different piping arrangements, and multiple refrigerant metering devices. (Chapter 22)

•

Different types of HVACR systems can be designed for multistage operation. Thermostats control the operation of components to manage heating or cooling capacity. (Chapter 36)

•

R-717 (ammonia, NH) has a low boiling point, making it ideal for low-temperature refrigeration. R-717 refrigeration systems are constructed of iron or steel, as ammonia attacks copper in the presence of moisture. (Chapter 9)

•

A chiller system uses water as its secondary refrigerant and another fluid for its primary refrigerant. A secondary refrigerant absorbs heat from the conditioned space and transfers it to a primary refrigerant to reject outside the conditioned space. (Chapter 33)

Review of Key Concepts Reviewing these concepts from earlier chapters will help you understand new concepts presented in this chapter: • A crankcase pressure regulator (CPR) is a valve with an adjustable pressure setting that prevents crankcase pressure from exceeding a preset safe value. It throttles suction pressure above its setting to maintain a safe pressure level in the compressor crankcase. (Chapter 19) •

•

•

Since high head pressure can cause problems for a compressor, a normally closed pressure switch is installed in the compressor discharge line between the compressor and condenser. If head pressure rises too high, the discharge line pressure switch opens to shut off the system. (Chapter 19) A mechanical air-conditioning or refrigeration system can modulate its cooling capacity through different means. A method called variable refrigerant flow (VRF) involves changing the amount of refrigerant pumped through the system. In single-compressor systems, this is usually done with an inverter-driven compressor and electronic expansion valve (EEV). (Chapter 32) An evaporator pressure regulator (EPR) is a pressureregulating valve that restricts the flow of refrigerant coming out of the evaporator. It maintains a set minimum pressure in the evaporator, which corresponds to a desired temperature setting. (Chapter 22)

1333

Introduction Because of the broad range of applications for commercial refrigeration, a wide variety of system configurations have been developed. Commercial refrigeration systems may be custom designed and constructed to meet the specific needs of the customer. These systems may use any of a number of methods to achieve the desired cooling capacities under the expected operating conditions.

1334

Modern Refrigeration and Air Conditioning

49.1 Commercial Systems Configuration Overview There are two basic types of commercial systems: packaged and split. Packaged systems are whole refrigeration units designed, built, and shipped by the manufacturer. They include all of the major refrigeration components, piping, and electrical wiring of a complete system. Such units arrive charged, tested, and ready for operation. Before using a packaged system, check manufacturer instructions to see if any further tasks are necessary, such as connection to a drain for condensate. Split systems are "site engineered." Components, such as the condensing unit, conditioned cabinets, tubing, and various control components, are purchased separately and assembled at the jobsite. Split systems are often custom designed for specific applications. Earlier chapters of this text showed many compression refrigeration systems that were quite simple. For real-world applications however, operating conditions and refrigeration requirements may require that additional components be installed in a system. Various pressure regulators, flow controls, and sensors make refrigeration systems efficient and safe. A small commercial refrigeration system equipped with a variety of typical commercial refrigeration components is shown in Figure 49-1. These components have been explained separately in various earlier chapters. Refer back to these chapters as referenced in the Review of Key Concepts section at the beginning of this chapter. An understanding of their purpose and operation is necessary when servicing and maintaining complex commercial refrigeration systems.

49.2 Multiple-Evaporator Systems A multiple-evaporator system is a refrigeration system with two or more evaporators connected to only one condensing unit. These systems are commonly used in commercial refrigeration applications, Figure 49-2. Liquid refrigerant flows through the thermostatic expansion valves (TXVs) to the evaporators. Each evaporator will have its own TXV. The evaporators may have identical or different evaporator temperatures. If the evaporator temperatures are identical, the system may use only low-side floats or TXVs to control the refrigerant. If the evaporators are intended to maintain different temperatures, the higher-temperature branch of the system must be equipped with an evaporator pressure regulator (EPR). Figure 49-2.

In a fixed volume, temperature and pressure both rise and fall together. If one evaporator operates at a higher temperature than another equally sized evaporator, the higher temperature evaporator must operate at a higher pressure also. Since evaporators in a multiple-evaporator system share a suction line, they would normally have the same pressure. In order for one of these evaporators to have a higher pressure, a restriction must be placed between the low-pressure suction line and the evaporator. When suction line pressure drops below the pressure setting of an evaporator pressure regulator (EPR), the EPR closes or throttles its opening. Since less refrigerant can pass out through the EPR, evaporator pressure rises. Keeping an evaporator at a higher pressure will cause evaporator temperature to rise correspondingly. In this manner, EPRs allow evaporators sharing the same suction line to operate at different temperatures. Just as the higher temperature evaporator must maintain a higher pressure than the lower temperature evaporator, the lower temperature evaporator must maintain a lower pressure than the higher temperature evaporator. During the Off cycle, refrigerant from the warmer evaporator can backflow into the colder evaporator. This could raise the temperature, which could short-cycle the system or begin thawing frozen products. To prevent the warmer evaporator's refrigerant from flowing into the cold evaporator, a check valve is installed. This will allow refrigerant to flow out of the cold evaporator but not back into it.

49.3 Modulating Refrigeration Systems Most refrigeration systems are designed to have enough cooling capacity to maintain the desired temperature under the heaviest load. To maintain this temperature, the compressor operates when cooling (heat removal) is required and shuts off as soon as the desired temperature is reached. In other words, most refrigeration systems are either on or off. This means full power cooling operation or no cooling. However, if the heat load is light, a simple on-off system may be oversized for the job. The operating cost for this oversized system is higher than for a system whose capacity more closely matches the needed load. The overcapacity system tends to cool the conditioned area too fast, resulting in short cycling. The term short cycling refers to a refrigeration system turning on and off too quickly. Short cycling causes extra wear and tear on the equipment, consumes excessive energy,

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 49 Commercial Refrigeration System Configurations

1335

Evaporator pressure regulator

L __________________

j

,---------------,

--=====:;====--

I I I

Check valve

TXV Liquid

Iinjection II valve, _ - - 1 1 - ~ 1 1 (-18°C) I L ______________ _JI

I

0°F

~:::::::::

c"'::::~~~~

Liquid line manifold

Hot-gas defrost valve

Hot-gas bypass valve

Accumulator Oil separator Sight glass

Control

.---

Hi-lo pressure control

Condenser

Filterdrier

Crankcase pressure regulator Liquid receiver Crankcase heater

Compressor

D D

High-pressure vapor •

High-pressure liquid

Low-pressure vapor

Low-pressure liquid

•

Goodheart-Willcox Publisher .

Figure 49-1. The combination of components in this commercial refrigeration system makes the unit work more efficiently. The inclusion of certain components, like various service valves, makes this system easier to service.

and prevents the system from providing proper dehumidification. A modulating refrigeration system is a system that is able to adjust its capacity to more closely match

a variable heat load. Several methods of modulating refrigeration are available. The most common methods of modulating refrigeration system capacity will be addressed in the following sections.

Copyright Goodheart-Willcox Co., Inc. 2017

1336

Modern Refrigeration and Air Conditioning

,--------------, Evaporator pressure regulator

I I

j

I

I

I

25°F (-4°C)

TXV

c::::::::::: c::::::~~~~~,_______+1

Higher pressure

1::i=========~==~=========~ ______________ _]I

,--------------, Check valve

i

evaporator

TXV

I I I I I I - Lower pressure

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _]

~~~r

Liquid line manifold

Pressure motor control

Sight glass

Condenser

i Liquid

~=====,:/ receiver -.::::=====" service

Accumulator

D •

D •

High-pressure vapor High-pressure liquid Low-pressure vapor Low-pressure liquid

valve

Liquid receiver Compressor Goodheart-Willcox Publisher

Figure 49-2. This multiple-evaporator system maintains a temperature of 0°F (-18°C) in one evaporator and 25°F (-4°C) in the other evaporator. The 0°F (-18°C) evaporator operates at a lower pressure than the 25°F (-4°C) evaporator.

49.3.1 Multiple-Compressor Systems A multiple-compressor system is a modulating refrigeration system in which two or more compressors operate in parallel. The compressors are said to operate in parallel because each compressor provides a separate path for carrying refrigerant from the evaporator to the condenser. This is similar to the way two or more resistors wired in parallel allow current to follow separate paths from one end of a voltage source to the other, Figure 49-3. Each of these parallel compressors is operated by a separate contactor or motor starter. By turning the individual compressors on and off, more or less refrigerant

can be pumped, and different operating pressures can be achieved in the evaporators. As a result, the system can operate at different cooling capacities as needed. In a multiple-compressor system, if the heat load remains constant and the temperature holds steady, a single compressor may provide adequate capacity. However, if the temperature rises to the second motor control's set point, a second compressor will start to operate along with the first. Additional compressors continue to cut in until enough cooling capacity is obtained. Figure 49-4 shows a typical refrigeration cycle for a multiple-compressor modulating refrigeration system. Notice how this installation has three compressors

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 49 Commercial Refrigeration System Configurations

1337

~

TXV

~

Evaporator

Condenser

t

~

High-pressure liquid

•

.....

~

~

t

~

t

.....~ ~

D High-pressure vapor D Low-pressure vapor •

~

Low-pressure liquid

Multiple-Compressor System

~

.....

Parallel Electrical Circuit Gaadheart-Willcax Publisher

Figure 49-3. A multiple-compressor system allows refrigerant to carry heat from the evaporator to the condenser along separate parallel paths. Each path leaves the evaporator and arrives at the condenser. Likewise, a parallel electrical circuit allows current to flow from one end of a voltage source to the other through separate parallel paths.

TXV

t Evaporator

Condenser

Liquid receiver

•

High-pressure liquid

D Low-pressure vapor

•

Low-pressure liquid Gaadheart-Willcax Publisher

Figure 49-4. Modulating refrigeration system that uses a single pressure motor control to operate up to three compressors to maintain the appropriate temperature.

Copyright Goodheart-Willcox Co., Inc. 2017

I

1338

Modern Refrigeration and Air Conditioning

but only one evaporator and one condenser. The compressors are cycled on and off based on signals from a pressure motor control connected to the suction line. The control contains a special switching device that alternates the operation of the compressors, so that each compressor runs for about the same amount of time. The modulating cycle maintains uniform temperatures and operates economically. Many conventional refrigerant controls can be used. However, thermostatic expansion valves (TXVs) are the most common type. The same evaporator is connected to all the compressors. The same condenser and liquid receiver may be used by all the compressors, or each compressor may have its own condenser and receiver.

49.3.2 Variable-Capacity, SingleCompressor Systems A second method of modulating system capacity is to vary the output from a single compressor. This can be accomplished in a number of ways, depending on the type of compressor installed in the system. Common methods of varying capacity in single compressor systems are as follows: • Change the speed of the compressor (variable speed motor) • Unload one of the cylinders (of a reciprocating compressor) • Change the effective displacement of the compressor • Bypass some of the refrigerant to the low side of the system. Detailed information about variable-capacity compressors can be found in Chapter 18, Compressors and Chapter 32, Residential Central Air-Conditioning Systems.

temperature. The result is more precise temperature and humidity control than would be attained by normally cycling the system on and off. In other systems, like the one shown in Figure 49-5B, hot gas is bypassed into the suction line. This prevents the refrigerant in the suction line from condensing as the flow of refrigerant through the evaporator slows down. In many cases where the hot gas is bypassed to the suction line, a desuperheating circuit is combined with the hot-gas bypass circuit. In these designs, a liquid injection (desuperheater) valve injects a small amount of liquid refrigerant into the bypass line as needed. The liquid evaporates in the bypass line, cooling the hot bypass gas, which in turn prevents the compressor from overheating. More information on hot-gas bypass and desuperheating is provided in Chapter 22, Refrigerant Flow Components. Pro Tip

Hot-Gas Bypass In most cases, hot-gas bypass to the evaporator inlet is preferred over bypassing to the suction line. If hot gas is bypassed to the suction line instead of the evaporator, flow rates through the evaporator and suction line can slow to the point where the refrigerant flow is unable to help distribute oil. In such cases, the system relies on gravity alone to return oil to the compressor. Any hot-gas bypass system must be carefully designed to provide adequate oil flow under all operating conditions.

Because the bypass gas produces no cooling effect, both methods of hot-gas bypass reduce the overall efficiency of the system. For this reason, a system modulated by hot-gas bypass may be more expensive to operate than systems that use other methods to modulate refrigeration effect.

49.3.3 Hot-Gas Bypass Capacity Control

49.4 Multistage Systems

The variable speed, variable displacement, and compressor unloading methods of adjusting capacity all work by changing the amount of refrigerant that passes through the compressor. The hot-gas bypass method of varying system capacity does not vary the output of the compressor. Instead, the system capacity is reduced during low-load periods by allowing some of the discharge line refrigerant vapor to bypass the condenser and metering device or evaporator. See Figure 49-5. In some systems, like the one shown in Figure 49-5A, when the heat load in the conditioned space decreases and the temperature starts to decrease, hot gas can be diverted directly into the evaporator, bypassing the metering device. The hot gas entering the evaporator creates an additional heat load. This allows the system to continue running while maintaining the desired

Some refrigeration systems must produce temperatures that are so low they can only be obtained using multiple stages of compression or refrigeration. A single compressor would simply be unable to achieve the compression ratios required. In such cases, a multistage system is used. A multistage system is any refrigeration system with more than one stage of compression. There are two general types: cascade and compound. Either type of multistage system can be used to achieve much lower temperatures than can be reached with a single-stage system.

49.4.1 Compound Refrigeration Systems A compound refrigeration system is a multistage system that has two or more compressors connected

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 49 Commercial Refrigeration System Configurations

1339

Solenoid valve

Evaporator

Temperature _ _ _ __.

Distributor

control

-+- Sight glass

t

Hot-gas bypass valve

Compressor

D •

D

High-pressure vapor High-pressure liquid Low-pressure vapor

Condenser

Liquid receiver

A

Solenoid valve

Evaporator

TXV Hot-gas bypass valve

~----1

Temperature control

t

I

Compressor

D •

D

High-pressure vapor High-pressure liquid Low-pressure vapor

Condenser

Liquid receiver

B Gaadheart-Willcax Publisher

Figure 49-5. The two types of hot-gas bypass for system capacity control. A-Hot gas from the compressor is bypassed directly to the evaporator inlet. B-Hot gas from the compressor is bypassed into the suction line.

Copyright Goodheart-Willcox Co., Inc. 2017

1340

Modern Refrigeration and Air Conditioning

in series. In a compound system, each successive compressor builds on the work performed by the previous compressor. Figure 49-6 shows the refrigeration cycle of a compound system. The compressor connected to the suction line that pumps the lower temperature and lower pressure refrigerant is the low-stage compressor. The compressor that discharges into the condenser and pumps higher temperature and higher pressure refrigerant is the high-stage compressor. As shown in Figure 49-6, the low-stage compressor discharges into the intake of the high-stage compressor. The low-stage compressor discharge line carries the vapor through an intercooler and then into the high-stage compressor. An intercooler is a heat exchanger in a compound refrigeration system that removes the superheat from the low-stage discharge vapor. The refrigerant vapor is cooled (but not condensed) between the compressors. Pro Tip

Desuperheating in Compound Refrigeration Systems The intercooler between low-stage and high-stage compressors in a compound refrigeration system is a desuperheating heat exchanger. Superheat (all sensible heat) in the refrigerant is absorbed and carried away by a coolant flowing through the intercooler. However, some systems remove superheat from the low-stage using direct liquid injection rather than an intercooler. Liquid injection desuperheating of discharge line hotgas vapor bypassed into the suction line is covered in Chapter 22, Refrigerant Flow Components. The same principles apply to liquid injection desuperheating of vapor between compressor stages in a compound refrigeration system.

The high-stage compressor discharges the refrigerant into the condenser, where it condenses into a high-pressure liquid and flows into the liquid receiver. From the liquid receiver, the liquid refrigerant flows through the liquid line to the TXV. The TXV regulates how much liquid refrigerant enters the evaporator. In the evaporator, the low-pressure liquid refrigerant absorbs heat and boils into low-pressure vapor. From the evaporator, the refrigerant vapor flows back to the low-stage compressor, and the cycle repeats. A compound system provides increased capacity, which is needed in applications where extremely low evaporator pressures (and corresponding low temperatures) are required. A compressor's efficiency is inversely proportional to its compression ratio. In other words, as the compression ratio of a compressor increases, its volumetric efficiency decreases. A single compressor

would have difficulty producing the pressure differences required to achieve extremely low evaporator temperatures. However, a compound system divides the required work between two compressors. For example, imagine a two-stage compound system in which the low-stage compressor has a 4.5:1 compression ratio and the high-stage compressor has a 4:1 compression ratio. The vapor enters the low-stage compressor at 15 psia and leaves the compressor at a pressure of 67.5 psia (15 psia x 4.5 = 67.5 psia). The refrigerant is cooled to remove superheat and then enters the highstage compressor. The vapor leaves the high-stage compressor at a pressure of 270 psia (67.5 psia x 4 = 270 psia). The net result is a compression ratio of 18:1 (4 x 4.5 = 18, or 270 psi divided by 15 psi equals 18). If a single compressor was capable of achieving this compression ratio, it would operate at extremely low efficiency. However, since the two compressors in the compound system have relatively low compression ratios, they work together to achieve the 18:1 compression with relatively high efficiency. Another problem of using a single compressor to reach very high compression ratios is that the discharge temperature would be excessively high. The discharge temperature may be so high that it would cause the oil in the compressor to vaporize, ruining its lubricating properties. When two compressors are used to achieve the same very high compression ratio, the refrigerant can be cooled between the two stages. In a typical compound system, a single temperature motor control operates all motors. A thermostatic expansion valve controls the flow of liquid refrigerant into the evaporator. Refrigerants commonly used include R-22, R-404A, and R-507. The pressures do not balance during the Off cycle, and the system remains pressurized, with a low side and a high side isolated from each other. Therefore, the compressor motors used in compound systems must be capable of starting under load. Compound installations usually operate under heavy service requirements. Condensers and refrigerant must be kept clean. Compressor valves must be kept in good condition. Thinking Green

Intermediate Pressure in a Compound Compression System The compressors in a compound compression system maintain three different pressures: the low-stage intake pressure, the intermediate pressure (low-stage discharge and high-stage intake), and the high-stage discharge pressure. To maximize the compressors' energy efficiency, the intermediate pressure should be set so that the percentage of pressure increase at compressor outlet is the same for both compressors.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 49 Commercial Refrigeration System Configurations

1341

TXV

Evaporator

Temperature motor control

Temperature sensor

- ----- ----------------.

Cooling water outlet

....__

Oil separator

Cooling water inlet

Oil return -line

11+-

Oil return line

Low-stage compressor

Cooling water inlet

Cooling water outlet

Liquid receiver

ID High-pressure vapor

•

High-pressure liquid

D Low-pressure vapor

•

Low-pressure liquid

I

Goodheart-Willcox Publisher

Figure 49-6. These two compressors are connected in a series, making this a compound refrigeration system.

Copyright Goodheart-Willcox Co., Inc. 2017

I

1342

Modern Refrigeration and Air Conditioning

49.4.2 Cascade Refrigeration Systems Cascade systems are often used in industrial processes where objects must be cooled to temperatures below -50°F (-46°C). A cascade refrigeration system is a multistage system that consists of two or more separate refrigeration subsystems with separate, isolated refrigerant circuits that work together to multiply cooling effect. The evaporator of the first-stage (high-stage) subsystem cools the condenser of the second-stage (low-stage) subsystem. The refrigerant circuit in each subsystem is completely separate, so the refrigerant in one subsystem never mixes with the refrigerant in the other subsystem. Although the refrigerant lines of the subsystems are not interconnected, the subsystems are said to be in series because the flow of heat has only one path to follow on its way from the conditioned space to the place where it is rejected. Like compound refrigeration systems, a cascade refrigeration system is divided into low and high stages based on pressure and temperature.

Figure 49-7 shows a cascade refrigeration system. Note that the two separate refrigeration subsystems interface at the cascade heat exchanger. The cascade heat exchanger contains the low-stage condenser and the high-stage evaporator. As the low-pressure liquid refrigerant in the high-stage evaporator vaporizes, it absorbs heat from the refrigerant in the low-stage condenser. This causes the low-stage refrigerant to condense. Pro Tip

Moisture in Cascade Systems Since cascade systems operate at very low temperatures, the refrigerant must be very dry. Any moisture in a cascade system would freeze at the needle seat of the TXV, stopping refrigerant flow.

The biggest advantage of cascade systems is that they have two separate refrigerant circuits, each charged with a different refrigerant. The refrigerant used in

Low-stage

High-stage evaporator

High-stage condenser

Low-stage evaporator

Liquid

exchanger Motor 1------' control

-+-

Oil separator

--~, ____ 1

compressor

ID High-pressure vapor

•

High-pressure liquid

D Low-pressure vapor

•

High-stage compressor

Low-pressure liquid Goodheart-Willcox Publisher

Figure 49-7. A cascade refrigeration system combines two complete refrigeration subsystems in order to achieve temperatures below -50°F (-46°C).

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 49 Commercial Refrigeration System Configurations the high-stage has a higher condensing and evaporating temperature. The refrigerant in the low-stage has a lower condensing and evaporating temperature. The refrigerant in the low-stage subsystem is called the secondary refrigerant. This refrigerant provides cooling to the conditioned area. The refrigerant in the high-stage subsystem is called the primary refrigerant. This refrigerant cools the secondary refrigerant and rejects the heat from the system. Having two refrigerants allows the use of the best refrigerant for each application. As a result, the compressors in the two subsystems each operate at an efficient compression ratio.

Pro Tip Cascade System Refrigerants In many cascade systems, CO 2 (R-744) is used as the secondary refrigerant and ammonia (R-717) is used as the primary refrigerant. Other popular secondary refrigerants include R-23 and R-508h. Other popular primary refrigerants include R-134a, R-22, and R-507.

Both the low-stage subsystem and the high-stage subsystem of a cascade system operate at the same time. The low-pressure liquid refrigerant in the high-stage evaporator cools the high-pressure vapor in the low-stage condenser. The evaporator of the low-stage subsystem supplies the cooling effect to the conditioned space.

Pro Tip Cascade Heat Exchangers Although there are different designs for cascade heat exchangers, the most common combine the functions of a shell-and-tube condenser and a flooded evaporator.

Each subsystem in a cascade system has a thermostatic expansion valve (TXV) for refrigerant control. TXVs close during the Off cycle; therefore, the pressures on the high and low sides of the subsystems do not balance. This means that the compressor motors used on cascade systems must be capable of starting under load. A temperature-sensing bulb on the lowstage evaporator provides input to the motor control, which is used to control both compressor motors.

Pro Tip Off Cycle Balancing When replacing a TXV in a cascade system, be certain to look at all its specifications. If the subsystems are not to balance during the Off cycle, ensure that the replacement TXV does not have a bleed port or means of balancing pressures. These features may be internal and difficult or impossible to tell by looking at the valve. Thoroughly review system requirements and product specifications.

1343

The low-stage subsystem must use special refrigerant oil that is wax-free, moisture-free, and able to flow at extremely low temperatures. Oil separators should be installed in the discharge lines on both subsystems. This will help keep adequate oil in the compressors. Caution

Oil Return Piping Practices The oil that is carried by the refrigerant must be returned to the compressor. Since the viscosity of oil increases inside an evaporator, special attention should be given to proper suction piping installation. The piping design should allow oil to gradually flow back to the compressor oil sump due to gravity.

49.5 Secondary Loop Refrigeration Systems An emerging trend in commercial refrigeration installations and retrofits is secondary loop refrigeration systems. These are comparable to cascade refrigeration systems; however, the secondary refrigerant is generally a fluid that does not change phases during normal operation. A secondary loop refrigeration system is a commercial refrigeration system in which a secondary loop circulates a nonphase-changing fluid for absorbing heat from a conditioned space and transfers that heat through a heat exchanger to a phasechanging refrigerant in a direct expansion refrigeration circuit, Figure 49-8. Secondary loops stretch from a heat exchanger in the mechanical room throughout the entire building to each of the different conditioned spaces, such as display cases and walk-in coolers. From the mechanical room, the secondary refrigerant is pumped and distributed to each cooling coil where it absorbs heat from the conditioned spaces. When the secondary refrigerant flows back to the mechanical room, it passes through a heat exchanger that functions as the primary loop's evaporator. Heat absorbed into the secondary loop from the conditioned spaces is absorbed into the primary loop in the heat exchanger. A compressor in the primary loop pumps the primary refrigerant to a condenser that displaces the heat. A traditional direct expansion commercial refrig- . eration system has only a single refrigerant circuit with a large charge of expensive refrigerant. A secondary loop refrigeration system uses an expansive, buildingwide secondary loop. This means that the primary loop (a direct expansion circuit) can be confined to a smaller area, such as the mechanical room or roof. Reducing the size of the primary loop greatly reduces the amount of direct expansion (phase changing)

Copyright Goodheart-Willcox Co., Inc. 2017

1344

Modern Refrigeration and Air Conditioning

Oil reservoir

Plate heat exchanger

Wiring and controls panel

Compressors Zero Zone, Inc.

Figure 49-8. A secondary loop refrigeration system is often paired with a parallel compressor rack to provide cooling for the secondary refrigerant. This system has multiple compressors, secondary loop pumps, heat exchangers, and numerous control devices.

refrigerant necessary for a secondary loop refrigeration system. Since secondary loops use a refrigerant that costs much less than a primary loop direct expansion refrigerant, the total refrigerant cost for the entire system is greatly reduced. This lower cost for refrigerant is a great incentive for using a secondary loop refrigeration system. While primary loops are built with copper ACR tubing, secondary loops are often built using plastic piping, such as ABS. This results in far fewer copper joints, which reduces the chance of a direct expansiontype refrigerant leak. Also, the cost of plastic pipe is less than copper tubing. However, to maximize efficiency by minimizing the absorption of heat from an unconditioned space, secondary loop piping should be insulated, Figure 49-9. Secondary loop pipes operate under lower pressure than copper pipes. The secondary loop usually circulates a nonphase-changing refrigerant, such as a glycol solution, that absorbs the heat from the conditioned spaces. The secondary refrigerant does not evaporate, but it still absorbs sufficient heat for refrigeration. Instead of an expansion valve or traditional

Zero Zone, Inc.

Figure 49-9. The piping and components circulating the secondary refrigerant are heavily insulated to maximize system efficiency.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 49 Commercial Refrigeration System Configurations refrigerant control, secondary loops use balance valves that control the flow of the secondary refrigerant through the flooded evaporators. Rather than compressors, secondary loops use circulating pumps to move the secondary refrigerant, Figure 49-10. In a secondary loop refrigeration system, the temperature of a conditioned space is controlled by the flow of the secondary refrigerant. The flow of secondary refrigerant and the temperature of a conditioned space have an inverse relationship. As one increases, the other decreases. The greater the flow of secondary refrigerant, the greater the heat absorption and the lower the conditioned space temperature. Reducing the flow reduces heat absorption and allows conditioned space temperature to increase, Figure 49-11. The secondary loop has a few additional components not normally found in direct expansion systems. One of these is an air separator, which traps air that

1345

is circulating with the secondary refrigerant. Another secondary loop component is an expansion tank, which is a cylinder containing a pressure responsive bladder that is used to account for changes in secondary loop pressure, Figure 49-12. Secondary loop refrigerant pressure can change due to thermal expansion, a small leak, or during service. Thermal expansion occurs when a secondary refrigerant warms during the Off cycle or long shutdowns when operating temperature may rise to ambient temperature. Secondary refrigerants are often dyed so that leak detection is less difficult. Thinking Green

GreenChill Certification The EPA's GreenChill Store Certification Program for Food Retailers is a voluntary program that recognizes stores that use environmentally friendly refrigeration systems. The refrigerant emission rates for GreenChill certified stores are estimated to be only 50% of the industry average. To participate in the program, the store must submit information about its heat load, refrigeration system, and refrigerant usage and loss. The EPA will review the application and certify the store if it is GreenChill compliant. The certification lasts for one year, and stores can refer to the certification as part of their marketing strategy to environmentally conscious customers.

Fill tank

Air separator

Zero lane, Inc.

Figure 49-10. These two pumps circulate a nonphasechanging refrigerant throughout the building to individual refrigerated display cases.

Secondary Loop Refrigerant and Balance Valve Relationship Balance Valve Position

Open more Close more

Refrigerant Flow

Heat Absorption

Conditioned Temperature

• •

• •

• •

Expansion tank

Gaadheart-Willcax Publisher

Figure 49-11. This chart shows the relationship of balancing valve position, secondary refrigerant flow, and conditioned space temperature.

Zero lane, Inc.

Figure 49-12. These secondary loop components have been incorporated into the refrigeration system on the parallel compressor rack in the building's mechanical room.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter Review •

Summary •

•

•

•

•

•

•

1346

The two basic types of commercial systems are packaged systems and split systems. Packaged systems are refrigeration units designed, built, and shipped by the manufacturer. Split systems are assembled at the jobsite. Large commercial refrigeration systems use various pressure regulators, flow controls, and sensors to achieve accurate temperature control and efficient operation. Understanding the operation and purpose of the many components used in a commercial refrigeration system is necessary to service and maintain these complex systems. Commercial refrigeration systems with more than one evaporator are called multipleevaporator systems. These may be designed so one evaporator maintains a lower temperature than the other evaporator. A higher temperature evaporator must be equipped with an evaporator pressure regulator (EPR). Modulating refrigeration systems are designed to handle varying heat loads. Instead of merely cycling on or off, a modulating refrigeration system operates at different capacities based on the heat load. One method of modulating refrigeration is to use multiple compressors operating in parallel. The compressors are cycled on and off in combination as needed to match the load. Another method of modulating refrigeration is to use a single, variable capacity compressor. A third method of modulating refrigeration is to use hot-gas bypass to reduce the capacity of the system as needed to match the heat load. Multistage refrigeration systems have more than one stage of compression and are used to reach extremely cold temperatures. The two types of multistage refrigeration are cascade and compound. A compound refrigeration system uses two compressors connected in series. The lowstage compressor discharges into the inlet of the high-stage compressor. An intercooler between stages reduces high superheat. This arrangement achieves a higher compression ratio than a single compressor with fewer complications and can produce extremely low temperatures.

•

•

A cascade refrigeration system consists of two or more refrigeration subsystems with separate, isolated refrigerant circuits connected in series. They are joined in a heat exchanger in which the high-stage evaporator cools the low-stage condenser. A secondary loop refrigeration system has two refrigerant loops. The secondary loop absorbs heat from a conditioned space into a nonphasechanging fluid. The primary loop circulates a phase-changing refrigerant that absorbs heat from the secondary loop. The benefits of a secondary loop refrigeration system include a reduced direct expansion refrigerant charge, less chance of refrigerant leaks, and a lower cost of refrigerant and piping.

Review Questions Answer the following questions using the information in this chapter. 1. A commercial refrigeration system that is

designed, built, charged, tested, and shipped ready for operation by the manufacturer is referred to as a _ _ system. A. bundled B. compound C. packaged D. split 2. What is the benefit of using an EPR on one of two evaporators that share a suction line and a liquid line? A. An EPR eliminates the need for pressure and temperature controls. B. Each evaporator can maintain a different temperature. C. The EPR will prevent backflow during the Off cycle. D. The two evaporators can share a single TXV.

3. A multiple-compressor system used for modulating refrigeration _ _. A. actually only includes one compressor B. can only operate all or none of the compressors at once C. uses two or more compressors connected in series D. uses two or more compressors connected in parallel

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 49 Commercial Refrigeration System Configurations 4. Unloading one or more compressor cylinders in a variable-capacity, single-compressor system can only be done in a _ _ compres.5or. A. centrifugal B. reciprocating C. screw D. scroll 5. A primary method of modulating a system's capacity using a single compressor is by A. B. C. D.

closing the liquid line solenoid valve changing the speed of the compressor restricting flow into the condenser reversing the flow of refrigerant

6. The hot-gas bypass method of capacity control is mainly used for _ _. A. high-load periods B. high-side pressure relief C. low-load periods D. Off cycle system balancing

7. Which of the following statements about compound refrigeration systems is not true? A. Compound systems can achieve high compression ratios more efficiently than single compressor systems. B. A compound system may have a single evaporator. C. Multiple compressors are used in parallel. D. All of the above.

1347

9. Which of the following statements regarding cascade refrigeration systems is not true? A. The high-stage and low-stage subsystems typically use different refrigerants. B. The high-stage evaporator cools the lowstage condenser. C. The high-stage subsystem is normally off and only switched on if the heat load increases. D. A single motor control controls both the high-stage and low-stage compressors. 10. A commercial refrigeration system that absorbs heat from a conditioned space into a nonphase-changing fluid and transfers that heat through a heat exchanger into a separate circuit that circulates a phase-changing refrigerant is a(n) _ _ system. A. compound refrigeration B. multiple-compressor C. multiple-evaporator D. secondary loop refrigeration

8. Which of the following best describes the purpose of an intercooler in a compound refrigeration system? A. The intercooler performs the same function as the condenser (subcooling) on a single-compressor refrigeration system. B. The intercooler performs the same function as the evaporator on a singlecompressor refrigeration system. C. The intercooler removes superheat from the suction gas before it enters the lowstage compressor. D. The intercooler removes superheat from the compressed gas as it flows from the low-stage compressor into the high-stage compressor.

Copyright Goodheart-Willcox Co., Inc. 2017

I

CHAPTER so

Understanding Heat Loads and Systetn Ther1nodyna1nics

Learning Objectives Information in this chapter will enable you to: • Understand the relationship between total heat load, service heat load, and heat leakage load. •

Compute a system's heat leakage load.

•

Calculate a system's service heat load manually and by using manufacturer's tables.

•

Use heat leakage load, service heat load, and all applicable miscellaneous heat loads to calculate a system's total heat load.

•

Compute the total heat load for a water cooler.

•

Summarize the thermodynamic principles at work in the basic refrigeration cycle.

•

Identify the various lines on a pressure-enthalpy diagram.

•

Interpret the graphs of different refrigeration cycles on pressure-enthalpy diagrams.

Chapter Outline 50.1 Heat Loads 50.1.1 Heat Leakage (Thermal Conduction) Load 50.1.2 Service Heat Load 50.1.3 Calculating the Total Heat Load 50.1.4 Heat Loads for Water Coolers 50.2 Thermodynamics of the Basic Refrigeration Cycle 50.2.1 Reading a Pressure-Enthalpy Diagram 50.2.2 Practical Pressure-Enthalpy Cycles

Chapter 50 Understanding Heat Loads and System Thermodynamics

Technical Terms adiabatic expansion effective latent heat heat leakage load miscellaneous heat loads product heat load

refrigerant quality respiration heat saturated liquid service heat load total heat load

Review of Key Concepts Reviewing these concepts from earlier chapters will help you understand new concepts presented in this chapter: • The amount of heat energy required to raise the temperature of one pound of a substance by 1° Fahrenheit or one kilogram of a substance by 1 Kelvin is the substance's specific heat capacity. (Chapter 4) •

The specific heat capacity of a substance is the amount of heat added or released to change the temperature of one pound of a substance by 1°F. In the SI system, specific heat capacity is the amount of heat needed to change one kilogram of a substance by one degree Kelvin. (Chapter 4)

•

Enthalpy is the total heat energy a substance contains, measured from an accepted reference temperature. (Chapter 4)

•

Sensible heat is the heat energy absorbed or released to change the temperature of a substance. Latent heat has no effect on the temperature of a substance, because it is heat absorbed or released as a substance changes state. For example, the latent heat added to a liquid to change it into a gas or the latent heat removed from a gas to change it into a liquid is the latent heat of vaporization. (Chapter 4)

•

Adiabatic compression is the compressing of a gas without gaining heat from or losing heat to its surroundings. This occurs in the cylinder of reciprocating compressors. (Chapter 5)

•

Boyle's law states that if temperature is held constant, volume varies inversely with pressure. Charles' law states that if pressure is held constant, volume varies in direct proportion with temperature. Gay-Lussac's law states that if volume is held constant, pressure and temperature vary directly. The combined gas law states that the ratio among a gas's volume, temperature, and pressure remains constant. (Chapter 5)

1349

•

A material's K-value is a measure of its thermal conductivity. It indicates how much heat will transfer through one square foot of the material one inch thick in one hour when there is a 1°F temperature difference between the two sides of the material. (Chapter 37)

•

A material's thermal conductance, or C-value, is similar to its K-value, but it is not dependent on the thickness of the material. (Chapter 37)

•

A material's R-value is a measure of its thermal resistance. It is the reciprocal of the material's K-value. The higher the material's R-value, the slower heat will transfer through the material. (Chapter 37)

•

The U-value of a component is a measure of its thermal transmittance. It is similar to a component's C-value, but takes into account the insulating effect of boundary air films. (Chapter 37)

Introduction The two basic types of commercial refrigeration systems are packaged (unitary) and split (site-engineered) systems. Packaged systems, such as ice machines and dispensing freezers, are delivered from the manufacturer with the refrigerated enclosure and all components preassembled. The manufacturer has chosen the correct evaporator and condensing unit and has designed the cabinet to handle a specific refrigeration job. Split or "site engineered" refrigeration systems are units such as walk-in coolers or multiple-evaporator display cases. These units require that the service technician select the components (such as the evaporator and condensing unit) that will be used for the specific application. To properly design and construct a split refrigeration system, the technician must understand the heat loads that the system will be required to remove. The components must be properly sized to match the system to its intended use. When selecting components, the technician must choose a condensing unit and evaporator with enough capacity to remove the heat load while still providing adequate defrosting and humidity control. Also, because refrigeration systems are not designed to operate continuously, the system must have sufficient capacity to handle the maximum heat load within its normal operating cycle.

1350

Modern Refrigeration and Air Conditioning

This chapter will explain heat loads and the thermodynamics of system operation. Chapter 51, Commercial Refrigeration Component Selection will explain how to use that information to properly size a system. The heat load calculations presented in this chapter are given in US Customary units (such as pounds, Btu, and feet). SI conversion factors are explained in the Appendix.

50.1 Heat Loads Simply put, heat load is the amount of heat that must be removed from a conditioned space over a set period of time in order to maintain the desired temperature in that space. To determine the heat load, a technician must calculate the total amount of heat that must be removed every 24 hours in order to hold the temperature at its desired level. The heat load is measured in Btu (British thermal units). A Btu is the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit. When determining the total heat load, heat leakage into the cabinet and the service heat load must be considered. If the inside of the cabinet is cooler than the air outside of the cabinet, heat from the surrounding air is transferred through the materials of the cabinet to the space inside. The amount of heat that is transferred into the cabinet is referred to as the heat leakage load. The amount of exposed surface, the thickness and type of insulation, and the temperature difference between the inside and outside of the cabinet all affect the amount of heat leakage that occurs. The service heat load accounts for the temperature of articles put into the cabinet, their specific heat, any heat they generate, and any latent heat they contain. It also accounts for any additional heat brought into the cabinet due to operation. This includes air changes and the heat generated inside the cabinet by fans, lights, and other electrical devices. The total heat load is the sum of the loads resulting from heat leakage, air changes, stored products, and miscellaneous heat sources. After determining the total heat load, a technician can determine the size of condensing unit and evaporator needed for the system.

50.1.1 Heat Leakage (Thermal

Conduction) Load The heat leakage load is the total amount of heat that leaks through the walls, windows, ceiling, and floor of the cabinet per unit of time (usually 24 hours). Research organizations, manufacturers, and refrigeration associations

have determined the rate at which heat leaks through various materials. Charts and tables based on these calculations are used by engineers and technicians. The following are five variables that affect heat leakage: 1. Time. Heat leakage is a process that occurs over time. Therefore, the total amount of heat leakage increases with time. The standard time period that is used to measure heat leakage rates in refrigeration applications is 24 hours. A one-hour period is used to measure heat leakage rates in air conditioning applications. 2. Temperature difference. The difference in temperature between the inside and outside of the cabinet is directly proportional to the rate of heat leakage. The greater the temperature difference, the more heat will transfer through the wall in a given period of time. The room temperature used to calculate heat leakage rates is the average summer temperature. In the United States, most locations have summer design temperatures ranging between 90°F and 105°F (32°C and 40°C). See Figure 50-1. This value can be reduced to 75°F or 80°F (25°C or 27°C) if the cabinet is located in an air-conditioned space. 3. Type of material. The rate at which heat leaks through a wall also depends on the materials the wall is made of. Expanded polystyrene (foam), for instance, will insulate approximately six times better than wood. Some insulating materials, however, are more costly than others. 4. Thickness of materials. The thicker the material the less heat will flow through it in a given period of time. For a given length of time, nearly twice as much heat will leak through a wall with 1" (2.5 cm) insulation than through a wall having 2" (5 cm) of the same insulation. 5. External area of cabinet. A larger surface area allows greater heat flow. The common unit used to measure surface area in heat flow calculations is the square foot. The surface area is always calculated using the outside dimensions of the cabinet.

K-Values, C-Values, A-Values, and U-Values As mentioned earlier, the type and thickness of the materials used to construct the cabinet are key factors in determining the rate at which heat can leak through it. In order to compare the materials, a standardized method of measuring their thermal characteristics must be used. In refrigeration work, K-values, C-values, R-values, and U-values are the measurements used to describe thermal characteristics of construction materials. However, each of these values measures thermal characteristics in

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 50 Understanding Heat Loads and System Thermodynamics

1351

Summer Design Temperatures State

Design Dry Bulb

OF

oc

Alabama

95

29

Alaska

74

Arizona

105

23 41

98

37

Arkansas California

State

Design Dry Bulb

OF

oc

Nevada

95

35

New Hampshire

32

New Jersey

90 92

New Mexico

95

35

New York

90

32

33

lower

86

30

North Carolina

95

35

middle

94

34

North Dakota

93

34

upper

28

Ohio Oklahoma

Connecticut

88

33 31

90 102

32

Colorado

83 92

Delaware

93

34

Dist. of Col.

94

34

96 93

36 34

Hawaii

95 87

35 31

Idaho

94

34

Florida upper lower

Georgia

Illinois upper

95 97

39 32

Pennsylvania

90 92

33

Rhode Island

87

31

South Carolina

95

35

South Dakota

95 96

35 36

Texas

101

38

Utah

95

35

Vermont

87

31

Virginia

95

35

Oregon

Tennessee

32

35

Washington West Virginia

90 94

Wisconsin

90

34 32

Wyoming

90

32

Indiana

95

36 35

Iowa

95

35

upper

97

36

lower

100

38

Alberta

78

26

35 37

British Columbia

75

Louisiana

95 98

Manitoba

Maine

88

31

New Brunswick

80 82

24 27

Maryland

94

34

Newfoundland

72

22

Massachusetts

90

32

Northwest Territory

65

18

Michigan

88

31

Nova Scotia

78

26

Minnesota

32

Ontario

79

26

Mississippi

90 97

77

25

98

36 37

Prince Edward Island

Missouri

Quebec

74

23

Montana

88

31

Saskatchewan

83

28

Nebraska

97

36

Yukon Territory

71

lower

Kansas

Kentucky

Canadian Provinces and Territories

Design Dry Bulb

oF

oc

28

22

Gaadheart-Willcax Publisher

Figure 50-1. Table listing summer design temperatures. If the space containing the refrigeration system is unconditioned or outdoors, these values can be used as ambient temperatures for calculating heat loads. Copyright Goodheart-Willcox Co., Inc. 2017

I '

1352

Modern Refrigeration and Air Conditioning

a slightly different way. Each of these values was thoroughly explained in Chapter 37, Heating and Cooling Loads. K-Values (Thermal Conductivity) and C-Values (Thermal Conductance) A material's K-value represents that material's thermal conductivity. It is a measure of how much heat can pass through one square foot of the material, one inch thick, in an hour when there is a temperature difference of 1°P (0.56°C) between one side of the material and the other. A material's K-value does not account for any boundary air film or liquid film on either side of the material. As you might expect, the symbol used to represent thermal conductivity (K-value) in formulas and equations is K. Remember that a material's K-value represents the rate at which heat is transferred through a one-inch thickness of the material. If the material is thicker than l': it will take more time for the heat to travel through it. If the material is thinner than 1", it will take less time for the heat to pass through it. Therefore, if the material is not exactly 1" thick, the material's K-value must be divided by the actual thickness of the material to accurately calculate how long it takes for heat to pass through it. The resulting value is known as the material's thermal conductance, or C-value, and is measured in units of Btu/hr-ft2- F. For materials that are exactly 1" thick, the K-values and the C-values are equal.

Dividing a value by a fraction is awkward. To change the fraction into a decimal value, divide its two values. One divided by two (1 + 2) equals 0.5. K

C=-

.5,,

To find the heat leakage of walls made of multiple materials (composite walls), the first step is to find the thermal conductivity of each material used in the construction. The thermal conductivity of some common materials is shown in the Appendix. R-Values (Thermal Resistance) When calculating the heat leakage rates for walls (or other components) made from multiple materials, technicians base their calculations on the thermal resistance of the materials rather than their thermal conductivity (K-value) or thermal conductance (C-value). A material's or component's resistance to heat flow is commonly known as its R-value and is represented by the letter R in formulas and equations. Resistance to heat flow is the inverse of thermal conductance. It is measured in units of hr-ft2- P/Btu. 0

Formula: Thermal resistance (R)

0

Thermal conductance (C)

K

=-

X

where

= thermal conductivity (in Btu-in/hr-ft2- P) X = thickness of the material (in inches) 0

K

Example: If the insulation of a cabinet box is 4", what is the thermal conductance? Solution: K

C=X K

4"

Example: If the insulation of a cabinet box is 1/2': what is the thermal conductance? K

C=X K

1/2"

=

1 C

Total resistance to heat flow in a composite wall (or other component) is the sum of the thermal resistance of each material in the wall. The example below shows the equation for a wall constructed of two materials, but the equation can be expanded for any number of materials: where

= total thermal resistance R1 = thermal resistance of material 1 R 2 = thermal resistance of material 2

~

Example: A custom-built cabinet wall is constructed of 1" thick particle board, 1/2" Celotex, and 1/4" polyurethane insulation as shown in Figure 50-2. The K-value for the particle board is 0.94, the K-value for Celotex is 0.31, and the K-value for polyurethane is 0.16. Determine the total thermal resistance (~) in the composite wall. Solution: The K-values provided indicate how quickly heat would flow through a 1" thickness of the materials, given a 1°P temperature difference from one side of the material to the other. However, the Celotex is only 1/2" thick and the polyurethane is only 1/4" thick. Each material's K-value must be divided by its thicknesses to determine the C-value (thermal conductance) of the material:

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 50 Understanding Heat Loads and System Thermodynamics

1/2" Celotex 1/4" polyurethane 1" particle board

Goodheart-Willcox Publisher

Figure 50-2. Cross section of a custom-built cooler wall.

C1

= K1 X

o.~4 1

= 0.94 Btu/hr-ft2-°F C z

= K2

x2

0.31

-1/2

= 0.62 Btu/ hr-ft2-°F C 3

= 0.64 Btu/hr-ft2-°F The reciprocals of the individual C-values provide the thermal resistance for each material: 1 R =cl

1 0.94 = 1.06 hr.ft2-°F/Btu 1

R =c2

1 0.62

= 1.61 hr.ft2-°F/Btu

=2_

R 3

U-Values (Thermal Transmittance) In the above example, the R-value was calculated for a cabinet constructed of polyurethane, particle board, and Celotex. The value calculated above represents the rate at which heat leaks through the materials in the cabinet wall. But, the materials used to construct the door are not the only factors to consider when determining how quickly heat leaks through the wall. There are also thin films of stagnate air that cling to the outside and inside surfaces of the wall. These air films are attracted to the wall surfaces and are known as inside and outside boundary air films. Because of the attraction, the air films do not move as quickly as the surrounding air. This stagnation causes the boundary air films to act like thin layers of insulation that reduce the rate at which heat is transmitted through the wall. The measurement that takes these factors into account is called thermal transmittance or U-value. A material's thermal transmittance is equal to the inverse of the sum of all thermal resistances in the material, including the thermal resistance of the boundary air films:

x3

0.16

2

The thermal resistances are then added together to calculate the total resistance in the wall: ~ = R1 + R2 + R3 = 1.06 + 1.61 + 1.56 = 4.23 hr.ft2-°F/Btu

Formula:

= K3 -1/4

1

1353

where R 01 = thermal resistance of outside boundary air film R1 = thermal resistance of material 1 Ri1 = thermal resistance of inside boundary air film Example: The wall in Figure 50-3 includes an outside film of air with a thermal conductance (C-value) of 6.0 Btu/hr.ft2-°F and an inside film of air with a thermal conductance of 1.65 Btu/hr.ft2-°F. The K-value for the particle board is 0.94, the K-value for Celotex is 0.31, and the K-value for polyurethane is 0.16. What is the U-value of the wall? Solution: Begin by determining the thermal resistances of the materials and boundary air films: Resistance of outside boundary air film: 1 Roi=c of

c3

1 6.0 Btu/ hr.ft2-°F

1 0.64 = 1.56 hr.ft2-°F/Btu

= 0.17 hr.ft2-°F /Btu Copyright Goodheart-Willcox Co., Inc. 2017

I

1354

Modern Refrigeration and Air Conditioning

Resistance of 1/4" polyurethane: C =K1 1

~ 1/2" Celotex

1/4" polyurethane

x1

~

0.16 0.25"

- - ~ - 1" particle board

= 0.64 Btu/ hr.ft2-°F

Outside air film

~

1 R =1

~ Inside air film

cl

1 0.64 Btu/ hr.ft2-°F

Goodheart-Willcox Publisher

Figure 50-3. Cross section of a cooler wall, including inside and outside boundary air films.

= 1.56 hr.ft2-°F /Btu Resistance of 1" particle board:

~

C =K2 2

x2

0.94 1"

The U-value for the composite wall is the reciprocal of the total thermal resistance of the wall:

= 0.94 Btu/hr-ft2-°F R = 2

= Roi + R1 + R2 + R3 + Ri1 = 0.17 + 1.56 + 1.06 + 1.61 + 0.61 = 5.01 hr.ft2-°F/Btu

u =2_

1

T

~

1 5.01 hr.ft2-°F /Btu

c2

1 0.94 Btu/ hr.ft2-°F

= 0.20 Btu/ hr-ft2-°F

= 1.06 hr-ft2-°F/Btu Calculating Area Resistance of 1/2" Celotex: C =K3 3

x 3

0.31 0.5''

= 0.62 Btu/ hr.ft2-°F

=2_

R 3

c3

1 0.62 Btu/ hr.ft2-°F

= 1.61 hr.ft2-°F /Btu Resistance of inside boundary air film: 1

R=if C if

1 1.65 Btu/ hr.ft2-°F

The area of a cabinet is measured from the outside. There are six surfaces: four walls, the ceiling, and the floor, as shown in Figure 50-4. Usually the floor and ceiling have the same area, as do the areas of walls opposite each other. To determine the total outside area of a typical rectangular box cabinet: 1. Multiply the width of the cabinet by the length of the cabinet. Then, multiply the result by two. This is the combined area of the floor and ceiling of the cabinet. 2. Multiply the width of the cabinet by the height of the cabinet. Then, multiply the result by two. This is combined area of the ends of the cabinet. 3. Multiply the length of the cabinet by the height of the cabinet. Then, multiply the result by two. This is the combined area of the sides of the cabinet. 4. Add these three values to determine the total external area of the cabinet. Pro Tip

= 0.61 hr.ft2-°F/Btu

Calculating Cabinet Area

Next, the resistances of the individual materials are added together to calculate the total resistance for the wall:

Most companies compute total area based on the outside of the cabinet. The exterior is easier to measure and the results are on the safe side.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 50 Understanding Heat Loads and System Thermodynamics

1355

Calculating Heat Leakage Load After computing the area of the cabinet's exterior, subtract the window area to obtain the area of the insulated surface. Window areas are calculated from the measurements of the outside edges of the window frame. Since the heat transfer rate through a window is different than the heat transfer rate through the insulated walls, each type of surface must be considered separately. To find the area of the cabinet's insulated walls, subtract the window area from the total area of the cabinet's exterior. Technicians can use a table like the one shown in Figure 50-5 to find the amount of heat leakage per 24-hour period through a square foot of a given material at a given temperature difference. Consider a wall made of steel paneling on both sides with a 4" core of extruded polystyrene insulation in between. The table in Figure 50-5 reveals that, at a temperature difference of 60°F (95°F - 35°F), 49.8 Btu will leak through every square foot during a 24-hour period. The glass leakage rates listed at the bottom of the table show how much heat will leak through one square foot of glass in a 24-hour period for a given temperature difference. If the cabinet has double-pane glass, 660 Btu will leak through each square foot of glass when the temperature difference is 60°F. This rate is then multiplied by the surface area of the glass to determine the total amount of heat leakage through the glass. Next, the heat leakage through the insulated portion of the cabinet is added to the heat leakage through the glass. The sum of the two equals the total heat leakage into the cabinet. Example: The walk-in cooler shown in Figure 50-6 is 10' x 9' x 8' high. It has two double-pane glass windows measuring 1 1/2' x 2'. The box is kept at 35°F in a room with a summer design temperature of 95°F. The cabinet's wall construction consists of 4" extruded polystyrene with metal on each side. The windows are double-pane construction. The temperature difference is 95°F - 35°F = 60°F.

Solution: Total area:

10' x 9' x 2 = 180 ft 2 (ceiling and floor) 9' x 8' x 2 = 144 ft2 (ends) 10' x 8' x 2 = 160 ft2 (sides) Total area = 484 ft2

T __

... -·········

w x Ix 2 = area of tne ceiling ancl floor

wx h x 2 = area of the ends Ix h x 2 = area of the sides Total external area= sum of the three areas Goodheart-Willcox Publisher

Figure 50-4. The area of each surface of the cabinet is calculated by multiplying the two dimensions that define the edges of that surface.

Area of insulated walls: 484 ft2 - 6 ft2 = 478 ft2 of insulated wall Figure 50-5 shows that 4" thick extruded polystyrene insulation and a 60°F temperature difference will result in a transfer of 49.8 Btu of heat through each square foot of insulated wall during a 24-hour period. Multiplying this heat transfer rate by the square footage of the insulated walls calculates the total heat transferred through the walls in 24 hours:

49.8 Btu/24 hr.ff x 478 ft2 = 23,804 Btu/24 hr through the walls Figure 50-5 also shows that 1 ft2 of double-pane glass allows heat transfer of 660 Btu/24 hr when there is a 60°F temperature difference. The heat leakage rate is multiplied by the total surface area of the windows:

660 Btu/24 hr.ff x 6 ft2 = 3960 Btu/24 hr through the windows To calculate the total heat leakage for the cabinet, the heat leakages through the insulated walls and through the glass are added together. The sum is the total amount of heat that leaks into the cabinet over a 24-hour period when there is a 60°F temperature difference between the inside and outside of the cabinet:

Area of windows:

+

1 1/2' x 2' x 2 = 6 ft2 of window

23,804 Btu/24 hr 3960 Btu/24 hr 27,764 Btu/24 hr

Copyright Goodheart-Willcox Co., Inc. 2017

I

.48

7"

--J

~

I\)

ri

:J

?

(")

0 X

~ Cl

;cl.

p,

(1)

G) 0 0 a. :::,

16.8

280

Triple-pane glass 320

500

1220

12.6

13.5

14.9

16.7

18.9

21.6

25.2

30.2

37.4

49.5

350

550

1350

14.0

15.0

16.5

18.5

21.0

24.0

28.0

33.5

41.5

55.0

390

610

1490

15.4

16.5

18.2

20.4

23.1

26.4

30.8

36.9

45.7

60.5

Figure 50-5. Heat gain factors for walls, floor, and ceiling.

7.0

440

_ Double-pane I 11 0 glass

I

1080

11.2

12.0

13.2

14.8

Single-pane I _ 27 0 glass

.30

.28

11"

.33

.37

12"

10"

9"

.42

22.4

.56

tl5" ;:?;

-0

8"

26.8

.67

5"

6"

'!:;

19.2

33.2

.83

4"

(")

0

44.5

1.11

3"

420

660

1620

16.8

18.0

19.8

22.2

25.2

28.8

33.6

40.2

49.8

66.0

454

715

1760

18.2

19.5

21.5

24.1

27.3

31.2

36.4

43.6

54.0

71.5

490

770

1890

19.6

21.0

23.1

25.9

29.4

33.6

39.2

46.9

58.1

77.0

525

825

2030

21 .0

22.5

24.8

27.8

31.5

36.0

42.0

50.3

62.3

82.5

560

880

2160

22.4

24.0

26.4

29.6

33.6

38.4

44.8

53.6

66.4

88.0

595

936

2290

23.8

25.5

28.1

31.5

35.7

40.8

47.6

57.0

70.6

93.5

630

990

2440

25.2

27.0

29.7

33.3

37.8

43.2

50.4

60.3

74.7

99.0

665

1050

2560

26.6

28.5

31.4

35.2

39.9

45.6

53.2

63.7

78.9

104.5

700

1100

2700

28.0

30.0

33.0

37.0

42.0

48.0

56.0

67.0

83.0

110.0

740

1160

2840

29.4

31.5

34.7

38.9

44.1

50.4

58.8

70.4

87.2

115.5

810

1270

3100

32.2

34.5

37.8

42.6

48.3

55.2

64.4

77.1

95.5

126.5

840

1320

3240

33.6

36.0

39.6

44.4

50.4

57.6

67.2

80.4

99.6

132.0

Goodheart-Willcox Publisher

770

1210

2970

30.8

33.0

36.3

40.7

46.2

52.8

61.6

73.7

91.3

121.0

...

(JQ

g: ::s ::r

0..

§

n"'

~

0..

::s

p,

g·

"' ~"'a

~ ....,

"'::s

/l)

0..

0

~

0)

U1

Co)

Chapter 50 Understanding Heat Loads and System Thermodynamics

1357

sunlight and has a light surface, add 3°F (2°C) to the ambient air temperature.

50.1.2 Service Heat Load The service heat load, sometimes called the usage heat load, is the sum of the various heat loads that result from operation of the unit for a given period of time, usually 24 hours. The following are a few of the individual heat loads that typically contribute to the service heat load:

Goodheart-Willcox Publisher

•

Cooling the contents to cabinet temperature.

•

Cooling of air changes.

•

Removing respiration heat from fresh vegetables and meat.

•

Removing the heat generated by electric lights and motors.

Figure 50-6. Sample walk-in cooler dimensions.

Removing the heat given off by people entering or working in the cabinet. To accurately calculate a service heat load, a technician must consider all potential heat load sources and perform the required calculations with care. The technician must know the amount of food put into the conditioned space, how many times the door is opened, the number of people working in the cabinet, the amount of time they spend in the cabinet, and the type of work they are performing. •

Pro Tip

Determining Cabinet Surface Area and Volume Rather than manually calculating the exterior surface area and interior volume of cabinets, technicians can refer to a table like the one shown in Figure 50-7. This table lists the exterior surface area for cabinets with different dimensions. It also lists the interior volume of different size cabinets based on exterior dimensions, wall thickness, and ceiling height.

Using Tables to Calculate Service Heat Load

Federal Requirements for Walk-In Coolers and Freezers The Energy Independence and Security Act of 2007 requires all walk-in coolers manufactured after January 1, 2009 to have wall, ceiling, and door insulation of at least R-25. Walk-in freezers must have wall, door, and ceiling insulation of at least R-32 and floor insulation of at least R-28. The requirement does not apply to windows, glass display doors, or structural members.

Adjusting for Heat from the Sun If part or all of the cabinet is exposed to direct sunlight, adjustments must be made to account for the solar heat load. If the cabinet is in direct sunlight and has a dark surface, add 10°F (6°C) to the ambient temperature when calculating the heat leakage load. If the cabinet is in direct sunlight and has a medium-colored surface, add 5°F (3°C) to the ambient air temperature. If the cabinet is in direct

Refrigeration equipment manufacturers have developed tables that help technicians estimate service heat loads accurately and more easily. In these tables, a cabinet is classified according to how it will be used. Some tables list specific classifications, such as florist's cabinets, grocery boxes, normal market coolers, fresh meat cabinets, and restaurant short-order cabinets. Other tables have more general classifications, such as average service, heavy service, and long storage. From experience, the manufacturers have found that cabinets used for the same general type of service will have roughly equivalent service heat loads when all other factors are equal. The service heat load is affected by four basic factors: •

Temperature difference between the exterior and interior of the cabinet.

•

Volume of the cabinet's interior.

• •

How the cabinet is used. Length of time for which the heat load is calculated.

Copyright Goodheart-Willcox Co., Inc. 2017

I '

1358

Modern Refrigeration and Air Conditioning

5x5

210

137

131

124

112

101

90

71

250

174

167

159

144

131

117

93

5x6

236

169

161

154

140

127

114

91

280

215

206

197

180

164

148

120

5x7

262

201

194

184

168

153

138

111

310

256

248

236

216

198

179

146

5x8

288

233

224

214

196

179

162

131

340

296

286

274

252

232

210

172

6x6

264

209

201

193

175

160

145

119

312

266

256

247

225

207

188

156

6x7

292

248

238

228

210

193

176

146

344

316

304

292

270

249

228

192

6x8

320

286

277

267

245

226

207

173

376

364

353

342

315

292

269

228

6x9

348

325

315

305

280

259

238

200

408

414

402

390

360

335

309

263

6x10

376

364

353

343

315

292

269

227

440

463

451

439

405

378

350

299

6x12

432

444

432

419

385

358

331

281

504

555

546

536

495

463

430

370

7x7

322

294

283

272

254

234

214

180

378

374

361

348

326

302

278

237

7x8

352

341

329

317

294

273

252

214

412

434

420

406

378

353

328

281

7x9

382

386

374

362

334

312

290

248

446

492

477

463

430

403

377

326

7x10

412

433

420

407

374

346

318

282

480

551

536

521

481

448

413

371

7x12

472

527

512

497

454

414

374

348

548

670

653

635

583

535

486

458

8x8

384

394

382

369

343

320

296

253

448

501

487

473

441

413

385

333

8x9

416

448

434

420

392

367

341

294

484

570

553

538

504

474

443

386

8x10

448

503

587

471

441

413

385

335

520

641

748

603

567

534

500

441

8x12

512

610

591

573

539

506

473

417

592

776

755

734

692

653

615

548

8x14

576

718

697

675

637

594

561

499

664

914

889

864

818

768

730

656

9x9

450

510

489

469

448

420

392

341

522

649

623

600

576

543

510

449

9x10

484

570

554

537

504

473

443

386

560

725

706

686

647

612

575

508

9x12

552

694

674

654

616

581

545

476

636

883

859

836

792

752

708

626

9x14

620

814

793

771

728

687

647

566

712

1035

1011

987

935

888

840

745

10x10

520

638

620

602

567

534

500

440

600

870

790

770

729

680

650

579

10x12

592

776

755

734

693

655

617

547

680

988

962

939

890

847

802

720

10x14

664

912

889

866

818

775

733

654

760

1158

1132

1110 1050 1005

954

860

12x12

672

946

919

893

848

804

760

680

768

1203

1172

1144 1090 1038

988

895

12x14

752

1110

1086

1052

1001

951

900

809

856

1411

1382

1348 1289 1230

14x14

840

1304

1269

1235

1180

1126

1072

968

952

1660

1619

1568 1518 1458 1394 1272

1170 1060

Goodheart-Willcox Publisher

Figure 50-7. This table can be used to determine the exterior surface area and interior volume of cabinets with different footprints, wall thicknesses, and ceiling heights.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 50 Understanding Heat Loads and System Thermodynamics

Using Tables to Determine Service Heat Load To determine the service heat load, a technician would look up vital information in a variety of tables. The following is the general procedure a technician would use to calculate service heat loads using tables: 1. Determine the type of service for which the cabinet is being used, such as average, heavy, or long storage. The type of use has a greater effect on the service heat loads of smaller cabinets than it does on the service heat loads of larger ones. 2. Find the volume of the cabinet using the cabinet's inside dimensions. Refer to the "Volume of Interior" columns in Figure 50-7. 3. After the total volume of the cabinet has been found, determine the usage heat gain per cubic foot of cabinet space using a table like the one in Figure 50-8. When looking up the usage heat gain, use the same temperature difference that was used to calculate heat leakage into the cabinet. 4. Multiply the volume of the cabinet's interior by the usage heat gain. The result is the estimated service heat load. Example: Use tables to find the estimated service heat load for a 9' x 10' x 8' cabinet with 4" thick walls. The cabinet will be subjected to average service. Use a temperature difference of 60°F (33°C) for the calculations. Solution: The volume of the cabinet is looked up in the table in Figure 50-7 and found to be 570 ft 3 (approximately 600 ft 3). Next, the usage heat gain is looked up on the table in Figure 50-8. Since 570 ft3 is not listed on the table, the usage heat gain for a 570 ft3 must be estimated using the two closest values. The difference between the usage heat gain for a 500 ft 3 cabinet and a 600 ft3 cabinet is 2.6 Btu/24 hr. Since the 570 ft 3 cabinet is 30 ft 3 smaller than the 600 ft 3 cabinet and 70 f t3 larger than the 500 ft3 cabinet, the difference between the service loads for the 500 ft3 cabinet and the 600 ft3 cabinet is multiplied by 30% and added to the usage heat gain for the 600 ft 3 cabinet:

Usage heat gain for 600 ft 3 cabinet = 70 Btu/24 hr-ft 3 Usage heat gain for 500 ft 3 cabinet = 72.6 Btu/24 hr-ft 3 Difference in cabinet volumes = 100 ft 3 Difference in usage heat gain = 2.6 Btu/24 hr-ft 3

1359

Usage heat gain for 570 ft 3 cabinet = 70 Btu/24 hr-ft 3 + (0.30 x 2.6 Btu/24 hr-ft 3) = 70.8 Btu/24 hr-ft 3 Multiply this value by the total volume in cubic feet to calculate an accurate estimate of the cabinet's service heat load: Service heat load = usage heat gain per cubic foot xvolume. = 70.8 Btu/24 hr-ft 3 x 570 ft3 = 40,356 Btu/24 hr

Manually Calculating Service Load The preceding section explained how to use tables to quickly estimate the service load for a cabinet with a fair amount of accuracy. Occasionally, a technician may need to manually calculate the service load for a commercial refrigeration system. The total service load includes the air change heat load, the product heat load, and miscellaneous heat loads. The following sections explain how those individual heat loads are determined and then added together to calculate the service heat load. Calculating Cabinet Volume The first step in manually calculating a service heat load is to determine the cabinet volume. Cabinet volume is based on the inside dimensions of the cabinet. For standard size cabinets, the volume can be looked up in a table like the one shown in Figure 50-7. If a table is not available, the volume of the cabinet can be calculated manually. The volume of the cabinet interior is calculated using the exterior dimensions minus the wall thickness, as shown in Figure 50-9. This volume is used to calculate the number of air changes that the cabinet is likely to undergo in a day. The volume of a rectangular box or a cube is found simply by multiplying the length of the box by the width of the box and the height of the box.

Formula for Calculating the Volume of Rectangular Boxes: V=lxwxh Example: In the sample cabinet, which is 10' long x 9' wide x 8' high, the walls are 4" thick. Find the volume of the cabinet. Solution: Inside dimension = outside dimension - (wall thickness x 2) Inside length= 10' - 8" = 9'-4" or 9 1/3'

Copyright Goodheart-Willcox Co., Inc. 2017

I

1360

Modern Refrigeration and Air Conditioning

Usage Heat Gain, Btu/24 hr for 1 ft 3 Interior Volume (ft 3)

Temperature Difference, °F (ambient temp. minus storage room temp.)

Service* 1

20

Average

30

Heavy

50

75

40

50

55

60

65

70

75

4.68

187

234

258

281

305

328

351

Heavy

5.51

220

276

303

331

358

386

Average

3 .30

132

165

182

198

215

231

4.56

182

228

251

274

297

319

342

80

90

100

374

421

468

413

441

496

551

248

264

297

330

365

410

456

Average

2 .28

91

114

126

137

148

160

171

182

205

228

Heavy

3 .55

142

177

196

213

231

249

267

284

320

355

Average

1.85

74

93

102

111

120

130

139

148

167

185

Heavy

2.88

115

144

158

173

188

202

216

230

259

288

100

Average

1.61

64

81

84

97

105

113

121

129

145

161

Heavy

2.52

101

126

139

151

164

176

189

202

227

252

200

Average

1.38

55

69

76

83

90

97

103

110

124

138

Heavy

2.22

90

111

122

133

144

155

166

178

200

222

300

Average

1.30

52 .0

65.0

71 .5

78.0

84.5

91 .0

97.5

104.0

117

130

Heavy

2.08

83.2

104.0

114.0

125.0

135.0

146.0

156.0

166.0

187

208

400

Average

1.24

49.6

62 .0

68.2

74.4

80.6

86.8

93 .0

99.2

112

124

Heavy

1.96

78.4

98.0

108.0

118.0

128.0

137.0

147.0

157.0

176

196

500

Average

1.21

48.4

60.5

66.6

72.6

78.7

84.7

90.7

96.8

109

121

Heavy

1.87

74.8

93.5

103.0

112.0

122.0

131.0

140.0

150.0

168

187

600

Average

1.17

46.8

58.5

64.0

70.0

76.0

82 .0

88.0

94.0

105

117

Heavy

1.85

74.0

92 .5

102.0

111 .0

120.0

130.0

139.0

148.0

167

185

800

Average

1.11

44.4

55.5

61 .1

66.6

72 .2

77.7

83.3

88.8

100

111

Heavy

1.76

70.4

88.0

96.8

106.0

115.0

123.0

132.0

141 .0

158

176

1,000

Average

1.10

44.0

55.0

60.5

66.0

71 .5

77.0

82 .5

88.0

99

110

Heavy

1.67

66.8

83.5

91 .9

100.0

108.0

117.0

125.0

134.0

150

167

Average

0.995

39.8

49.8

54.7

59.7

64.7

69.7

74.7

79.6

89.6

99.5

Heavy

1.580

63.2

79.0

86.9

94.8

103.0

111.0

119.0

126.0

142.0

158.0

1,500

Average

0.920

36.8

46.0

50.6

55.2

59.8

64.4

69.0

73.6

82.8

92.0

Heavy

1.500

60.0

75.0

82 .5

90.0

97.5

105.0

113.0

120.0

135.0

150.0

2,000

Average

0.835

33.4

41 .8

45.9

50.1

54.3

58.5

62 .7

66.8

75.2

83.5

Long storage

0.775

31 .0

38.8

42 .6

46.5

50.4

54.3

58.1

62 .0

69.8

77.5

Average

0.750

30.0

37.5

41.3

45.0

48.8

52.5

56.2

60.0

67.5

75.0

Long storage

0.576

23.0

28.8

31.7

34.6

37.3

40.3

43.2

46.1

51.8

57.6

Long storage

0.403

16.1

20.2

24.2

26.2

28.2

30.2

32 .2

36.3

40.3

1,200

3,000

5,000

22 .2

Goodheart-Willcox Publisher

Figure 50-8. A table like this one can be used to determine usage heat gain. It provides different values for the different types of storage the cabinet may be used for: average storage, heavy storage, and long storage. (continued) Copyright Goodheart-Willcox Co., Inc. 2017

1361

Chapter 50 Understanding Heat Loads and System Thermodynamics

Usage Heat Gain, Btu/24 hr for 1 ft 3 Interior Volume (ft 3)

Temperature Difference, °F (ambient temp. minus storage room temp.)

Service* 1

40

50

55

60

65

70

75

80

90

100

7,500

Long storage

0.305

12.2

15.3

16.8

18.3

19.8

21.4

22 .9

24.4

27.5

30.5

10,000

Long storage

0.240

9.6

12.0

13.2

14.4

15.6

16.8

18.0

19.2

21 .6

24.0

20,000

Long storage

0.187

7.48

9.35

10.30

11 .2

12.2

13.1

14.0

15.0

16.8

18.7

50,000

Long storage

0.178

7.12

8.90

9.79

10.7

11.6

12.5

13.4

14.2

16.0

17.8

*For average and heavy service, product load is based on product entering at 10°F above the refrigerator temperature. For long storage, the entering temperature is approximately equal to the refrigerator temperature. Where the product load is unusual, do not use this table. Goodheart-Willcox Publisher

Figure 50-8. Continued.

,•'

-------------

-------- - / --------_

----------r,

TT1·•· a·

li

:

__ . /

-------~ ~s::~;;,1/ D

_D

Goodheart-Willcox Publisher

Figure 50-9. To calculate the volume of a cabinet, the interior dimensions of the refrigerated space are multiplied together. Typically, an interior dimension is equal to the corresponding exterior dimension minus two times the thickness of the walls.

Inside width = 9' - 8" = 8'-4" or 8 1/3' Inside height = 8' - 8" = 7'-4" or 7 1/3' Inside volume = 9 1/3' x 8 1/3' x 7 1/3' = 28/3' X 25/3' X 22/3' = 15400/27 ft 3 = 570.4 ft 3

This allows the warmer room air to move into the refrigerated space. This air movement is sometimes called infiltration. The air that enters a refrigerated space must be cooled. According to Charles' law, the air entering the cabinet is reduced in pressure as it cools. This creates a pressure difference between the outside of the cabinet and the inside. If the cabinet is not airtight, air will continue to leak in due to the pressure difference. This infiltration of warm air continues until the pressure inside the cabinet equalizes with pressure outside the cabinet. Figure 50-10 lists the approximate number of air changes that can be expected to occur in cabinets of various volumes. Figure 50-11 shows the rate at which heat (sensible heat + latent heat) must be removed from infiltrating air in order to cool it to the same temperature as the refrigerated space. The amount of heat that must be removed depends on outside conditions and refrigerator temperatures. To calculate the total heat load due to air changes, the required heat removal rate must be multiplied by the number of air changes, and the volume of the cabinet, in cubic feet. Formula: Air change heat load

= heat removal rate x volume of cabinet x number of air changes

Air Change Heat Load

Product Heat Load

Each time a service door or a walk-in door is opened, the cold air inside the refrigerated space, being heavier, spills out through the bottom of the opening.

Any substance that is warmer than its surroundings will lose heat. This will continue until the substance cools to the ambient temperature. Product heat load

Copyright Goodheart-Willcox Co., Inc. 2017

I ,

1362

Modern Refrigeration and Air Conditioning

Air Changes by Volume Volume (ft 3 )

Air Changes per 24 hr

Volume (ft 3)

Air Changes per24 hr

200

44.0

6,000

6.5

300

34.5

8,000

5.5

400

29.5

10,000

4.9

500

26.0

15,000

3.9

600

23.0

20,000

3.5

800

20.0

25,000

3.0

1,000

17.5

30,000

2.7

1,500

14.0

40,000

2.3

2,000

12.0

50,000

2.0

3,000

9.5

75,000

1.6

4,000

8.2

100,000

1.4

5,000

7.2

Note: For heavy usage, multiply the above values by 2. For long storage, multiply the above values by 0.6. Reprinted by permission of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, Georgia

Figure 50-10. Table listing the average number of storage

room air changes per 24 hours. The values listed are based on volume and take into account door openings and air filtration.

the temperature of one pound of that substance by 1°F. The specific heat of various products can be looked up on a table like the one shown in Figure 50-12. Formula for Sensible Heat Load:

Q = W x sp ht x (T1 - T2) where

Q = sensible heat to be removed W = weight of the product sp ht = specific heat capacity of the product T1 = temperature of the product T2 = cabinet temperature When a material changes its physical state (such as changing from liquid to solid, solid to gas, or viceversa) the specific heat of the material also changes. It is important to use the correct specific heat in the load calculations. If the product is going to be cooled below its freezing point, two separate sensible heat calculations are performed. The first calculation determines the sensible heat that must be removed to cool the product from its initial temperature to its freezing point. This calculation requires the technician to use the specific heat value for above-freezing temperatures. The second calculation determines how much sensible heat must be removed after the product is frozen to cool it to its final temperature. For this calculation, the below-freezing specific heat value is used. Pro Tip

refers to the heat contained in products that are placed in a refrigerated space. Some of the product's heat must be removed in order to cool the product to the desired temperature. This heat is transferred to the air surrounding the product. If this heat is not transferred outside of the cabinet by the refrigeration system, the temperature inside the cabinet will gradually rise. When product heat loads are being discussed, three different types of heat must be considered: sensible heat, latent heat, and respiration heat. Sensible Heat Sensible heat is heat that results in a temperature change in the product. To lower the temperature of the product, sensible heat must be transferred to the air surrounding the product and rejected from the cabinet by the refrigeration system. To calculate the amount of sensible heat that must be removed to bring the product down to cabinet temperature, the weight of the product is multiplied by the specific heat capacity of the product and the temperature difference between the product temperature and the cabinet temperature. The specific heat capacity of a substance is the amount of heat needed to raise

Specific Heat of Vapors Every substance has four different values for its specific heat, one value for the solid state, one for the liquid state, and two for the vapor state. The following are the two different values for specific heat of a vapor: •

Specific heat when under a constant pressure.

Specific heat when confined to a constant volume. A vapor under constant pressure has a greater specific heat value than the same vapor under constant volume. The vapor heated with a constant pressure upon it will expand and do external work, such as increasing the size of a balloon. This external work naturally requires an additional quantity of heat. •

Latent Heat Latent heat is the heat that results in a phase change in the product. For example, if a bowl of soup is put in a freezer, sensible heat is removed to bring the soup down to its freezing temperature, and then latent heat must be transferred out of the soup to freeze it solid. If the soup were steaming when it was put into the freezer, additional latent heat would need to be removed from the steam in order to condense it.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 50 Understanding Heat Loads and System Thermodynamics

1363

Heat Removed from Replacement Air to Reach Cabinet Temperature (Btu/ft3) Temperature of Outside Air Storage Room Temp. °F @80% RH

85°F

90°F

100°F

95°F Relative Humidity

50%

60%

50%

60%

50%

60%

50%

60%

65

0.45

0.64

0.68

0.91

0.93

1.20

1.21

1.51

60

0.66

0.85

0.89

1.12

1.14

1.41

1.42

1.71

55

0.85

1.04

1.08

1.31

1.33

1.60

1.61

1.91

50

1.03

1.22

1.26

1.49

1.51

1.78

1.79

2.09

45

1.19

1.39

1.43

1.66

1.68

1.94

1.95

2.25

40

1.35

1.55

1.59

1.81

1.83

2.10

2.11

2.41

35

1.50

1.70

1.74

1.96

1.99

2.25

2.26

2.56

30

1.64

1.84

1.88

2.10

2.13

2.39

2.40

2.70

Temperature of Outside Air Storage Room Temp. °F @80% RH

40°F

50°F

100°F

90°F Relative Humidity

70%

80%

70%

80%

50%

60%

50%

60%

25

0.39

0.43

0.69

0.75

2.02

2.24

2 .54

2.84

20

0.52

0.56

0.82

0.89

2 .15

2.38

2.68

2.97

15

0.65

0.69

0.95

1.01

2.28

2.50

2.80

3.10

10

0.77

0.82

1.08

1.14

2.40

2.63

2.93

3.22

5

0.89

0.94

1.20

1.26

2.52

2 .75

3.05

3.34

0

1.01

1.05

1.31

1.38

2.64

2.86

3.16

3.46

-5

1.13

1.17

1.43

1.49

2.76

2.98

3.28

3.58

-10

1.24

1.29

1.55

1.61

2.88

3.10

3.40

3.70

-15

1.36

1.41

1.67

1.73

2.99

3.22

3.52

3.81

-20

1.48

1.52

1.78

1.85

3.11

3.34

3.64

3.93

-25

1.60

1.64

1.90

1.97

3.23

3.45

3.75

4.05

-30

1.72

1.76

2.03

2.09

3.35

3.58

3.88

4.17

Reprinted by permission of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, Georgia

Figure 50-11. Table listing the heat that must be removed, per cubic foot, to cool replacement air to cabinet temperature. The values are given in Btu/ft3.

To calculate the latent heat that must be removed from a product in order to freeze it, multiply the weight of the product by its latent heat of fusion. The latent heat of fusion of most food products is equal to the percentage of water in the food times the latent heat of fusion for water, which is 144 Btu/lb. The latent heat of fusion for various products can be looked up in a table like the one shown in Figure 50-12.

Formula for Latent Heat Load

where

Q = total heat released during freezing W = weight of the product L1 = the product's latent heat of fusion

Copyright Goodheart-Willcox Co., Inc. 2017

I

1364

Modern Refrigeration and Air Conditioning

Heat Load of Various Refrigerated Products

Product

Apples Asparagus

Storage Temp. (OF)

Quick Freeze Temp. (OF)

Long

Short

-15 -30

30-32 32

38-42 40

Relative Humidity (%)

Specific Heat (Btu/lb· °F) Above Freezing

Below Freezing

85-88 85-90

0.92 0.95

0.39 0.44 0.31

Latent Heat (Btu/lb)

Freezing Point (°F)

Respiration (Btu/lb per day)

92 134

28.4 29.8

0.75

0-5

36-40

80

0.55

Bananas

56-72

56-72

85-95

0.81

Beans, Green

32-34

40-45

85-90

0.92

36-40

50-60

70

0.30

0.24

18

30-32 30-32

38-42 38-42

84 85

0.60 0.77

0.35 0.40

79 100

32-35

45-50

95-98

0.90

Bacon, Fresh

Beans, Dried Beef, Fresh, Fat Beef, Fresh, Lean

-15 -15

Beets, Topped

0.47

30

25.0

108

30.2

4.18

128

29.7

3.30

26.9

2.00

28.9

31-32

42-45

80-85

0.89

32-35

40-45

90-95

0.93

32 32

40-45 45 40-45

90-95 95-98

0.64 0.93 0.87

32

40-45

85-90

0.90

31-32 32-38

45-50 39-45

90-95

0.95 0.70

Cherries

31-32

40

80-85

0.85

Chocolate Coatings

45-50

Corn, Green

31-32

45

85-90

Cranberries

36-40

40-45

85-90

34

40-45

45-50

45-50

80-85

0.93

28

55-60 38-45 46-50

50-60

0.83

0.44

104

0.40

98

85-90

0.76 0.88

31.0 30.4

0.82

0.41

105

30.0

0.56

0.34

65

Blackberries

-15

Broccoli Butter Cabbage Carrots, Topped

+15 -30 -30

Cauliflower Celery Cheese

-30 +15

Cream Cucumbers Dates, Cured Eggs, Fresh

-10

Eggplants

-15

25

Fish, Dried

30-40

Furs

125

29.2 0.34 0.47 0.45

15 130 120 135

29.7

2.27

118

28.0

6.60

0.86

29.0

4.10

0.91

27.3

0.48

0.88

0.37

84 30.5

85-90 25-30 60-70

32-34

40-42

15

15

Grapefruit

32

32

85-90

0.92

30-32

35-40

80-85

0.92

28

36-40

80

0.68

0.38

87

31-32

45-50

0.35

0.26

26

Ham, Fresh Honey

1.73

30.1

Furs, To Shock

Grapes

15.0 31 .2 29.6

0.3

35-40

Flowers Fish, Fresh, Iced

30-31 45-50

0.46

40-60 111

28.4

0.50

111

27.0

0.50

Dunham-Bush, Inc.

Figure 50-12. Table listing the temperature, specific heat, and latent heat data for some common foods. These values can be used in heat loads calculations. (continued) Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 50 Understanding Heat Loads and System Thermodynamics

1365

Heat Load of Various Refrigerated Products Quick Freeze Temp. (OF)

Storage Temp. (OF)

Long

Short

Relative Humidity (%)

Lard

32-34

0-10 40-45

80

Lemons

55-58

Lettuce

32

45

32-34

36-38

Product

Ice Cream

-20

Liver, Fresh

Specific Heat (Btu/lb· °F)

Latent Heat (Btu/lb)

Above Freezing

Below Freezing

.50-.80 0.52

0.45 0.31

96 90

80-85

0.91

0.39

190

90-95

0.90

83

0.72

0.42

94

25

36-40

0.81

0.42

105

31-32

45

0.24

0.22

7

Meat, Brined

31-32

40-45

Melons

34-40

40-45

Milk

34-36

40-45

Mushrooms

32-35

55-60

80-85

0.90

Nut Meats

32-50

35-40

65-75

32

50-60 50

Lobster, Boiled Maple Syrup

Onions

32-34

Oranges

Respiration (Btu/lb per day)

28.1

0.40

31 .2

8.00

1.00

0.75

0.36

75

0.92

0.35

115

28.5

0.92

0.46

124

31.0

0.30

0.24

14

20.0

70-75

0.91

0.46

120

30.1

1.00

85-90

0.89

0.40

91

27.9

0.70

0.85

0.45

120

75-85

32-35

Oysters

Freezing Point (°F)

30.2

Peaches, Fresh

32-34 31-32

34-40 50

90-95 85-90

0.82 0.92

0.45 0.42

120 110

28.9 29.4

1.00

Pears, Fresh

29-31

40

85-90

0.90

0.43

106

28.0

6.60

Peas, Green

32

40-45

85-90

0.80

0.42

108

30.0

Peas, Dried

35-40

50-60

0.28

0.22

14

32

40-45

85-90

0.90

Parsnips

-30

Peppers

30.1

Pineapples, Ripe

40-45

50

85-90

0.90

127

29.9

Plums

31-32

40-45

80-85

0.83

115

28.0

Pork, Fresh

30 36-50 28-30

36-40 45-60 29-32

85 85-90

0.38 0.44 0.41

66 105 99

Pumpkins

50-55

55-60

70-75

0.60 0.77 0.80 0.90

28.0 28.9 27.0 30.2

Raspberries

31-32

40-45

80-85

0.89

0.46

125

30.0

Sausage, Fresh

31-36

36-40

80

0.89

Sauerkraut

33-36

36-38

85

0.91

0.47

128

Squash

50-55

55-60

70-75

0.90

32 31-32 40-50 28-30

45-50 42-45 55-70 36-40

85 80-85 85-90

0.92 0.92 0.95 0.71

Potatoes, White Poultry, Dressed

-30 -10

Spinach Strawberries

-15

Tomatoes, Ripe Veal

-15

Figure 50-12. Continued.

Copyright Goodheart-Willcox Co., Inc. 2017

2.35

0.85

3.30

29.3 0.48 0.39

129 135 91

30.8 30.0 30.4 29.0

3.30 0.50

o,,,;~:;;,, '"'

I

1366

Modern Refrigeration and Air Conditioning

Respiration Heat When fruits and vegetables are put into cold storage, their biological processes continue. As produce ripens and ages, it absorbs oxygen and releases carbon dioxide, ethylene gas, and heat. This heat is called respiration heat and must be removed by the refrigeration system to prevent the cabinet from warming. The respiration heat of various products can be looked up on a table similar to the one shown in Figure 50-12. Total Product Heat Load The total product heat load is the sum of the sensible heat loads, latent heat loads, and respiration heat loads for every item in a refrigerated space. Figure 50-12 shows the specific heat and latent heat of various refrigerated products. In addition, it recommends temperatures and relative humidity for storing various items.

Miscellaneous Heat Load All sources of heat not covered by heat leakage, product cooling, and respiration heat loads are usually categorized as miscellaneous heat loads. Some of the more common miscellaneous heat loads are lights, electric motors, people, and defrosting heat sources.

Heat from Electric Motors The heat released by any electric motors operating in the conditioned space must also be determined. On the average, electric motors release heat at a rate of approximately 2550 Btu/hp-hr. The exact amount of heat released depends on motor efficiency. The larger the motor, the more efficient it is. Figure 50-13 shows the heat given off by motors and the devices they drive. Forced-draft evaporators usually have motors and fans. Heat is released at a rate of 4600 Btu/hp-hr for motor sizes ranging from 1/8 hp to 1/3 hp. To calculate the total heat released by a particular motor, the horsepower rating of the motor and the number of hours the motor operates in a 24-hour period are multiplied by the heat release rate. For example, the following calculates the heat released by a continuously operating 1/8 hp fan motor in one day:

Total heat released in 24 hr = heat release rate x horsepower of motor x hours of operation Q = 4600 Btu/hp-hr x 1/8 hp x 24 hr = 13,800 Btu

Heat from Lights Lights located in the refrigerated space will release heat. Light emitting diodes (LED) and compact fluorescent (CFL) lights use about 1/4 of the energy of traditional incandescent lights and produce much less heat. A typical 100 W incandescent lightbulb can be replaced with a 20 W LED or CFL light and emit the same amount of light, while emitting far less heat. For example, a 100 W lamp will give off 341 Btu in one hour: 1 W = 3.41 Btu/hr 100 W = 341 Btu/hr If the same 100 W incandescent light is replaced with a 20 W LED light, it will give off only 68 Btu/hr. To find the amount of heat released by a light source, multiply the number of Btu released in one hour by the number of hours per day the light is on. For example, if a 100 W incandescent bulb runs continuously, the heat load would be:

341 Btu/hr x 24 hr/24 hr = 8184 Btu/24 hr If the workday is eight hours (only time light is on), the heat load would be:

341 Btu/hr x 8 hr/24 hr = 2728 Btu/24 hr Thinking Green

High-Efficiency Lighting Use of high-efficiency lighting in a walk-in freezer or cooler can improve the overall energy efficiency of a system by as much as 10%.

Heat Equivalent of Electric Motors

Motor Size (hp)

Connected Load in Refrigerated Space•

Motor Losses Outside Refrigerated Spaceb

Connected Load Outside Refrigerated Spacec

Btu/hp·hr

Btu/hp·hr

Btu/hp·hr

1/8 to 1/3

4600

2550

2100

1/2 to 3

3800

2550

1300

Sto 20

3300

2550

800

• For use when both useful output and motor losses are dissipated within refrigerated space; motors driving fans for forced circulation unit coolers. For use when motor losses are dissipated outside refrigerated space and useful motor work is expended within refrigerated space; pump on a circulating brine or chilled water system; fan motor outside refrigerated space driving fan circulating air within refrigerated space. b

cFor use when motor heat losses are dissipated within refrigerated space and useful work expended outside of refrigerated space; motor in refrigerated space driving pump or fan located outside of space. Reprinted by permission of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, Georgia, from the 1994 ASHRAE Handbook-Refrigeration

Figure 50-13. Table listing the approximate amount of heat released by operating electric motors. Note the different operating conditions and how they affect the amount of heat released.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 50 Understanding Heat Loads and System Thermodynamics

Heat from Defrosting Heat Sources

Thinking Green

High-Efficiency Motors Use of high-efficiency compressors in a walk-in cooler or freezer can increase system efficiency by up to 10%. Use of high-efficiency evaporator fan motors can further improve energy efficiency up to an additional 10%. Use of high-efficiency condenser fan motors can boost energy efficiency another 5%.

Heat from People People inside a refrigerated space release heat at varying rates. This depends on what they are wearing (insulation), the temperature of the cabinet, and on how hard they are working. Figure 50-14 shows that occupants add 720 Btu/hr of heat load per person at 50°F (l0°C) and 1400 Btu/hr heat load per person at-10°F (-23°C). To find the heat load resulting from people working in the refrigerated space, use the table to find the heat equivalent of occupancy based on the temperature inside the refrigerated space. Next determine the total number of hours worked. For example, if three people work in a space for four hours and then two people work in the space for an additional two hours, the total number of hours worked would be 16. The next step in determining the heat load from occupants is to multiply the heat equivalent of occupancy by the total number of hours worked. For example, if one person worked in a 30°F (-l C) refrigerator for eight hours, the heat load would be: 0

950 Btu/hr x 8 hr = 7600 Btu

Heat Equivalent of Occupancy Refrigerated Space Temperature

1367

(OF)

Heat Equivalent per Person (Btu/hr)

50

720

40

840

30

950

20

1050

10

1200

0

1300

-10

1400

Note: Heat equivalent may be estimated by qP = 1295 -11 .5t (°F) Reprinted by permission of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, Georgia, from the 1993 ASHRAE Handbook-Fundamentals

Figure 50-14. Table indicating the approximate amount of heat released each hour by a person in a refrigerated space, based on the temperature of the space.

Many refrigerating units have defrosting heat sources, especially if the operating temperature is 32°F (0°C) or lower. Whether the defrost heat source is electric, hot gas, or water, the defrosting operation adds heat to the interior of the refrigerator. The amount of heat is difficult to determine because most of the defrosting heat is removed in the defrost drain water. Add approximately 10% of the defrosting heat input as part of the service heat load.

50.1.3 Calculating the Total Heat Load Total heat load is the sum of the heat leakage load and all of the individual heat loads that make up the service heat load. The information presented in the previous sections of this chapter can be used to find the total heat load in the following example: Example: A metal-sheathed walk-in cabinet is 10' long x 9' wide x 8' high. The wall construction consists of 4" extruded polystyrene with metal on each side. The cabinet has two double-pane glass windows measuring 1 1/2' x 2' each. It is in a room with a temperature of 95°F and a relative humidity of 60%. It cools 2000 lb of fresh lean beef from 60°F to 35°F each day. The evaporator has two 1/8-hp motors and the cabinet has two 40-watt lamps that operate 8 hours each day. One person works in the cabinet 8 hours each day. Solution, Step 1: Calculate the Heat Leakage Load The first step in calculating the total heat load is to calculate the total heat leaking through the cabinet. To find this, the surface area of the entire cabinet is calculated. Then, the surface area of the windows is subtracted from the total surface area to determine the surface area of the insulated walls. Next, the heat leakage rates through the various materials of the cabinet are determined, based on the temperature difference between the inside and outside of the cabinet. The heat leakage rates can be determined using the material's R- or U-values or by looking them up on a table similar to the one shown in Figure 50-5. Next, the heat leakage rate for the insulated walls of the cabinet is multiplied by the surface area of the walls, and the heat leakage rate through the windows is multiplied by the surface area of the windows. Finally, the heat leakage through the walls is added to the heat leakage through the windows. The result is the total heat leakage into the cabinet. The total heat leakage for the cabinet described in the example is 28,063 Btu/24 hr. Solution, Step 2: Calculating the Air Change Heat Load Portion of the Service Heat Load The next step in calculating total heat load for the cabinet is to find the heat load that results from air

Copyright Goodheart-Willcox Co., Inc. 2017

I '

1368

Modern Refrigeration and Air Conditioning

changes inside the cabinet. The volume of the cabinet can be calculated manually or can be looked up on a table like the one shown in Figure 50-7. The volume of the cabinet in the example is 570 ft3. The number of air changes, based on the volume of the cabinet, can be looked up on a table like the one shown in Figure 50-10. The cabinet in the example will undergo roughly 24 air changes per day. The volume of air exchanged each day equals the volume of the cabinet times the number of air changes per day, or 13,680 ft3 in the case of the example. Next, the amount of heat that must be removed in order to cool the replacement air to the cabinet temperature can be calculated manually using the specific heat of air and the latent heat of vaporization of water vapor, or it can be looked up on a table similar to the one shown in Figure 50-11. For the cabinet in the example, 2.25 Btu must be removed for every cubic foot of replacement air. The last step in determining the air change load is to multiply the heat removal rate by the volume of the cabinet. For the cabinet described in the example, this equals 30,780 Btu/24 hr. Solution, Step 3: Calculating the Product Heat Load Portion of the Service Heat Load The first step in calculating the product heat load is to determine the amount and weights of products that will be stored in the cabinet. Next, the specific heat of each produce must be looked up on a table like the one shown in Figure 50-12. Then, the temperature difference between the inside of the cabinet and the product entering the cabinet is multiplied by the product's specific heat and the weight of the product. In the case of the 2000 lb of beef in the example, the specific heat is 0.77 Btu/lb and the temperature difference is 25°F. Therefore, the product heat load is 38,500 Btu per day. The beef does not freeze and there is no respiration heat associated with its storage, so no further product heat load calculations are required. Solution, Step 4: Calculating the Miscellaneous Heat Load Portion of the Service Heat Load The miscellaneous heat load is the sum of all the heat generated from electric motors, lights, people inside the refrigerated space, and heat added during defrosting cycles. The heat from electric motors can be looked up on a table like the one shown in Figure 50-13. The two 1/8 hp motors described in the example produce 27,600 Btu of heat every 24 hours. To calculate the load from lights inside the refrigerated space, the wattage of each bulb is multiplied by the time of operation and a conversion factor of 3.41 Btu/watt. The two bulbs in the example generate a heat load of 2182 Btu per day.

An estimate of the amount of heat that is added by a person working in the refrigerated space can be looked up in a table like the one shown in Figure 50-14. The heat equivalent found on the table is then multiplied by the total number of hours worked by all occupants. The cabinet temperature in the example is 35°F, which is halfway between the 30°F and 40°F values listed on the table. The heat equivalent of occupancy is found by averaging the heat equivalents for 30°F and 40°F, which result in a heat equivalent of 895 Btu/hr. Since the worker is in the cabinet 8 hours a day, the total heat load due to occupants is 7160 Btu per day. Solution, Step 5: Combining the Various Heat Loads Once the heat leakage load and all of the individual heat loads that make up the service heat load are calculated, they are added together to find the total heat load. Since the loads are being added together, it is important that each load be expressed in the same terms, typically Btu/24 hr. The following shows how the various heat loads are added to arrive at the total heat load for the cabinet described in the example:

Total heat load =

28,063 Btu/24 hr 30,780 Btu/24 hr 38,500 Btu/24 hr 27,600 Btu/24 hr 2182 Btu/24 hr + 7160 Btu/24 hr

Total heat load (daily) = or

134,285 Btu/24 hr 5,595 Btu/hr

50.1.4 Heat Loads for Water Coolers So far, the discussion of heat loads has been limited to coolers, walk-in freezers, and similar systems. The same principles can be applied to refrigeration systems that are used to cool water rather than to cool air. Finding the total heat load of a water-cooling system is a combination of a specific heat and a heat leakage problem: 1. Water is cooled to temperatures that vary upward from 35°F. Calculating the amount of heat removed from the water to cool it to a certain temperature is a specific heat-related problem. 2. Because the water in the system is stored at a low temperature, heat from the room leaks into the stored water. The rate of heat leakage in a water cooler is calculated in essentially the same way that heat leakage into a refrigerated cabinet is calculated. The first step in installing a water cooling system is to determine what the cooling capacity of the system must be in order to effectively and efficiently handle

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 50 Understanding Heat Loads and System Thermodynamics

the total heat load. As with total heat load for a walk-in cooler, the total heat load for a water cooler is a combination of the heat leakage load and the service heat load.

Calculating the Service Heat Load for a Water Cooler For a water cooler, the service heat load is simply the heat that must be removed to cool the water. The service heat load that can be expected depends on the way the system will be used. The following are the common variables that will affect the service heat load: • The rate at which water from the system will be consumed. The rate at which water will be consumed varies greatly depending on the types of activities the consumers will be performing. For example, a worker in a machine shop or factory can be expected to drink about twice the amount of water that a worker in an office building would drink. The number of people that are expected to use the system must also be known. • The temperature of the water leaving the cooler. Drinking water temperatures should be regulated based on the type of work the consumers are doing. The heavier the work, or the warmer the room temperature, the warmer the drinking water must be.

1369

The temperature of the water entering the cooler. In order to calculate the amount of heat that must be removed to cool the water, the temperature difference between the incoming and outgoing water must be known. The table in Figure 50-15 lists different types of water cooler applications, the rates of consumption that can be expected for those applications, and the temperature of water that should be supplied for each application. These values, in combination with the temperature of the supply water and the number of people expected to use the system, can be used to quickly calculate the service heat load for a water cooling system. •

Example: The "heavy manufacturing" usage listing on the table in Figure 50-15 indicates that drinking water should be kept at a temperature between 50°F and 55°F. Also, 1/4 gallon of water per hour per person will be consumed. A production foundry can be classified as heavy manufacturing. If the foundry employs 50 workers for a period of eight hours, the water load per day would be 50 people x 8 hours x 1/4 gal/hr-person, or 100 gal of water. In other words, the system must be capable of cooling 100 gallons of water during the 8-hour workday.

Water Requirements Usage

Final Temp. Required

(OF)

Total Amount of Water Used and Wasted

Office building-employees

50

1/8 gallon per hour per person

Office building-transients

50

1/2 gallon per hour for each 250 persons per day

Light manufacturing

50-55

1/5 gallon per hour per person

Heavy manufacturing

50-55

1/4 gallon per hour per person

Restaurant

45-50

1/1 0 gallon per hour per person

Cafeteria

45-50

1/12 gallon per hour per person

Hotels

50

1/2 gallon per room (14 hr. day)

Theaters

50

1 gallon per hour per 75 seats

Stores

50

1 gallon per hour per 100 customers per hour

Schools

50-55

1/8 gallon per hour per student

Hospitals

45-50

1/12 gallon per day per bed

Note: The total amount of water used and wasted varies with the type of installation and kind of service. This table will serve as a basis for determining the cooler capacity required . Dispensed Water Div. of Elkay Mfg. Co.

Figure 50-15. Table listing the recommended temperature and quantity of cooled drinking water that should be provided in various public places and places of work. Copyright Goodheart-Willcox Co., Inc. 2017

I '

1370

Modern Refrigeration and Air Conditioning

If the incoming water in the supply pipe is 75°F, the cooler must reduce its temperature 20°F in order to provide water that is 55°F. Since there are 8.34 lb of water in 1 gallon, and the specific heat for water is 1 Btu/lb-°F, the service heat load can be computed as follows:

Formula: Service heat load

effective latent heat.

= specific heat x weight x temp. difference

Solution: Service heat load

The remainder of the refrigerant absorbs heat from the evaporator and, therefore, from the cabinet, until that refrigerant also vaporizes. The rate at which heat is absorbed by the refrigerant as it vaporizes is known as the latent heat of vaporization. The amount of heat absorbed from the cabinet and evaporator is the

= 1 Btu/lb-°F x (100 gal X

8.34 lb/gal)

X

20°F

= 16,680 Btu Calculating the Heat Leakage Load for a Water Cooler The amount of heat leakage is determined by the materials used to construct the insulated water-storage parts of the system, the thickness of the materials, and their exterior surface area. Insulation 1" to 3" thick is common for water-cooling insulations. Ice water insulation is typically 1 1/2" thick. The heat leakage load for standalone water-cooling systems is calculated the same way as the heat leakage load for cabinets.

50.2 Thermodynamics of the Basic Refrigeration Cycle Thermodynamics is the science that describes the relationships between heat and mechanical action. The compression refrigeration cycle is a good example of the interaction of heat and mechanical action in a closed system. Pressurized refrigerant that is in the liquid state and near room temperature passes through a metering device, which reduces the pressure on the refrigerant. The temperature of the refrigerant also decreases. However, because the boiling point of the refrigerant decreases along with the pressure, the latent heat in the refrigerant increases as sensible heat decreases. As a result, the enthalpy remains constant. The metering device does not remove or add heat. It only converts some sensible heat to latent heat. This process results in a large pressure and temperature drop without gaining or losing heat (Btu) and is known as adiabatic expansion. The drop in pressure and corresponding drop in boiling point causes some of the refrigerant to immediately vaporize as it passes through the metering device. When this refrigerant vaporizes, it absorbs heat from the remaining refrigerant. The refrigerant that immediately vaporizes as it passes through the metering device is referred to as flash gas.

When the refrigerant vapor leaves the evaporator, it travels along the suction line. During this movement, the pressure of the vapor decreases slightly (usually 2 psi [14 kPa]). It increases in temperature by about 10°F (6°C). The term "superheat" refers to heat added to the refrigerant after it has vaporized. The amount of superheat is all sensible heat and amounts to the difference between the refrigerant's evaporating temperature and the temperature of the vapor at the compressor's inlet. The compressor then takes the slightly superheated vapor and compresses it. It is compressed to a high-temperature, high-pressure vapor. Because of the pressure applied to the refrigerant, the condensing temperature of the refrigerant can rise to as high as 250°F (121°C), depending on the type of refrigerant and the pressure generated in the compressor. Pro Tip

Refrigerant Compression The term adiabatic compression refers to compressing a vapor without adding or removing heat energy from the vapor. When a compressor compresses refrigerant vapor, it decreases the volume of the vapor, which in turn increases the temperature and pressure of the vapor. Because this process is done relatively quickly, only a small amount of heat from the compressor is added to the vapor. As a result, the process is nearly adiabatic.

The superheated and compressed vapor passes to the condenser. If the refrigerant's temperature is higher than ambient temperature, it loses some of its heat to the air or water surrounding the condenser. This lowers the vapor temperature of the refrigerant. Eventually the refrigerant in a condenser loses enough sensible heat to completely desuperheat the refrigerant. At this point, it is at its condensing temperature. If the refrigerant loses any more heat, it will begin to condense from a vapor into a liquid. Since a high-side refrigerant's condensing temperature is higher than the temperature of any water or air surrounding the condenser, the refrigerant vapor starts losing some of its latent heat of vaporization and begins condensing. The amount of heat the vapor loses determines how much of it condenses into a liquid. After the refrigerant condenses into a liquid, it may

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 50 Understanding Heat Loads and System Thermodynamics

continue to lose heat to the condenser. This additional heat loss is in the form of sensible heat, meaning the liquid refrigerant's temperature will drop. This is known as subcooling. The liquid refrigerant returns to the metering device, where the refrigeration cycle is repeated. Figure 50-16 shows this cycle taking place. It also indicates the temperatures in various parts of the refrigerating system.

Q)

C ::=

t e

Condenser

0

All super-

>

heated

"O

vapor area

g-

Liquid-vapor mixture area

Q)

:::, Ill Ill

~ :::,

e

ll.

50.2.1 Reading a Pressure-Enthalpy Diagram A graph that plots refrigerant properties against pressure and heat conditions is commonly known as a pressure-enthalpy diagram, or pressure-heat diagram. Detailed instructions for reading pressure-enthalpy diagrams were presented in Chapter 9, Introduction to Refrigerants. The following is a brief overview. A pressure-enthalpy diagram can be divided into three main areas, Figure 50-17. Graph points that fall

1371

50%

75%

~ 50%

CJ CJ 25%

90%

~

10%

Heat Btu/lb Goodheart-Willcox Publisher

Figure 50-17_ The saturated liquid line and saturated vapor line divide a pressure-enthalpy diagram into three distinct areas. Note that, as heat is removed from saturated refrigerant, more of it becomes liquid. As heat is added to saturated refrigerant, more of it becomes vapor.

between the saturated liquid line and the saturated vapor line represent refrigerant that is a mixture of saturated liquid and saturated vapor. Graph points to the left of the saturated liquid line indicate that the refrigerant is all subcooled liquid. Graph points to the right of the saturated vapor line indicate the refrigerant is all superheated vapor. Many facts can be determined from the simplified pressure-enthalpy diagram in Figure 50-18.

Q) :

Q)

~

~ :C

C Q) :

.9:5 : f/)

-

t

C. :

c

;sv ~CJ;

E ' ~-

2 :0/b' :

Q)

..c

),._'

§ ~ : 0

CO

,~0

I

----·

D ,,'

,

.$ Cl)

§

C o ::··/------------------, -------

i ---/i:' ·---·, c-;;~~~~--r·-: Jj ~ 0

, ,._ m-

pressure line I '

/cJ &~ Compressor

D D

A!

:c

Heat Btu/lb -

High-pressure vapor •

High-pressure liquid

Low-pressure vapor

Low-pressure liquid

•

Goodheart-Willcox Publisher

Figure 50-16. Refrigeration system schematic showing the approximate temperatures of refrigerant in various parts of the system.

Goodheart-Willcox Publisher

Figure 50-18. Simplified pressure-enthalpy diagram showing the different types of lines included in a pressure-enthalpy diagram. Line A indicates constant heat condition with pressure change. Line B shows constant pressure and condition of refrigerant with changing heat content. Line C indicates constant temperature with changing pressure and heat.

Copyright Goodheart-Willcox Co., Inc. 2017

I ,

1372

Modern Refrigeration and Air Conditioning

Along any vertical line, such as A, the heat in one pound of refrigerant is constant. Along any horizontal line, such as B, the refrigerant has a constant pressure. Along stepped lines, like C, the temperature reading is constant. Note that the constant temperature line is vertical in the subcooled liquid area. It is horizontal in the liquid-and-vapor area. It slopes down and slightly to the right in the superheated vapor area. Refrigerant quality refers to how much of the mass of a refrigerant is in the liquid state and how much is in the vapor state. A 10% quality means that the refrigerant is 10% vapor and 90% liquid. Line D is a line of constant quality.

Saturated Vapor A saturated vapor is a refrigerant vapor under conditions that would cause some of the vapor to condense if any amount of heat were removed from it or if pressure was increased on it. Often saturated vapor is in the presence of some of its own liquid. For example, the vapor in a refrigerant cylinder that is half-full of liquid refrigerant would be a saturated vapor.

Superheated Vapor A superheated vapor is a vapor whose temperature has increased above its 100% saturated condition for the pressure that it is under. When additional heat is added to a 100% saturated vapor, a superheated vapor is created, and its temperature increases above its saturation temperature. A superheated vapor will abide by Charles' and Boyle's gas laws. Superheat is the sensible heat (as measured in degrees) above the vapor's saturation temperature. When refrigerant vaporizes in an evaporator, it is a saturated vapor at first. However, as this vapor leaves the evaporator and passes through the suction line to the compressor, it usually becomes warmer by 5°F to 15°F (3°C to 8°C). This increase in temperature is called "superheating the low-side vapor." In a compressor, the low-pressure, superheated vapor gets compressed. Most of the mechanical energy of compression is used to reduce the volume of the refrigerant vapor, which increases its temperature and pressure. Some of the mechanical energy of compression is converted to heat energy due to friction. The heat from friction is absorbed by the refrigerant, further increasing its superheat. Excessive superheating of refrigerant vapor lowers the efficiency of the system. The less superheating that takes place, the more efficient the system will be. As mentioned earlier, the amount of heat added by a refrigeration compressor is relatively small. On a pressure-enthalpy diagram, like the one shown in

Figure 50-19, the superheat added by the compressor causes the compression line (B to C) to slant to the right. There are three general places in the refrigeration system where refrigerant is a superheated vapor: • In the suction line. • In the compressor. • In the discharge line. In Figure 50-19, the portions of the refrigeration cycle where the refrigerant is a superheated vapor are shown in red. When heat is removed from a superheated vapor, its volume and/or pressure decreases without condensation until the refrigerant's temperature reaches its condensation point. In Figure 50-19, this cool down is denoted by line from C to D.

Saturated Liquid The term saturated liquid refers to a refrigerant in liquid form under conditions that would cause some of the liquid to vaporize if any amount of heat were added or if pressure was decreased. A refrigerant with a quality of 99% is composed of 1% saturated liquid and 99% saturated vapor. As the percentage of saturated vapor decreases, the percentage of saturated liquid increases.

Subcooled Liquid If a refrigerant consists of 100% saturated liquid and additional heat is removed, the refrigerant is referred to as a subcooled liquid. Refrigerant is

Heat Btu/lb Goodheart-Willcox Publisher

Figure 50-19. The superheated vapor portions of the refrigeration cycle are shown in red. From A to B, superheat is added to refrigerant vapor as it travels through suction line from evaporator to intake valve of compressor. From B to C, superheat is added as the vapor is compressed. From C to D, superheat is removed in the top portion of the condenser.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 50 Understanding Heat Loads and System Thermodynamics subcooled before it enters the refrigerant metering device. Subcooling the refrigerant before it enters the metering device increases the refrigerant's effective latent heat (heat-absorbing capacity).

Effective Latent Heat The amount of latent heat contained in vaporizing and in condensing refrigerant changes at different pressures. At a lower pressure, more latent heat must be added to a saturated liquid at any given temperature in order to vaporize it. In a refrigeration system, the high-pressure liquid refrigerant leaving the condenser is throttled down to a low-pressure, low-temperature liquid by the metering device. Some of the liquid forms flash gas as it passes through the metering device, cooling the remaining liquid. The formation of the flash gas causes the refrigerant to become a mixture of liquid and vapor refrigerant as it enters the evaporator. The refrigerant that has been converted to flash gas can no longer cool effectively. Therefore, the latent heat absorbed to form the flash gas must be subtracted from the total latent heat when determining the cooling ability of the refrigerant at a given pressure. In Figure 50-20, the latent heat absorbed to create the flash gas is indicated by the dotted green line. Only the latent heat to the right of the dotted green line contributes to the cooling effect of the system, and it is therefore referred to as the effective latent heat.

1373

The effective latent heat is an average value because low-side pressure and high-side pressure vary somewhat during the operation of the system. Conditions such as oil in the system and system efficiencies can cause variations from the ideal cycle. For R-134a, the total latent heat of vaporization at 5°F (-15°C) is 88.30 Btu/lb. However, after accounting for the latent heat used to form flash gas in the refrigerant, its actual heat-absorbing ability, or effective latent heat, is only about 62.33 Btu/lb, as shown in Figure 50-20. These values are based on the standard evaporating temperature of 5°F (-15°C) and standard condensing temperature of 86°F (30°C).

50.2.2 Practical Pressure-Enthalpy Cycles The refrigeration cycles for different systems vary depending on applications and conditions. A graph showing two different refrigeration cycles used in freezers is shown in Figure 50-21. The cabinet is kept at 0°F (-17.8°C). The refrigerant in the evaporator is at -m°F (-23.3°C). The air surrounding the condenser is 95°F (35°C). With insulation on the suction line, superheat can be minimized for efficient operation. See the compression/superheat line beginning at point A in Figure 50-21. With only m°F (6°C) of superheat, refrigerant enters the condenser at 180°F (82°C). The condenser must remove 55°F (30°C) of superheat before refrigerant can begin condensing.

Condenser C - - - - - - - - - - --c:- - - - - - - - - - - - - - - -- --c:- - -;

_y

t

i~

~:o Q) :o

: &

:' J'J

.g,:E

-·o ~:u

: §.

---------- ------ ...------------~---Evaporator A

:~ __ ! (j

t

Effective latent heat (62.33 Btu/lb)

e:::,

Total latent heat (88.30 Btu/lb) 13.59

39.56

Ill Ill

101.89

113.0

e ll.

Heat Btu/lb Gaadheart-Willcax Publisher

Figure 50-20. Pressure-enthalpy diagram plotting the operation of R-134a system. The total latent heat in 1 lb of refrigerant entering the compressor is 88.30 Btu/lb (101.89 Btu/ lb - 13.59 Btu/lb). However, 25.97 Btu/lb (39.56 Btu/lb 13.59 Btu/lb) of the total latent heat was used to create flash gas, and does not contribute to cooling the cabinet. As a result, the effective latent heat is only 62.33 Btu/lb (88.30 Btu/lb 25.97 Btu/lb).

Heat Btu/lb Gaadheart-Willcax Publisher

Figure 50-21. Pressure-enthalpy diagram showing the typical refrigeration cycles of an air-cooled system for storing frozen foods.

Copyright Goodheart-Willcox Co., Inc. 2017

I '

1374

Modern Refrigeration and Air Conditioning

If the suction line was not insulated, the cool suction line would continue to absorb heat along its entire length. This would increase the amount of superheat. See the compression/superheat line that begins at point B in Figure 50-21. With 60°F (33°C) of superheat, the compressor would pump the refrigerant vapor to the proper pressure at 240°F (116°C) at the condenser inlet. The condenser would need to remove 115°F (64°C) of superheat before the refrigerant could begin condensing. Not insulating a suction line results in unnecessarily high superheat. As shown in this example, increasing superheat by 60°F (33°C) means that the condenser has to remove 60°F (34°C) more superheat than if the suction line had been insulated properly. Minimizing superheat results in the condenser providing the best possible subcooling. The better the system subcooling, the better the effective latent heat for the evaporator. A typical air conditioning (comfort cooling) cycle is shown in Figure 50-22 for comparison.

Cascade System To produce extremely low temperatures efficiently, two refrigeration systems with separate refrigerant circuits may be used instead of one. The two systems are connected by a shared heat exchanger that transfers the heat of one circuit's condenser into the evaporator of the other circuit. The resulting arrangement is called a cascade system.

Condenser

t

First stage -1QOF - - -

Second stage -100°F Heat Btu/lb Goodheart-Willcox Publisher

Figure 50-23. Pressure-enthalpy diagram showing the

operation of cascade system used to obtain ultralow temperatures. Refrigerant passing through the first-stage evaporator (A1 to B1) removes heat from the second-stage condenser (D to E).

In a cascade system, the evaporator of the higherpressure subsystem (first stage) removes the heat from the condenser of the lower-pressure subsystem (second stage). Figure 50-23 shows the principle of this type system on a pressure-enthalpy diagram. Many cascade systems use a different refrigerant for the lower pressure subsystem than the refrigerant used in the higher pressure subsystem.

Compound Systems

125°F

t

- - - 86°F

Some refrigeration systems, especially ultralowtemperature systems, use two compressors connected in series. They pump the very low-pressure suction line vapor up to the condensing pressure and temperature condition. In the first stage, a compressor pumps the vapor up to a midpoint on the compression curve. Then, the compressed vapor is cooled (desuperheated) but remains in vapor form. The second compressor further compresses the cooled intermediate vapor to the final pressure-temperature condition. Figure 50-24 shows the operation of a compound refrigeration system graphed on a pressure-enthalpy diagram.

40°F Evaporator

e:::, Ill Ill

e C.

Hot-Gas Cycles Heat Btu/lb Goodheart-Willcox Publisher

Figure 50-22. Pressure-enthalpy diagram showing the typical cycle of a comfort cooling system with an evaporator temperature of 40°F (4.4°C) operating in an ambient temperature of 95°F (35°C).

A compressor's discharge hot gas may be used for any of the following purposes: • Defrosting evaporators. • Preventing suction pressure from dropping too low (when cooling load decreases).

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 50 Understanding Heat Loads and System Thermodynamics

1375

86°F

t

Compressor 2

t

Compressor 1

1

'

~

'--y--'

Heat Btu/lb~

Heat Btu/lb

~

Goodheart-Willcox Publisher

Figure 50-24. Pressure-enthalpy diagram showing the typical cycle of a compound refrigeration system. Compressor 1 (first stage) compresses the vapor partway to its final pressure (A to B). The vapor passes through a heat exchanger, where it is desuperheated (B to C). Compressor 2 (second stage) then compresses the vapor to its condensing pressure (C to D). Using a heat exchanger to cool the refrigerant between compressor stages reduces the amount of heat of compression at the final stage (D1 to D) and reduces the temperature at the second-stage compressor's exhaust valve but still allows for a high compression ratio needed for lowtemperature operation.

2 Goodheart-Willcox Publisher

Figure 50-25. Pressure-enthalpy diagram plotting a hot-gas defrosting cycle. The heat lost from A to B, as shown at 1, is the heat used to defrost the evaporator.

t

f:::, •

Keeping liquid refrigerant from entering compressor. Many refrigeration systems use an automatic bypass system. A graph of hot-gas bypass being used to defrost an evaporator is shown in Figure 50-25. The refrigerant is compressed normally. The hot gas is then bypassed through the evaporator, as shown by the lines from A to B to C to D. If the hot gas used for defrosting is cooled too much (as indicated by the line from A to B1), it will become partly liquid. This could cause liquid to enter the compressor. To prevent this, an accumulator is installed on the suction line to ensure that only vapor can reach compressor. The maximum heat for defrosting is shown at 2, unless bypass gas is allowed to condense and is then vaporized (in another evaporator of a multiple system or in a special defrost evaporator). The cycle for hot-gas bypass for low-pressure control is shown in Figure 50-26. The bypass line (controlled by a solenoid valve and a pressure-responsive valve) is piped from the discharge line into the suction line. The

Ill Ill

G)

C:

Heat Btu/lb

~

Goodheart-Willcox Publisher

Figure 50-26. A hot-gas bypass system helps maintain a normal low-side pressure in the low side of the system. If the low-side pressure drops to point B1, the bypass circuit A to B opens and brings low-side pressure up to normal, at C1. Without the bypassed hot gas, the low-side would operate at a lower pressure.

bypass circuit is controlled by a sensor or sensing bulb connected to the suction line. The bypass action will return the compression line to approximately C1 to 0 1• The four horizontal evaporator lines represent how the low-pressure side changes from cut-in pressure to cutout pressure.

Copyright Goodheart-Willcox Co., Inc. 2017

I '

Chapter Review •

•

•

•

•

•

•

1376

A system's total heat load is the sum of its heat leakage load and its service heat load. A properly sized system will remove the total heat load during the appropriate operating cycle. • Calculating the total heat load for a water cooler is similar to calculating the total heat load for a refrigerated cabinet. The service heat load is determined by the temperature difference between the incoming water and the cooled water and the rate at which water is consumed. The heat leakage load for a standalone water-cooling system is calculated the same way as for a refrigerated cabinet. • Thermodynamics is the science that describes the relationship between heat and mechanical action. The compression and expansion of fluids in a closed system, such as a refrigeration system, shows thermodynamic principles in action. • A pressure-enthalpy diagram graphs the physical properties of refrigerant at different points in the refrigeration cycle. The graph can be divided into three distinct areas: subcooled liquid only, saturated liquid and vapor, and superheated vapor only. A refrigerant's heat content, pressure, and temperature throughout a refrigeration cycle are shown on a pressureenthalpy diagram.

•

Summary A heat load is the amount of heat that must be removed from the conditioned space over a set period of time in order to maintain the desired temperature in the conditioned space. Heat leakage load is the amount of heat that leaks through the cabinet over a specific period of time, usually 24 hours. The five variables that affect heat leakage load are the length of time through which leakage is measured, the difference between outside temperature and the temperature inside the cabinet, the thickness of the materials used to construct the cabinet, the type of materials used to construct the cabinet, and the surface area of the cabinet. A material's thermal conductivity, or K-value, represents the rate at which heat is transferred through a 1" thickness of the material. A material's thermal conductance, or C-value, is the material's thermal conductivity (K-value) divided by its thickness. A material's thermal resistance is equal to the inverse of its thermal conductivity, or C-value. A material's thermal transmittance, or U-value, is equal to the inverse of the sum of thermal resistances in the material, including boundary air films. Heat leakage load is calculated by multiplying the surface area of each material in the cabinet by the thermal transmittance, or U-value, of that material and then adding the results together. The service heat load, or usage heat load, is the sum of the various heat loads that result from operation of the unit for a given period of time, usually 24 hours. The individual heat loads that make up the service heat load include cooling the products in the cabinet, cooling air changes in the cabinet, removing respiration heat from the products, removing heat from lights and motors, and removing heat generated by people working in the conditioned space. The service heat load is affected by the temperature difference between the interior and exterior of the cabinet, the volume of the cabinet's interior, how the cabinet is used, and the length of time through which the load is measured. Service heat load can be calculated manually or estimated using manufacturer's tables.

Review Questions Answer the following questions using the information in this chapter. 1. A refrigeration system's heat leakage load

is determined by the materials the cabinet is made of, the surface area of the cabinet's exterior, and _ _. A. the number of air changes per hour B. the products stored in the cabinet C. the temperature difference between the inside and outside of the cabinet D. All of the above. 2. Which of the following measurements accounts for the boundary air films inside and outside of a cabinet? A. C-value. B. K-value. C. R-value. D. U-value.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 50 Understanding Heat Loads and System Thermodynamics 3. Which of the following variables does not affect the service heat load of a refrigeration system? A. The number of people working inside the cabinet. B. The number and size of electric motors operating inside the cabinet. C. The quantity and types of products stored in the cabinet. D. The thermal conductance of the cabinet's walls. 4. The three types of heat that must be considered when calculating the product heat load are A. latent heat, sensible heat, and superheat B. latent heat, respiration heat, and subheat C. latent heat, respiration heat, and sensible heat D. respiration heat, sensible heat, and superheat

1377

9. A line that is completely horizontal in a pressure-enthalpy diagram shows constant A. heat (enthalpy) B. pressure C. refrigerant quality D. temperature

10. On a pressure-enthalpy diagram, a line that is vertical in the liquid-only portion, horizontal in the liquid-vapor portion, and slants slightly to the right from vertical in the gas-only portion shows constant _ _. A. heat (enthalpy) B. pressure C. refrigerant quality D. temperature

5. Which of the following variables are used to calculate the sensible heat load of a product? A. Latent heat, product weight, and respiration heat. B. Latent heat, product weight, and temperature change. C. Product weight, specific heat, and temperature change. D. Respiration heat, specific heat, and temperature change. 6. A refrigerant quality of 30% means that the refrigerant is _ _. A. 30% liquid and 70% vapor B. 70% liquid and 30% vapor C. subcooled by 30°F D. superheated by 30°F 7. When a refrigeration system's suction line that is cooler than ambient temperature is not insulated, heat absorbed into the suction line A. B C. D.

does not affect the system at all increases superheat increases system efficiency All of the above.

8. A line that is completely vertical in a pressure-enthalpy diagram shows constant A. B. C. D.

heat (enthalpy) pressure refrigerant quality temperature

I Copyright Goodheart-Willcox Co., Inc. 2017

CHAPTER51

Cotntnercial Refrigeration Cotnponent Selection

Chapter Outline 51.1 Sizing Compressors, Condensers, and Evaporators

51.1.1 Selecting a Compressor 51.1.2 Selecting a Condenser 51.1.3 Selecting an Evaporator 51.1.4 Liquid Receiver Sizing 51.2 Calculating Theoretical Compressor Volume 51.2.1 Volumetric Efficiency 51.2.2 Factors Affecting Volumetric Efficiency 51.3 Designing Piping 51.3.1 Pressure Drop 51.3.2 Refrigerant Velocity 51.3.3 Oil Circulation 51.3.4 Condenser Condensate Line 51.3.5 Liquid Line 51.3.6 Suction Line 51.3.7 Compressor Discharge Line

Learning Objectives Information in this chapter will enable you to: • Identify the factors that affect compressor sizing. •

Calculate a compressor's required capacity based on its total heat load and operating cycle.

•

Summarize the factors that affect the heat transfer rates of evaporators and condensers.

•

Use tables from manufacturers to size compressors, condensers, and evaporators.

•

Calculate a compressor's theoretical volume and volumetric efficiency.

•

List the basic criteria that must be considered when sizing refrigerant lines.

Chapter 51 Commercial Refrigeration Component Selection

Technical Terms bore condenser condensate line operating cycle static loss stroke

total equivalent length total heat of rejection (THR) volumetric efficiency

Review of Key Concepts Reviewing these concepts from earlier chapters will help you understand new concepts presented in this chapter: • A commercial refrigeration system's heat load is the amount of heat that it must remove from a conditioned space over a set period of time in order to maintain the desired temperature in the space. (Chapter 50) •

Relative humidity is the amount of moisture in an air sample compared to the total amount of moisture the same sample would hold if it were completely saturated at the same temperature. Relative humidity is stated as a percentage. (Chapter 27)

•

A saturated vapor produces condensate (liquid) if it loses heat (Btu) or applied pressure increases. A saturated vapor's temperature is equal to its boiling point at the given pressure. (Chapter 5)

•

Air-cooling evaporators, such as natural-draft and forced-draft evaporators, are evaporators that are designed to directly cool the air in a conditioned space. Liquid-cooling evaporators are designed to cool a liquid rather than air. (Chapter 21)

•

A modulating refrigeration system is able to adjust its capacity to more closely match a variable heat load. Modulating systems may use two or more compressors operating in parallel, one single compressor capable of varying its speed or output, or hot-gas bypass for capacity control. (Chapter 49)

1379

Introduction Commercial refrigeration often requires the installation of custom systems that are designed for a specific application. For instance, the existing space available in a building may dictate the size of a walk-in cabinet. The first step in sizing a custom refrigeration system is to determine the purpose of the system. The product to be refrigerated determines the type of refrigerant and the size of equipment to be used. Low-temperature applications, such as freezers that operate from 0°F to -60°F (-18°C to -51 °C), may use R-404A or R-508B with multiple compressors in a cascade system to achieve low temperatures. Midtemperature applications, such as grocery display cases that operate from 35°F to 45°F (2°C to 7°C), may use a secondary loop refrigeration system circulating a nonphase changing refrigerant that absorbs heat but does not vaporize. Higher-temperature applications, such as florist cabinets, may use R-134a with a hermetic compressor and require high humidity. Since each application is unique, the condensing and evaporating units must be matched to the specific refrigeration requirements of the conditioned space.

1380

Modern Refrigeration and Air Conditioning

51.1 Sizing Compressors, Condensers, and Evaporators A preliminary step in designing a commercial refrigeration system is estimating anticipated total heat loads. The total heat load determines how much cooling capacity the system must have in order to function effectively and efficiently. The total heat load represents the total amount of heat added to the conditioned space in a 24-hour period. The selection and installation of commercial refrigeration equipment requires a technical understanding of the variables involved. The capacity of the metering device, type of temperature control, method of air circulation, and specific duty of the system affect the selection of the compressor, condenser, and evaporator. These variables can have a profound effect on the system's operation.

Selecting Components for a Commercial Refrigeration System Commercial refrigeration components are selected based on the total heat load for the system. Many methods are used to select these components. The following is a basic overview of one method: 1. Select the compressor and condenser using capacity tables provided by manufacturers. 2. Select the evaporator to match the compressor capacity. 3. Select other system components based on compressor, evaporator, and system characteristics.

Basically, a compressor must remove vapor from the evaporator fast enough to enable the refrigerant to vaporize at the correct low pressure. Refrigerant vapor must be removed from the evaporator as fast as heat enters the evaporator in order to vaporize the refrigerant. A compressor is selected based on manufacturers' tables of compressor capacities. The compressor's capacity must be sufficient to move enough refrigerant through the system to remove the total heat load within the operating cycle. The capacity of a compressor varies based on the suction line temperature and the condensing temperature. The compressor selection also determines the refrigerant to be used to operate the system at maximum efficiency.

Suction Line Temperature The suction line temperature is determined based on the design temperature of the conditioned space and the temperature difference (TD) between the design temperature and the refrigerant in the evaporator. Other system conditions affecting compressor sizing include the desired relative humidity in the conditioned space and the defrost method used in the system. Relative humidity in a refrigerated storage space must be controlled. High relative humidity can promote the growth of mold and mildew in some products. Low relative humidity can cause some products to lose moisture and become too dry. Every product has an ideal relative humidity for refrigerated storage. When designing a refrigeration system, a technician determines the relative humidity based on the product to be stored in the space, Figure 51-1.

Storage Relative Humidity for Select Products

Pro Tip

Component Sizing The refrigeration component sizing and selection information in this chapter provides a simplified overview of the process. The type of refrigerant, compressor, condenser, and evaporator are unique to each application. Refer to manufacturers' information and advanced technical manuals for additional information.

51.1.1 Selecting a Compressor One of the first components selected for a system is the compressor. The compressor is the heart of a refrigeration system, using mechanical energy produced by an electric motor to pump refrigerant through the system. The refrigerant picks up heat in one place and releases it in another place.

Product

Onions Ham

Relative Humidity (%)

70-75 80

Grapes

80-85

Mushrooms

80-85

Beef

84-85

Apples

85-88

Broccoli

90-95

Turnips

95-98 Goodheart-Willcox Publisher

Figure 51-1. A refrigerated storage space must maintain the

relative humidity appropriate for the product being stored.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 51 Commercial Refrigeration Component Selection

Once the relative humidity is selected based on the product to be stored, the temperature difference (TD) can be determined. When the TD and the design temperature of the conditioned space are known, you can calculate the saturated suction line temperature. To do so, simply subtract the TD from the design temperature. For example, a 4S°F (7°C) refrigerated storage space with a m°F (S C) TD would have a suction line temperature of 3S°F (2°C). A table showing TDs and relative humidity levels is shown in Figure 51-2. 0

Operating Cycle The capacity of a compressor is determined in part by its operating cycle, which is the number of hours that it operates per day. A smaller compressor with a longer operating cycle can be as effective as a larger compressor with a shorter operating cycle. The operating cycle is partly based on the evaporator defrosting requirements. See Figure 51-3. Once the operating cycle is determined, the compressor capacity can be calculated by dividing the total heat load (which is based on 24 hours) by the number of hours in the operating cycle.

1381

Formula for required compressor capacity:

. (B /hr) = . d Total load (Btu/day) - -heat ----~ Requrre capaaty tu Operating cycle (hr/day) Example: A particular walk-in cabinet used to store lean beef at 3S°F (2°C) requires 160,S24 Btu/day. The system has a TD of m°F (S C) and, thus, a suction line temperature of 2S°F (-4°C). Based on the cabinet and suction line temperatures, warm air defrost can be used for the system. For a system with warm air defrost, the operating cycle is 16 hours. Determine the required compressor capacity. 0

Solution: Total load (Btu/day) . (B /hr) = . d - -heat ---~Requrre capacity tu Operating cycle (hr/day)

160,S24 Btu/day 16 hr/day = m,033 Btu/hr The required system capacity can also be expressed in tons of refrigeration effect.

Temperature Difference and Relative Humidity Relative Humidity (%)

Temperature Difference (°F)

90-95

8

80-90

10-12

65-80

12-15

50-65

17-22

Example: Given a system load of m,033 Btu/hr, determine the equivalent load in tons. Solution: One ton is equal to 12,000 Btu/hr. Therefore, divide the load by the number of Btu per hour in 1 ton of refrigeration as follows:

Gaadheart-Willcax Publisher

Figure 51-2. Relationship between relative humidity and temperature difference (TD) in the conditioned space.

Required capacity (tons)= m,033 Btu/hr .;-12,000 Btu/hr ton = 0.84tons

Operating Cycle Based on Defrost Requirements System Temperatures

Comment

Suction line temperature 30°F or higher.

No defrost cycle needed.

Suction line temperature below 30°F and cabinet temperature 35°F or higher.

Warm air defrost. Compressor stops and evaporator fans run to defrost the evaporator. One hour of Off cycle for every two hours compressor is run .

Suction line temperature below 30°F and cabinet temperature below 35°F.

Dedicated defrost system required.

Operating Cycle (hours per day) 20-22 16

18-22 Gaadheart-Willcax Publisher

Figure 51-3. The system temperature impacts the defrost method, which often determines the operating cycle for the system. Copyright Goodheart-Willcox Co., Inc. 2017

I

1382

Modern Refrigeration and Air Conditioning

Solution: Condensing temp

Thinking Green

Compressor Loads After calculating the compressor load based on the system operating cycle, some HVACR technicians in the past would add 5% to 10% to the load as a "safety factor" to ensure the system components would not be undersized. Generally, this extra safety factor is unnecessary. The resulting oversized components are an inefficient use of energy. Determining the load based on the operating cycle typically provides a sufficient safety factor. In addition, heat loads are calculated based on the most unfavorable ambient temperature conditions. This provides an additional safety factor for most days of the year.

= Ambient temp + TD = 100°F + 20°F = 120°F

Compressor Capacity Tables

Condensing Temperature Compressor capacity is affected by both the suction line temperature and the condensing temperature. The condensing temperature is determined from the characteristics of the condenser, including the condenser type and the ambient design temperature. Condensers can be air cooled or water cooled. For commercial refrigeration systems, air-cooled condensers are commonly used. Air-cooled condensers in commercial systems generally have a temperature difference (TD) of 10°F to 30°F (5°C to l7°C). The condensing temperature is the ambient design temperature plus the TD. Example: A rooftop air-cooled condenser has a TD of 20°F (11°C). The ambient design temperature is 100°F (38°C). Determine the condensing temperature.

Compressor manufacturers provide tables listing compressor capacities. The capacity of a compressor is determined by three factors: • Refrigerant type. • Suction line temperature. • Condensing temperature. To select a compressor for a given load and conditions, first locate tables for available compressors. Then check the compressor capacities based on the refrigerant type, suction line temperature, and condensing temperature. See Figure 51-4.

51.1.2 Selecting a Condenser A condenser is selected with the compressor. Sometimes, a single condensing unit (consisting of a compressor, condenser, and liquid receiver) is selected. In other cases, the compressor and condenser are selected as separate units. A condenser is selected based on the total heat of rejection (THR). THR comprises the total heat load for the system and the energy added to the refrigerant by the compressor. As a result, the required capacity for the condenser is greater than the required capacity for the compressor or the evaporator. An estimation is often used to determine the energy added to the refrigerant by the compressor.

2

5,200

4,500

9,100

8,200

18,000

16,800

22,800

21,200

5

14,200

12,500

24,800

22,400

41,700

39,300

62,400

58,500

10

31,000

26,000

44,800

43,600

81,000

75,000

120,000

112,000

25

70,000

56,000

96,000

85,000

188,000

174,000

283,000

263,000

50

122,000

100,000

188,500

159,500

375,000

350,000

585,000

550,000

Goodheart-Willcox Publisher

Figure 51-4. Compressor capacities for a given refrigerant at various suction line temperatures and at two different condensing temperatures. These capacities are only examples. Always refer to the capacity values provided by the compressor manufacturer. Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 51 Commercial Refrigeration Component Selection One estimating method multiplies the nominal horsepower of the compressor by 3,000 Btu/hr-hp. A second method of estimating the THR multiplies the compressor capacity by a factor to account for the added energy. The factor used in the calculation depends on the compressor type: For an open-drive compressor: THR = Compressor capacity x 1.25 For a hermetic compressor: THR = Compressor capacity x 1.30

Once the THR is estimated, the condenser is selected based on capacities provided in manufacturers' tables. The design of the condenser is also important. The refrigerant tubes at the core of the condenser reject heat to the surrounding air by convection through the condenser fins attached to the tubes. Condensers are categorized by size and the number of fins per inch (FPI) attached to the refrigerant tubes. See Figure 51-5. The following are some of the factors affecting a condenser's ability to transfer heat:

1383

•

Surface area. The greater the number of fins per inch (FPI), the greater the surface area of the condenser and the greater the heat transfer. • Temperature difference (TD). The greater the TD between the condenser and the cooling medium (air or water), the greater the heat transfer. • Air/water velocity. The greater the velocity of the cooling medium, the greater the heat transfer. Condensers are categorized by their cooling medium. Air-cooled condensers are commonly used for commercial refrigeration systems. Water-cooled condensers are also used. Water-cooled condensers have a much greater capacity than air-cooled condensers. The heat transfer rate for air-cooled condensers varies between 1 Btu/ hr·ft2·°F and 4 Btu/hr·ft2· F. The heat transfer rate for water-cooled condensers varies between 100 Btu/ hr·ft2·°F and 500 Btu/hr·ft2· F. The capacity of a watercooled condenser requires the good thermal contact between the shared heat exchanger surface area of the cooling medium and the refrigerant. The flow rate of the water also has a significant impact on the condenser's capacity. See Figure 51-6. 0

0

Condenser Heat Transfer Rates (Btu/hr· 0 TD) R-22 and R-410A

R-404A

Model 8 FPI

10 FPI

12 FPI

14 FPI

8 FPI

10 FPI

12 FPI

14 FPI

A

9,400

10,300

10,800

11 ,800

9,200

10,100

10,600

11 ,600

B

21,600

22,900

23,800

24,500

21,200

22,400

23,300

24,000

C

32,400

34,400

35,700

36,800

31,800

33,700

35,000

36,100 Gaadheart-Willcax Publisher

Figure 51-5. Table of heat transfer rates for three different condenser models. A condenser's ability to transfer heat varies based on the number of fins per inch (FPI) on the condenser and the type of refrigerant used in the system. Heat transfer rates are listed per degree of TD, so the values in the table must be multiplied by the condenser TD to determine the condenser capacity in Btu/hr.

Condenser Capacity (Btu/hr) Temperature Difference (°F)

Water Flow Rate (gpm)

15

20

25

30

35

40

2

11 ,000

14,500

17,900

21 ,300

24,500

27,600

6

23,800

31,500

38,900

46,100

53,100

59,900

10

31 ,500

41,700

51 ,500

61 ,200

70,600

79,700

Gaadheart-Willcax Publisher . Figure 51-6. Table of capacities for a specific water-cooled condenser model. Capacity increases as the water flow rate (listed in gallons per minute) increases and as the temperature difference (TD) increases. Copyright Goodheart-Willcox Co., Inc. 2017

1384

Modern Refrigeration and Air Conditioning

•

51.1.3 Selecting an Evaporator An evaporator removes heat from the conditioned space only when the compressor is running. Therefore, the heat-removing capacity of the evaporator is calculated on the same operating cycle as that of the compressor. Evaporators are selected using capacity data provided by the manufacturer. See Figure 51-7. The capacity of the evaporator should be matched to the capacity of the compressor. When balancing the capacity of the compressor and the evaporator, a technician must make calculations for each based on the same suction line temperature. As the suction line temperature decreases, the capacity of the evaporator increases to remove an increased amount of heat (greater TD).

Factors Affecting Evaporator Capacity When calculating the capacity of evaporators, a technician should rely on manufacturer specifications. Manufacturers obtain their heat capacity values from actual experiments. Such factors as poor circulation, frosted fins, air turbulence around the evaporator, and even the amount of moisture in the air can affect the capacity of the evaporator. One of the laws of thermodynamics is that heat always flows from an object at a higher temperature to an object at a lower temperature. The ability of an evaporator to absorb heat from the conditioned space depends on many variables, including the following:

EV-1

Surface area. The greater the number of fins per inch (FPI), the greater the surface area of the evaporator and the greater the amount of heat transfer. • Temperature difference (TD). The greater the TD, the greater the amount of heat transfer. • Heat conductivity of the material. The greater the conductivity of the material, the greater the heat transfer. • Thickness of material. The thinner the material, the greater the heat transfer. • Time. The greater the amount of time the compressor is running, the greater the heat transfer. Evaporators are made of materials that are good conductors of heat. For air-cooling evaporators, the heat must pass through an air film on the evaporator tubing. Then, the heat travels through the evaporator tubing and the oil or liquid refrigerant film on the inside of the tubing to the refrigerant. Heat is also absorbed by the metal fins of the evaporator and transferred by conduction to the tubing, Figure 51-8. If the air is moved rapidly, heat flow to the evaporator tubing is greater. More air contacts the tubing per unit of time, and the air film is thinner. If the oil or refrigerant film inside the tubing is moved faster, the film will be thinner. Generally, the denser the fluid is, the greater the heat flow. Likewise, the faster the fluid motion is, the greater the heat flow. Heat transfer rates for the three primary types of evaporators vary significantly:

825

220

4,100

4,300

4,700

7,100

EV-2

2

1,565

440

5,600

5,900

6,400

9,700

EV-3

3

2,340

660

12,200

12,800

14,000

21,000

EV-4

4

3,120

880

16,300

17,100

18,700

28,100

EV-6

6

4,575

1,320

24,500

25,700

28,000

42,300 Goodheart-Willcox Publisher

Figure 51-7. Sample of performance data for various forced-draft evaporator models. Manufacturers provide capacity ratings for their evaporators. Select an evaporator and compressor that have matching capacities.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 51 Commercial Refrigeration Component Selection Air film

Refrigerant

1385

comparison purposes. Always refer to the manufacturer's data for the specific unit.

Tubing

Conditioned space

Oil or liquid refrigerant film

Temperature Change

Example: What is the approximate capacity of a naturaldraft evaporator (a simple cold plate that absorbs heat from the surrounding area) with a surface area of 15 ft2, a refrigerant temperature of 22°F (-6°C), and an average cabinet temperature of 42°F (6°C)? Solution: To find the evaporator capacity, multiply the heat transfer rate for a natural-draft evaporator (1 Btu/ hr-ft2-°F) by the surface area (15 ft 2) and the temperature difference (20°F): Evaporator capacity= 1 Btu/hr-ft2-°F x 15 ft2 x 20°F Evaporator capacity= 300 Btu/hr

Forced-Draft Evaporator Capacities Conditioned space

Tubing

Air film

Refrigerant

Oil or liquid refrigerant film Goodheart-Willcox Publisher

Figure 51-8. Heat transfer from conditioned space to refrigerant through evaporator tubing. Heat passes through an air film, the tubing wall, and an oil or liquid refrigerant film.

•

Natural-draft evaporators-approximate rate of 1 Btu/hr.ft2- F. • Forced-draft evaporators-approximate rate of 3 Btu/hr.ft2- F. • Liquid-cooling evaporators-approximate rate of 15 Btu/hr.ft2-°F to 150 Btu/hr.ft2- F. The heat transfer rate of a specific evaporator may vary from these approximations, but the approximate value may be helpful for general estimates and

A forced-draft evaporator is an evaporator with an electric fan mounted near it to increase airflow. The evaporator fan is rated in size by how much air it is able to move in a given time. The common rating is cubic feet per minute (cfm). In a forced-draft evaporator, a large quantity of air flows over the evaporator. This airflow increases the capacity of the evaporator because a greater volume of air comes in contact with the evaporator over time. The table in Figure 51-9 shows a sample of typical forced-draft evaporator performance data.

0

0

0

Liquid-Cooling Evaporator Capacities The heat transfer rate of liquid-cooling evaporators can range from 15 Btu/hr-ft2-°F to 150 Btu/hr-ft2-°F. The heat transfer rate is affected by the cooled liquid velocity, evaporator construction, and temperature difference. Liquid velocity is usually measured in feet per minute (fpm). The table in Figure 51-10 illustrates the significant effect that water velocity and TD can

Forced-Draft Evaporator Performance Data Model

Fan Size (in)

Airflow (cfm)

Total Surface Area-Fins and Tubes (in 2 )

Cooling Capacity (Btu/hr) 15°F TD

25°FTD

EV-12

12

620

35

2,200

4,500

EV-15

15

1,200

78

5,200

9,000

EV-17

17

2,020

98

6,500

10,500 Geodhe,rt-W111w, P,,b/,he,

Figure 51-9. Sample of forced-draft evaporator cooling capacities and other performance information.

Copyright Goodheart-Willcox Co., Inc. 2017

I

1386

Modern Refrigeration and Air Conditioning

Liquid-Cooling Evaporator Heat Transfer Rates (Btu/hr-ft 2 ·°F) Water Velocity (fpm)

Temperature Difference (°F) 6

8

10

12

15

150

67

76

83

90

97

200

83

95

103

110

118

250

97

109

115

122

129

300

103

115

123

130

138 Goodheart-Willcox Publisher

Figure 51-10. As water velocity and temperature difference increase, the heat transfer rate also increases. For this particular evaporator, if the water velocity is reduced from 250 fpm to 150 fpm, the cooling capacity of the evaporator is reduced by about 30%.

have on the evaporator's heat transfer rate and, subsequently, on the evaporator's cooling capacity.

51.1.4 Liquid Receiver Sizing Liquid receivers for a commercial system should be selected to be 15% larger than the total liquid volume in the system. This practice is a safety measure in the event the refrigerant circuit becomes restricted. The restriction could be a clogged filter or frozen moisture stuck in a metering device's valve opening. Figure 51-11 shows liquid capacity for various models. Note that the capacity of a liquid receiver varies for different refrigerants.

51.2 Calculating Theoretical Compressor Volume In the previous section, compressor capacity was determined based on manufacturer information. The volume of refrigerant vapor moved through

a compressor can also be calculated mathematically. For a reciprocating compressor, the volume of vapor is determined by the bore (cylinder diameter), stroke (distance traveled by a piston), number of cylinders, speed of the compressor (rpm), and volumetric efficiency. Using this information, a technician can calculate the heat-removing capacity of the compressor. As the crankshaft of the compressor completes one revolution, the piston moves from the lowest point of its travel (bottom dead center) to the highest point (top dead center) and back to the lowest point again. On the downstroke, low-pressure refrigerant vapor is drawn into the cylinder. The vapor fills the space between the top of the piston and the head of the cylinder. The piston compresses this vapor on the upstroke. It pushes the vapor through the exhaust valve into the high-pressure side of the system. The volume displaced by the piston on each stroke (movement from bottom dead center to top dead center) is the volume of refrigerant vapor moved through the compressor. See Figure 51-12.

Liquid Receiver Data Capacity (lb) Model

Diameter (in)

Length (in)

Inlet/Outlet Size R-134a, R-22

R-404A, R-507

LR-A

4

10

1/4

4.0

3.6

LR-B

5

10

1/4

6.0

5.4

LR-C

5

10

3/8

6.0

5.4

LR-D

6

12

3/8

10.0

9.0

LR-E

6

18

1/2

16.0

14.4 Goodheart-Willcox Publisher

Figure 51-11. Liquid receiver capacities. Capacity depends on the type of refrigerant in the system.

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 51 Commercial Refrigeration Component Selection

1387

Solution:

Bore (D)

1tD2

V=-xSxNxR 4

3.1416 X (2") 2 ,, = x 2 x 2 x 400 rpm 4 = 3.1416 in 2 x 2" x 2 x 400 rpm = 5,027 in 3/min

Top dead center

t

Stroke (S)

i

•

This volume can be converted into cubic feet per minute (1,728 in3 = 1 ft 3): V = 5,027 in 3/min x

1ft3 1,728 in3

= 2.91 ft 3 /min

51.2.1 Volumetric Efficiency

Goodheart-Willcox Publisher

Figure 51-12. Stroke is the distance a piston travels from bottom dead center to top dead center. The volume of vapor displaced by the piston can be calculated by multiplying the stroke by the surface area of the piston head.

Formula for volume of vapor moved per minute through a compressor:

The actual volume of vapor pumped through a compressor is always less than the calculated theoretical volume. A compressor's volumetric efficiency is the actual volume of vapor pumped divided by the theoretical volume of the compressor cylinder. This value is then multiplied by 100 to obtain a percentage. Formula for volumetric efficiency:

. . . Actual volume Volumetnc efficiency= - - - - - - - - x 100 Theoretical volume

1tD2

V=-xSxNxR 4

where V = volume (in3) pumped per minute D = diameter of cylinder (inches) S = length of stroke (inches) N = number of cylinders R = revolutions per minute (rpm) This formula for volume simply calculates the surface area of the piston head, 1tD2/ 4, and multiplies it by the length of the stroke (S). The resulting product is the displacement volume. Next, this value is multiplied by the number of cylinders (N). Then it is multiplied by the revolutions per minute of the compressor (R). This gives the total volume in cubic inches pumped per minute. The formula presented in this section calculates the theoretical compressor volume. The actual volume of vapor moved through a compressor is always less than this theoretical volume as a result of manufacturing tolerances and friction losses.

Example: How much vapor will a two-cylinder compressor pump at 400 rpm if it has a 2" bore and a 2" stroke?

Example: A compressor is designed to pump 10 in3 of vapor each revolution. If it pumps only 6 in3 each revolution, what is the volumetric efficiency of the compressor? Solution:

Volumetric efficiency =

Actual volume h . x 100 T eorehca1 vo1ume

= 6 in3

X

100

10 in3 =60%

51.2.2 Factors Affecting Volumetric Efficiency For efficient compressor operation, the volumetric efficiency must be as high as possible. Several condi- . tions affect volumetric efficiency: • Head pressure. The greater the head (high-side) ' pressure, the lower the volumetric efficiency. The compressed vapor in the clearance space (space

Copyright Goodheart-Willcox Co., Inc. 2017

1388

Modern Refrigeration and Air Conditioning

within the cylinder when the piston is at top dead center) expands on the piston downstroke. The greater the head pressure, the greater the amount of vapor remaining in the cylinder and not being discharged. •

Low-side pressure. As low-side pressure decreases, so does volumetric efficiency. The lower pressure results in a lower amount of vapor in the cylinder for each stroke.

•

Clearance space. The larger the clearance space, the lower the volumetric efficiency. A larger clearance space allows a greater amount of compressed vapor to remain in the cylinder.

•

Valve openings. The size and condition of valve openings affect volumetric efficiency. The intake and exhaust valves offer restrictions to the vapor flow. If the intake valve reduces low-side vapor flow into the cylinder, the cylinder will not be completely filled and the efficiency will be lowered. If the exhaust valve sticks, the extra pressure generated in the cylinder will reduce the compressor's efficiency. A reduction in efficiency also occurs if the line from the compressor to the condenser is pinched or otherwise restricted.

•

Compressor speed. The compressor piston travels so fast that it prevents the vapor from filling the cylinder chamber completely. Therefore, there are losses in efficiency. The pressure in the cylinder never gets as high as the pressure in the suction line during the suction stroke. The greater the speed of the compressor, the less vapor pumped per stroke.

•

Compressor heat. The compressor runs at a warm temperature. Some of this heat warms the vapor as it enters the cylinder, causing the vapor to expand. This reduces the amount of vapor pulled into the cylinder.

•

Leaking vapor. Some vapor may leak past the piston into the crankcase, resulting in a reduction in the amount of vapor being discharged by the compressor. Small compressors used in domestic refrigeration have a bore and stroke of about 1 1/2" (4 cm). Their volumetric efficiency varies between 40% and 75%, with 60% being an average value. Larger commercial compressors have volumetric efficiencies from 50% to 80%. The average value is 70%. A volumetric efficiency of 60% to 65% is common in air-cooled units.

51.3 Designing Piping When designing piping for commercial refrigeration systems, technicians must take into account practical considerations. For example, refrigerant lines

should be arranged so that they do not interfere with normal inspection or service. Do not run lines that obstruct the view of sight glasses or interfere with the removal of compressor components, such as cylinder heads and end bells. There should also be adequate clearance between refrigerant lines and walls for installing support hangers or insulation. Before sealing insulation around a refrigerant line, test the line's joints and fittings for leaks. Commercial refrigeration systems have several different types of refrigerant lines, including the compressor discharge line, hot-gas bypass line, condenser condensate line, liquid line, and suction line. While these different lines are sized in a similar manner, the key sizing criteria vary based on the purpose of the line and the state of the refrigerant in it. The basic criteria that must be considered when sizing refrigerant lines include the following: •

Pressure drop.

•

Refrigerant velocity.

•

Oil circulation.

51.3.1 Pressure Drop As fluid travels through a pipe, the pressure of the fluid decreases due to friction between the fluid and the walls of the pipe. This pressure drop occurs in refrigerant piping, in airflow through duct systems, and in water flow through plumbing systems. In refrigeration systems, pressure drop in refrigerant lines should be kept to a minimum. This means that lines should be short and direct with as few fittings as possible. For most lines, the maximum allowable pressure drop is equal to a saturation temperature drop of 2°F (1°C). The actual amount of pressure drop (in psi or kPa) that is equal to a 2°F drop in saturation temperature varies based on the type of refrigerant. For example, two different refrigeration systems operate at an evaporator saturation temperature of 20°F (-7°C), but one uses R-134a as a refrigerant and the other uses R-404A. The pressure of R-134a at 20°F is 18.4 psi (127 kPa), and its pressure at 18°F (-8°C) is 17.0 psi (117 kPa). Thus, a 2°F drop in saturation temperature from 20°F to 18°F is equal to a pressure drop of 1.4 psi (10 kPa) for R-134a. For R-404A, however, the pressure at 20°F is 56.8 psi (392 kPa), and the pressure at 18°F is 54.2 psi (374 kPa). This means that R-404A can have a 2.6 psi (18 kPa) pressure drop for the same 2°F drop in saturation temperature. Pressure drop is affected by refrigerant line diameter, line length, and number and types of fittings used. Smaller lines have a greater pressure drop. If refrigerant lines are too small, they create excessive

Copyright Goodheart-Willcox Co., Inc. 2017

Chapter 51 Commercial Refrigeration Component Selection pressure drop and cause a reduction in system efficiency. Increasing the line size decreases the pressure drop. The length of the refrigerant line also affects pressure drop. As the length of the line increases, the pressure drop increases. Each bend, fitting, and valve in the line causes additional pressure drop. Pressure drop due to fittings is usually expressed as an equivalent length of line. See Figure 51-13. The equivalent length for each fitting and valve is added to the length of the line itself to find the line's total equivalent length. When determining pressure drops by looking up line lengths in a sizing table, remember to determine the line's total equivalent length, not just the length of the line itself. Neglecting the values of the valves and fittings can result in improper refrigerant line sizing.

51.3.2 Refrigerant Velocity The velocity of the refrigerant in the refrigerant lines is a significant concern when determining line sizes. The refrigerant velocity must be sufficient to ensure that oil is properly circulated throughout the system. If the refrigerant velocity is too low, excessive amounts of oil can be trapped in certain parts of the system. The required minimum velocity for a refrigerant line depends on the size of the line, the type of refrigerant, and the temperature of the refrigerant. Some manufacturers provide a chart for determining refrigerant velocity based on these three variables. This chart can be used to find the refrigerant velocity in different

1389

types of refrigerant lines, such as the suction line, liquid line, and compressor discharge line, Figure 51-14. For example, to use the chart in Figure 51-14 to determine suction line velocity, start by finding the system's capacity at the top of the chart, which is 5.5 tons. Next, draw a vertical line down from 5.5 tons until it intersects with the line that corresponds with the system's evaporator temperature, which is -40°F (-40°C) in this example. At the intersection of those two lines, draw a horizontal line across to the different tubing sizes on the left-hand side of the chart. After choosing one of the tubing sizes (5 1/8" for example), draw a vertical line down from where the horizontal line and tubing size intersect. At the bottom of the chart, the vertical line intersects with a line indicating the refrigerant velocity for that tubing size. Thus, the chart shows that a 5 1/8" suction line with an evaporator temperature of -40°F has a velocity of 730 fpm (feet per minute). If the velocity is too low or too high for a certain application, choose a different tubing size and draw a new line down to find the velocity for that size. There is a tradeoff between pressure drop and refrigerant velocity. Larger lines cause less pressure drop, but they require a higher velocity to ensure that oil flows with the refrigerant. Likewise, low-temperature lines require higher velocities than high-temperature lines because oil is more viscous and flows less easily at lower temperatures. In general, commercial refrigeration systems should be designed with low refrigerant velocities to ensure efficient compressor performance and low operating costs.

1/2

1.4

0.7

2.7

0.9

17

6

0.6

7/8

2.0

0.9

4.0

1.4

22

9

0.9

1 5/8

4.0

2.1

8.0

2.6

43

18

1.8

21/8

5.0

2.6

10.0

3.3

84

35

3.2

31/8

7.5

4.0

15.0

5.0

84

35

3.2

41/8

10.0

5.2

21 .0

6.7

120

47

4.5

61/8

16.0

7.9

30.0

10.0

170 70

7.0 Gaadheart-Willcax Publisher

Figure 51-13. Pressure drop in a refrigerant line is based on the total equivalent length of the line. To determine total equivalent length, add an equivalent length for each fitting and valve to the actual length of the line. Copyright Goodheart-Willcox Co., Inc. 2017

I '

1390

Modern Refrigeration and Air Conditioning HFC-134a Refrigerant Velocity in Lines (65°F Evap. Outlet)

Tons of Refrigeration 0.1

0.2

0.4 0.6

2

4

6

10

20

40 60

100

;

/

/

I/

V

;

/

/

//

//

; ;

V / i/ '

/ I/ / / V .,"/ /V , ; ,:,, /

/ / I/

/

/ //

'/ /

;

/

V/.,

V

/

; ;

/

I/

/

/

V

//

/

V/ V /

I/

/

/

;

/

/.,

/

// V /

/ / /., / / /

.,

/ /

;

/

/

;

/

/

/ V

/ / // / //

.,

V. /

'/

I:) / 1,.

/

/

/

/

o~ / 7-o'!- . ...., " '\ t. ~t..:..3..L.1 o.,o·,\. ·· , ~ ,1...: .Q~/ I . ·; , l.. f.f..lr ·;,' \J'

600

~

DuPont111 Suva' 95 (R-5088)

-

Pressure-Enthalpy Diagram

400

(Engineering UnilS)

L

~?, ,J.. · ·v \ 1

-c. _ ••• "'-·

!"'

I

-·

L...----r---r---r---,----,----,----,--~-Vy~ Y;;;r--;,tr!A .Ii r;

I I j /

I

I

//1,/1111,' l'J' 200 ~ +--+-+-+-+--t--t-.,l'+·-c/~ /:l'+-F......,'/~ _!.f. 'f-11-,t~,"'.,

•

.J ,;:,-i '\

f f, /\ A-X. ·/

1~( / / } ,'

\

~

1

.

• • • • • \ : '\

·-x ·\1\/

I I II

l \ ,., .

•a !:::,

•

'

100 80

/

I

60

/

I

I

I / II 1

/

i ' /I 1T

1..,1

~

/ / H

j

,

,

'/

J

I7.

20

~7>-~-rt/~,r-f 1 I I II ' I I '

:P

J I 'I I I I I I /I I I // I I I/ I V I -140

,~-.-j~-:t~

,-,-r.'i'