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Summary

SRC-W & SW series compressors Summary (WA-00-08-E)

Summary .......................................................................................................................WA-00-07-E General...........................................................................................................................WA-01-03-E Oil management ............................................................................................................WA-02-06-E Capacity Control...........................................................................................................WA-03-02-E Components...................................................................................................................WA-04-03-E Electrical devices...........................................................................................................WA-05-08-E Model Designation & Technical data..........................................................................WA-06-03-E Extend of delivery.........................................................................................................WA-07-04-E Dimensional Drawings .................................................................................................WA-08-03-E Performance data..........................................................................................................WA-09-03-E Application range .........................................................................................................WA-10-02-E Additional cooling.........................................................................................................WA-11-03-E Economiser ....................................................................................................................WA-12-03-E Parallel compounding...................................................................................................WA-13-03-E General design recommendation.................................................................................WA-14-02-E Maintenance ..................................................................................................................WA-15-02-E Capacity Control conversion .......................................................................................WA-16-03-E

SRC-WS & WS Series - Application and maintenance manual, Technical sheet WA-00-08-E– data subject to change

General

SRC-W & SW series compressors General (WA-01-03-E)

1. 1.1 1.2 1.3 1.4

GENERAL INTRODUCTION COMPRESSION PROCESS THE ROTORS BUILT-IN VOLUME RATIO

SRC-WS & WS Series - Application and maintenance manual, Technical sheet WA-01-03-E – data subject to change

2 2 3 4 4

1

General 1. General 1.1

Introduction

Including a total amount of 36 screw compressors in semi-hermetic execution with external oil separator, the SRC-W and SW series cover the power range from 30 to 240 Hp and a displacement range from 118 to 700 m3/h at 50 Hz. SRC-W and SW compressor series are available with two different intrinsic volumetric ratios (Vi) optimised either for low and medium/high evaporating temperature. The user has therefore the possibility of choosing the ideal compressor depending on the particular application. As a consequence, the maximum compression efficiency is always performed. The use of an external oil separator leads to the highest flexibility in the designing of racks with a number of compressors variable from 2 to 6 with one common oil separator (parallel compounding). Beside this, the oil cooling widens the application limits up to the hardest operating conditions. A complete series of accessories for the oil return line (between the oil separator and the compressor) is provided in the standard delivery. Furthermore, a complete range of oil separators and oil coolers is also available. The low vibrations level and the absence of discharge gas pulsation make the use of anti-vibrating dampers and flexible pipes not necessary. In addition, the extremely low noise level, concentrated in the medium-high frequency range, is very easy to insulate The SRC-W and SW series compressors feature an oil–injected helical twin screw design; the male rotor is directly connected to the electrical motor (2 poles, 3000 rpm) and drives in turn the female rotor. The perfect rolling behaviour leads to an extremely smooth running. The innovative design of rotors profile (5 lobes male rotor, 6 flutes female), the high quality production standard and the use of superior quality mechanical components lead to high compression efficiency, high reliability and long operating life. The hydraulic slide valve and by-pass capacity control lead this compressor series to a high compression efficiency during part load operation. This makes the SRC-W and SW series particularly suitable for applications where long periods of part load operation are required reducing also the number of starts. The use of the subcooling economiser circuit (ECO) leads to a further increase in the cycle efficiency (COP).

Picture 1-A: Section drawing of SW compressor model 1 Suction filter 2 Electric motor 3 Rolling bearings (suction side) 4 Slide valve capacity control (SW models) 5 Rotors

2

6 Rolling bearings (discharge side) 7 Hydraulic piston for capacity control (SW models) 8 Electronic protection module 9 Terminal box 10 Suction shut-off valve

SRC-WS & WS Series - Application and maintenance manual, Technical sheet WA-01-03-E – data subject to change

General

Picture 1-B: example of SW series compressor 11 Oil pressure connection 12 Solenoid valves 13 Oil inlet connection

1.2

14 Discharge shut-off valve 15 Low pressure gas connection 16 High pressure gas connection

Compression process

The rotors are located in a horizontal housing provided with a suction port (motor side) and a discharge port. The extremely small clearances between the rotors and the housing are dynamically sealed by an oil film, which is directly injected on the rotor profiles. The compression takes place by a volume reduction resulting from the rotary motion; between the rotors and the housing several compression chambers are generated that decrease their volume and move along the axis at the same time; basically the process can be divided into three phases, see Picture 1-C (the following description is related to a single lobe of the male rotor and a single flute of the female rotor):




Suction

When the male rotor lobe and the female rotor flute begin to unmesh, the compression chamber is open to the suction side and a gas flow is led through the suction port, due to the rotary motion the chamber increase its volume and more gas is led into the chamber until this is not any longer open to the suction side.




Compression

With a further rotation the volume of the compression chamber is reduced and at the same time it is moved along the axis toward the discharge port, increasing the pressure of the refrigerant contained in the chamber.




Discharge

At a point, which is fixed by the housing geometry, the discharge port is uncovered and the compressed gas is discharged thanks to the further meshing of the lobe and the flute. Since the tooth ratio is 5/6 (5 lobes on the male rotor - 6 flutes on the female rotor) and the rotation speed is about 3000 rpm, there are 3000 x 5 = 15000 discharge processes every minute, resulting in a very low gas pulsation (a reciprocating compressor running at 1500 rpm should have 10 pistons to reach the same result). Chamber at end of suction phase

Chamber in compression phase

Chamber at start of discharge phase

Picture 1-C: Compression phases in twin screws compressors

SRC-WS & WS Series - Application and maintenance manual, Technical sheet WA-01-03-E – data subject to change

3

General 1.3

The rotors

The rotors, as shown in Picture 1-D, feature a 5/6 asymmetric profile, which has been completely developed within RefComp. The perfect rolling behaviour leads to an extremely smooth running. Picture 1-D shows the correct revolving direction, too.

Picture 1-D: rotor view, with correct revolving direction

1.4

Built-in volume ratio

The dimensions and geometry of the discharge port determine the so-called “built-in volume ratio” Vi, defined as the ratio between the chamber volume at the beginning and the end of the compression phase. This ratio, that is not dependent upon the operating conditions, corresponds to an ideal pressure ratio performing the maximum compressor efficiency: the gas leaving the discharge port has the same pressure of the discharge side. When otherwise the discharge pressure is different from the pressure of the gas leaving the discharge port, an overcompression or undercompression takes place.

Vi = Vsuction / Vdischar ge Vi = Vaspir az. / Vscarico Pdischarge = Pcond. Pscarico = Pcond

Pressure Pressione

Ideal condition Condizione ideale

Pevap

Vdischarge Vscarico

Volume

Vsuction Vaspiraz.

Vi = Vsuction / Vdischarge Vi = Vaspiraz. / Vscarico

Vi = Vsuction / Vdischarge Vi = Vaspiraz. / Vscarico Pdischarge Pscarico

Pcond. Pdischarge Pscarico

Underc om pression work La voro di sottocompre ssione

Overcompression work Lavoro di s ovrac ompressione

Pressure Pressione

Pressure Pressione

Pcond

Pevap

Pevap

Vdischarge Vscarico

Volume

Vsuction Vaspiraz.

Vdischarge Vscarico

Vo lume

Picture 1-E: Compression process in the p-V diagram

4

SRC-WS & WS Series - Application and maintenance manual, Technical sheet WA-01-03-E – data subject to change

Vsuction Vaspiraz.

General SRC-W and SW compressors series have a built-in volume ratio suitable for medium / high and low temperature applications. The Vi are respectively:



Vi = 4.4: models SW1L e SRC-WL for low evaporating temperature applications; Vi = 2.6: models SW1H e SRC-WS for medium / high evaporating temperature applications (Vi = 3.2 only for models SRC-WS 70/80).

The following charts, see Picture 1-F and Picture 1-G, shows the application limit and the best built-in volume ratio “Vi” for the requested working conditions. The application limit are obtained in function of the refrigerant condensing and evaporating temperature.

Picture 1-F: Vi application range for the refrigerant R22

Picture 1-G: Vi application range for the refrigerant R404A / R507

SRC-WS & WS Series - Application and maintenance manual, Technical sheet WA-01-03-E – data subject to change

5

Oil Management

SRC-W & SW series compressors Oil management (WA-02-06-E) 2.

OIL MANAGEMENT

2

2.1 OIL CIRCUIT 2.2 OIL FLOW RATE 2.3 LUBRICANTS 2.3.1 Approved lubricants for HCFC (R22) 2.3.2 Approved lubricants for HFC (R407C, R134a, R404A, R507) 2.3.3 Standard lubricants: operating conditions range 2.4 OIL SEPARATOR 2.5 OIL FILTER 2.6 OIL HEATER 2.7 OIL LEVEL 2.8 LUBRICATION MONITORING 2.8.1 Oil temperature monitoring 2.8.2 Static pressure control 2.8.3 Level control 2.8.4 Flow control

2 3 4 4 4 5 5 6 7 7 7 7 7 9 9

SRC-W & SW Series - Application and maintenance manual, Technical sheet WA-02-06 E– data subject to change

1

Oil Management 2. Oil management 2.1

Oil circuit

In the W series compressors the oil fulfils the following purposes:





Dynamic seal between contiguous chambers Bearing lubrication Control of slide valve for capacity modulation Cooling

The following Picture 2-A shows the oil circuit and involved components:

Picture 2-A: example of oil circuit scheme 1) 2) 3) 4) 5)

Compressor Oil separator Check valve Oil cooler Sight glass

6) 7) 8) 9) 10) 11)

Solenoid valve Oil flow switch Oil filter Shut-off valve Shut-off valve Solenoid valve

SRC-W & SW Series - Application and maintenance manual, Technical sheet WA-02-06 E– data subject to change

2

Oil Management Referring to Picture 2-A, the lubricant is stored in an external oil separator (position 2) that permits the compressor oil supply to the compressor after the separation between oil and gas mixture. The oil circulation results from the pressure difference between the oil separator - at discharge pressure - and the injection points - where the pressure is slightly higher than the suction pressure. From the reservoir the oil flows in the oil cooler (position 4), in case of take heat away is necessary, then through a filter (position 8) to the suction bearings, the injection point on the rotor profiles and the discharge-bearing chamber. The oil feeding back line is monitored by means of a flow switch (position 7), a solenoid valve (position 6) and a sight glass (position 5). At the same time, for the SW series through an external capillary, the oil is led to the slide valve control cylinder. The oil leaving the slide valve control cylinder, the suction bearings and the discharge-bearing chamber is led to the suction side of the rotors and it is then compressed through the rotors together with the suction gas and the mixture flows in the oil reservoir. The oil and gas are separated in the upper part of this vessel (position 2). The oil proportion flows downwards to the reservoir space from where it again flows to the compressor (position 1). According to the application conditions the circulating oil has to cooled down by an oil cooler. Under certain conditions direct refrigerant injection can also be used as an alternative (consult RefComp). Picture 2-A illustrates also the equalization pressure line which permit to reduce the pressure value inside the oil separator during the stop time, so it’s possible:
to start the compressor at minimum capacity
to reduce the dilution of refrigerant in the lubricant. For this reason, it’s necessary to use a check valve between the oil separator and the condenser, and to use a solenoid valve to control the equalization pressure line from the oil separator to the compressor. This equalization line can be opened only when the plant is stopped or, in case of compounding system, when all the compressors are switched off.

2.2

Oil flow rate

Since the oil circulation is generated by a pressure difference, the oil flow rate depends upon the difference between discharge and suction pressure, according to the following equation:

VOIL = K ⋅ PS − PA Where: VOIL = volumetric oil flow rate through the oil filter

[l/min]

K = coefficient dependent upon compressor model (see Table A) PS = discharge pressure

[bar]

PA = suction pressure

[bar]

SW-1H SW-1L SRC-WS SRC-WL K

4000 3000 40 30

5000 4000 50 40 5,5

6000 5000 60 50

7500 6500 70

9000 8000 80 6

10500 9500

11500 10500

12500 11500

14000 13000

16000 15000

19000 17000

6,75

21000 20000

24000 22000

25000 23000

7,5

Table A: K coefficient

The minimum oil flow rate to fulfil all the purposes (lubrication - sealing - slide valve control) is ensured when the compressor operates within the fixed application range. During the starting phase, since pressures are always equalised in the compressor, there is no oil circulation; however bearings and rotors are designed to tolerate some seconds of dry operation before the necessary pressure difference is reached.

SRC-W & SW Series - Application and maintenance manual, Technical sheet WA-02-06 E– data subject to change

3

Oil Management Attention!  Within 20 seconds after start the compressor must operate inside the application range (minimum pressure difference)

In case of air-cooled condenser and low ambient conditions, and in general when the minimum pressure difference is not easily reached, it may be necessary to adopt special measures such as:




2.3

delayed start of condenser fans pressure regulating valve between compressor and condenser (consult RefComp for further information) external oil pump (consult RefComp for further information)

Lubricants

The lubricants have been chosen based on the following requirements:





Sealing of the gaps Adequate bearing lubrication Good viscosity behaviour at high temperatures and pressures Good miscibility at low temperatures Attention!  Do not charge other lubricants than the suggested ones. 

The following oils are hygroscopic and must not come in contact with air.

2.3.1 Approved lubricants for HCFC (R22) Brand/Supplier Marca/Fornitore

Name Nome

Chemical composition Composizione chimica

Density Densità 15°C [g/ml]

Cinematic viscosity Viscosità cinematica 40°C [cSt]

CPI

CP 4214150

TOTAL FINA ELF

ESSO

Flash point Pour point Punto di Punto di infiammabilità scorrimento [°C] [°C]

Flock point Punto di flocculazione [°C]

Complex ester Estere complesso

1.01

168

290

-43

None Nessuno

Lunaria SK 100

Alchilbenzene Alchilbenzenico

0.874

94

200

-33