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Engro Fertilizers Limited | Daharki Internship 2011 | Project Report

Cooling Tower Chemistry and Performance Improvement Prepared for Training Department Engro Fertilizers Limited (EFERT) Daharki, District Ghotki, Sindh

Prepared by Osama Hasan Operations (URUT III) Intern School of Chemical and Materials Engineering (SCME) National University of Sciences and Technology (NUST) Email: [email protected] Contact: 03453034516

August 2011

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

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1 Transmittal August 24, 2011 Mr. Jehangir Alam Khan Internship Coordinator Training Department Engro Fertilizers Daharki Limited Dear Sir Please find enclosed the internship report due August 24, 2011. The report as requisite by your office has been drafted on the assigned project “Study the Cooling Tower Chemistry and Identify Key Parameters for Improving Performance”. The report discusses the cooling tower design, chemistry and performance parameters along with the suitable recommendations for the assigned project. Feedback will be most appreciated. Kind Regards

Osama Hasan Intern Operation (URUT III) Undergraduate Student at School of Chemical and Materials Engineering (SCME) National University of Sciences and Technology (NUST) H – 12 Islamabad – 44000 2008 – NUST – BE – Chem – 27 Email: [email protected] Mobile: 03453034516

Countersigned Amer Ahmed (Mentor) Shift Supervisor URUT III

Asim Rasheed Qureshi (Group Leader) Unit Manager URUT III

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

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2 Acknowledgement Author is thankful to Almighty Allah, For His unlimited blessings and bounties, And for keeping him sane, sound and successful; His parents and friends, For all their support and trust in him and his aims; His teachers and guides, For teaching him things he knew not; NUST Career Development Centre, For bringing the opportunity of this excellent learning and exposure; And last and the most important Management and Employees of Engro Fertilizers Limited Especially his mentor Mr. Amer Ahmed and Unit Manager Mr. Asim Rasheed Qureshi And all the shift coordinators, supervisors, trainee engineers, boardmen and area operators at Plant II For their utmost help, guidance and time Which made author make most of his internship at plant site;

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

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3 Table of Contents 1

Transmittal............................................................................................................................... 2

2

Acknowledgement ................................................................................................................... 3

3

Table of Contents..................................................................................................................... 4

4

List of Figures ........................................................................................................................... 6

5

List of Tables ............................................................................................................................ 6

6

List of Equations ...................................................................................................................... 6

7

Abstract.................................................................................................................................... 7

8

Introduction ............................................................................................................................. 8

9

Cooling Tower .......................................................................................................................... 9 9.1

Components ..................................................................................................................... 9

9.2

Materials ........................................................................................................................ 11

9.3

Types .............................................................................................................................. 12

9.3.1

Natural draft cooling tower .................................................................................... 12

9.3.2

Mechanical draft cooling tower .............................................................................. 12

9.3.3

Open vs. Closed-Circuit Towers .............................................................................. 13

9.3.4

Hybrid Towers ......................................................................................................... 13

9.4

Performance ................................................................................................................... 15

9.5

Assessment..................................................................................................................... 18

9.6

Factors Affecting Performance ...................................................................................... 18

9.6.1

Design...................................................................................................................... 18

9.6.2

Fill media effects ..................................................................................................... 24

9.6.3

Water Distribution .................................................................................................. 25

9.6.4

Fans ......................................................................................................................... 25

9.7 10

General Improvement Procedures................................................................................. 26 Cooling Water Chemistry ................................................................................................... 28

10.1 Corrosion ........................................................................................................................ 28 10.1.1

Corrosion Control .................................................................................................... 29

10.1.2

Corrosion Inhibitors ................................................................................................ 29

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

Page 5 of 51 10.1.3

Inhibitor Selection ................................................................................................... 30

10.2 Scaling............................................................................................................................. 31 10.2.1

Types ....................................................................................................................... 31

10.2.2

Deposit Control Methods ....................................................................................... 32

10.3 Microbial Growth ........................................................................................................... 35 10.3.1

Problems ................................................................................................................. 35

10.3.2

Selection of Micro Biocides..................................................................................... 36

10.3.3

Oxidizing Toxicants ................................................................................................. 37

10.3.4

Non Oxidizing Biocides ............................................................................................ 40

10.4 Chemical Dosing at CT – 4 .............................................................................................. 40 11

Performance Improvement ............................................................................................... 42

11.1 Water Use....................................................................................................................... 42 11.1.1

Reduce water loss ................................................................................................... 42

11.1.2

Reduce blow down.................................................................................................. 43

11.1.3

Use alternative water supplies ............................................................................... 44

11.1.4

Reuse blow down .................................................................................................... 44

11.2 Water treatment ............................................................................................................ 44 11.2.1

Sulphuric “Acid” Treatment .................................................................................... 45

11.2.2

Side Stream Filtration ............................................................................................. 45

11.2.3

Ozone ...................................................................................................................... 46

11.2.4

Magnets .................................................................................................................. 46

11.2.5

Sonication ............................................................................................................... 47

11.2.6

Electro coagulation ................................................................................................. 47

11.2.7

Activated carbon ..................................................................................................... 47

11.2.8

Ultraviolet radiation (UV)........................................................................................ 47

11.2.9

Hydrocavitation....................................................................................................... 48

11.2.10

Radio frequencies................................................................................................ 48

12

Recommendation............................................................................................................... 49

13

References ......................................................................................................................... 50

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

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4 List of Figures Figure 1 Schematic of an Induced Draft Cooling Tower ................................................................. 9 Figure 2 Cooling Tower Types ....................................................................................................... 14 Figure 3 Range and approach schematic ...................................................................................... 16 Figure 4 Tower size v/s approach ................................................................................................. 22 Figure 5 Tower size v/s wet-bulb .................................................................................................. 22 Figure 6 Tower size v/s head load................................................................................................. 23 Figure 7 Tower size v/s range variance ......................................................................................... 23 Figure 8 Corrosion cell .................................................................................................................. 28 Figure 9 Biofouled Heat Exchanger............................................................................................... 35 Figure 10 Hierarchy of opportunities............................................................................................ 42 Figure 11 Hydrocavitation system ................................................................................................ 48

5 List of Tables Table 1 Types of Cooling Towers .................................................................................................. 15 Table 2 Design Values of Different Fills ........................................................................................ 24 Table 3 Chemical Dosing Rate ....................................................................................................... 41 Table 4 Chemical Dosing at CT 4 ................................................................................................... 41 Table 5 Treatment options comparison ....................................................................................... 46

6 List of Equations Equation 1 CT Range ..................................................................................................................... 15 Equation 2 CT Approach ............................................................................................................... 16 Equation 3 CT Effectiveness .......................................................................................................... 16 Equation 4 Evaporation Loss ......................................................................................................... 17 Equation 5 Blow down .................................................................................................................. 17 Equation 6 Liquid/Gas ratio .......................................................................................................... 17 Equation 7 CT Range Def. 2........................................................................................................... 18 Equation 8 Water losses ............................................................................................................... 42 Equation 9 Cycle of Concentration C.O.C. .................................................................................... 43

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

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7 Abstract Cooling towers are one of the most important industrial utilities used to dissipate the unwanted process heat to the atmosphere through the cooling water in the heat exchangers across the plant site. Cooling tower is one of the most expensive utility in terms of power consumption and water circulation. Maintaining water quality in the circulation loops is one of the major challenges in process optimization for most efficient performance. To identify the key performance parameters with respect to perspective of the operations’ team, the water chemistry is the most crucial level and demands proper understanding to maintain complete control over the variations. Latest technological developments have made the water conservation more efficient and use of chemicals more limited by introducing “Recycling / reusing water practices” and “Chemical free platforms”. With limited options available to the designed and operating cooling tower, these areas could be explored for better and cost effective performance and environment friendly impact.

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

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8 Introduction “You cannot create experience, you must undergo it” Industrial internships are incomparable experience for an undergraduate student. With fertilizer industry holding the maximum learning potential for a chemical engineer, Engro leaves an impact of its own. The six week internship experience is unique in every sense of the word. The learning opportunities and industrial exposure at the EFERT made not just possible to relate the book knowledge to field application but also in developing a thorough understanding of industrial practices and operating concepts. Enven 1.3 – the world largest single train ammonia urea complex was an amazing experience for the author. From the up to date urea complex technology to world’s tallest prilling tower, it added many landmarks in list of experience. With internship project over cooling water chemistry and performance improvement parameters, the author has compiled the information on cooling water design, chemistry and operation; which could serve as a comprehensive study aid on the subject. The recommendations generated are but most effective to date, which should be considered with economical feasibility.

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

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9 Cooling Tower Cooling towers are a very important part of many chemical plants. The primary task of a cooling tower is to reject heat into the atmosphere. They represent a relatively inexpensive and dependable means of removing low-grade heat from cooling water. The make-up water source is used to replenish water lost to evaporation. Hot water from heat exchangers is sent to the cooling tower. The water exits the cooling tower and is sent back to the exchangers or to other units for further cooling.Cooling towers are able to lower the water temperatures more than devices that use only air to reject heat, like the radiator in a car, and are therefore more costeffective and energy efficient.

Figure 1 Schematic of an Induced Draft Cooling Tower

9.1 Components The basic components of a cooling tower include the frame and casing, fill, cold-water basin, drift eliminators, air inlet, louvers, nozzles and fans. These are described below. a) Frame and casing: Most towers have structural frames that support the exterior enclosures (casings), motors, fans, and other components. With some smaller designs, such as some glass fibre units, the casing may essentially be the frame.

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

Page 10 of 51 b) Fill: Most towers employ fills (made of plastic or wood) to facilitate heat transfer by maximizing water and air contact. There are two types of fill: 

Splash fill: Water falls over successive layers of horizontal splash bars, continuously breaking into smaller droplets, while also wetting the fill surface. Plastic splash fills promote better heat transfer than wood splash fills.



Film fill: consists of thin, closely spaced plastic surfaces over which the water spreads, forming a thin film in contact with the air. These surfaces may be flat, corrugated, honeycombed, or other patterns. The film type of fill is the more efficient and provides same heat transfer in a smaller volume than the splash fill.

c) Cold-water basin: The cold-water basin is located at or near the bottom of the tower, and it receives the cooled water that flows down through the tower and fill. The basin usually has a sump or low point for the cold-water discharge connection. In many tower designs, the coldwater basin is beneath the entire fill. In some forced draft counter flow design, however, the water at the bottom of the fill is channelled to a perimeter trough that functions as the coldwater basin. Propeller fans are mounted beneath the fill to blow the air up through the tower. With this design, the tower is mounted on legs, providing easy access to the fans and their motors. d) Drift eliminators: These capture water droplets entrapped in the air stream that otherwise would be lost to the atmosphere. e) Air inlet: This is the point of entry for the air entering a tower. The inlet may take up an entire side of a tower (cross-flow design) or be located low on the side or the bottom of the tower (counter-flow design). f) Louvers: Generally, cross-flow towers have inlet louvers. The purpose of louvers is to equalize air flow into the fill and retain the water within the tower. Many counter flow tower designs do not require louvers. g) Nozzles: These spray water to wet the fill. Uniform water distribution at the top of the fill is essential to achieve proper wetting of the entire fill surface. Nozzles can either be fixed and spray in a round or square patterns, or they can be part of a rotating assembly as found in some circular cross-section towers.

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

Page 11 of 51 h) Fans: Both axial (propeller type) and centrifugal fans are used in towers. Generally, propeller fans are used in induced draft towers and both propeller and centrifugal fans are found in forced draft towers. Depending upon their size, the type of propeller fans used is either fixed or variable pitch. A fan with non-automatic adjustable pitch blades can be used over a wide kW range because the fan can be adjusted to deliver the desired air flow at the lowest power consumption. Automatic variable pitch blades can vary air flow in response to changing load conditions.

9.2 Materials Originally, cooling towers were constructed primarily with wood, including the frame, casing, louvers, fill and cold-water basin. Sometimes the cold-water basin was made of concrete. Today, manufacturers use a variety of materials to construct cooling towers. Materials are chosen to enhance corrosion resistance, reduce maintenance, and promote reliability and long service life. Galvanized steel, various grades of stainless steel, glass fibre, and concrete are widely used in tower construction, as well as aluminium and plastics for some components. a) Frame and casing. Wooden towers are still available, but many components are made of different materials, such as the casing around the wooden framework of glass fibre, the inlet air louvers of glass fibre, the fill of plastic and the cold-water basin of steel. Many towers (casings and basins) are constructed of galvanized steel or, where a corrosive atmosphere is a problem, the tower and/or the basis are made of stainless steel. Larger towers sometimes are made of concrete. Glass fibre is also widely used for cooling tower casings and basins, because they extend the life of the cooling tower and provide protection against harmful chemicals. b) Fill. Plastics are widely used for fill, including PVC, polypropylene, and other polymers. When water conditions require the use of splash fill, treated wood splash fill is still used in wooden towers, but plastic splash fill is also widely used. Because of greater heat transfer

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

Page 12 of 51 efficiency, film fill is chosen for applications where the circulating water is generally free of debris that could block the fill passageways. c) Nozzles. Plastics are also widely used for nozzles. Many nozzles are made of PVC, ABS, polypropylene, and glass-filled nylon. d) Fans. Aluminium, glass fibre and hot-dipped galvanized steel are commonly used fan materials. Centrifugal fans are often fabricated from galvanized steel. Propeller fans are made from galvanized steel, aluminium, or moulded glass fibre reinforced plastic.

9.3 Types 9.3.1 Natural draft cooling tower The natural draft or hyperbolic cooling tower makes use of the difference in temperature between the ambient air and the hotter air inside the tower. As hot air moves upwards through the tower (because hot air rises), fresh cool air is drawn into the tower through an air inlet at the bottom. Due to the layout of the tower, no fan is required and there is almost no circulation of hot air that could affect the performance. Concrete is used for the tower shell with a height of up to 200 m. These cooling towers are mostly only for large heat duties because large concrete structures are expensive. There are two main types of natural draft towers: 

Cross flow tower: air is drawn across the falling water and the fill is located outside the tower



Counter flow tower: air is drawn up through the falling water and the fill is therefore located inside the tower, although design depends on specific site conditions

9.3.2 Mechanical draft cooling tower Mechanical draft towers have large fans to force or draw air through circulated water. The water falls downwards over fill surfaces, which help increase the contact time between the water and the air - this helps maximize heat transfer between the two. Cooling rates of mechanical draft towers depend upon various parameters such as fan diameter and speed of operation, fills for system resistance etc.

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

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9.3.3 Open vs. Closed-Circuit Towers One of the primary differentiations between cooling towers is whether it is an open or closedcircuit tower. In open towers, the cooling water is pumped through the equipment where it picks up thermal energy and then flows directly to the cooling tower where it is dispersed through spray nozzles over the fill, where heat transfer occurs. Then, this same water is collected in the tower sump and is sent back to the equipment to begin the process again. In an open tower any contaminants in the water are circulated through the equipment being cooled. In a closed-circuit tower, sometimes referred to as a fluid cooler, the cooling water flows through the equipment as in the open tower. The difference is when the water is pumped to the cooling tower, it is pumped through a closed loop heat exchanger that is internal to the cooling tower, then returned to the equipment. In this application, water in the closed loop is not in direct contact with the evaporative water in the tower, which means contaminants are not circulated through the equipment. In a closed-circuit tower, a small pump, known as a “spray pump” circulates a separate body of evaporative water from the tower sump, through the spray nozzles and over the internal heat exchanger piping. This “open” evaporative body of water is contained within the tower and needs to be regularly made up to replenish evaporative and other losses. However, once water treatment in the closed cooling loop is stabilized, the only time it needs to be made up or adjusted is if there is a leak.

9.3.4 Hybrid Towers Hybrid towers are closed towers which can operate either in the sensible heat transfer mode only (without evaporation) or a combination of sensible and latent heat transfer (with evaporation). During periods of low load and/or low ambient temperature, the spray of water is stopped and heat is sensibly transferred to the flow of air across the fins of the coils containing the cooling fluid. During periods when this is not enough, a latent heat transfer system is activated by switching on an evaporative cooler or water is sprayed across the dry coils to allow for increased heat transfer through evaporation. These processes offer substantial savings in water.

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

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Figure 2 Cooling Tower Types

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

Page 15 of 51 Mechanical draft towers are available in a large range of capacities. Towers can be either factory built or field erected – for example concrete towers are only field erected. Many towers are constructed so that they can be grouped together to achieve the desired capacity. Thus, many cooling towers are assemblies of two or more individual cooling towers or “cells.” The number of cells they have, e.g., an eight-cell tower, often refers to such towers. Multiple-cell towers can be lineal, square, or round depending upon the shape of the individual cells and whether the air inlets are located on the sides or bottoms of the cells. Table 1 Types of Cooling Towers Type

Advantages

Disadvantages

 Suited for high air  Recirculation due to high airresistance due to entry and low air-exit centrifugal blower fans velocities, which can be solved by locating towers in plant  Fans are relatively quiet rooms combined with discharge ducts Induced draft cross flow  Less recirculation than  Fans and the motor drive  Water enters at top and passes over fill forced draft towers mechanism require weatherbecause the speed of proofing against moisture and  Air enters on one side (single-flow tower) or exit air is 3-4 times corrosion because they are in opposite sides (double-flow tower) higher than entering air the path of humid exit air  An induced draft fan draws air across fill towards exit at top of tower Induced draft counter flow  Hot water enters at the top  Air enters bottom and exits at the top  Uses forced and induced draft fans Forced draft Air is blown through the tower by a fan located in the air inlet

9.4 Performance These measured parameters and then used to determine the cooling tower performance in several ways. a) Range. This is the difference between the cooling tower water inlet and outlet temperature. A high CT Range means that the cooling tower has been able to reduce the water temperature effectively, and is thus performing well. The formula is: Equation 1 CT Range

𝑪𝑻 𝑹𝒂𝒏𝒈𝒆 (°𝑪) = 𝑪𝑾 𝒊𝒏𝒍𝒆𝒕 𝒕𝒆𝒎𝒑 (°𝑪) − 𝑪𝑾 𝒐𝒖𝒕𝒍𝒆𝒕 𝒕𝒆𝒎𝒑 (°𝑪) b) Approach. This is the difference between the cooling tower outlet coldwater temperature and ambient wet bulb temperature. The lower the approach the better the cooling tower

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

Page 16 of 51 performance; although, both range and approach should be monitored, the `Approach’ is a better indicator of cooling tower performance. Equation 2 CT Approach

𝑪𝑻 𝑨𝒑𝒑𝒓𝒐𝒂𝒄𝒉 (°𝑪) = 𝑪𝑾 𝒐𝒖𝒕𝒍𝒆𝒕 𝒕𝒆𝒎𝒑 (°𝑪) − 𝑾𝒆𝒕 𝒃𝒖𝒍𝒃 𝒕𝒆𝒎𝒑 (°𝑪)

Figure 3 Range and approach schematic

c) Effectiveness. This is the ratio between the range and the ideal range (in percentage), i.e. difference between cooling water inlet temperature and ambient wet bulb temperature, or in other words it is = Range / (Range + Approach). The higher this ratio, the higher the cooling tower effectiveness. Equation 3 CT Effectiveness 𝑪𝑻 𝑬𝒇𝒇𝒆𝒄𝒕𝒊𝒗𝒆𝒏𝒆𝒔𝒔 (%) =

(𝑪𝑾 𝒕𝒆𝒎𝒑 – 𝑪𝑾 𝒐𝒖𝒕 𝒕𝒆𝒎𝒑) × 𝟏𝟎𝟎 (𝑪𝑾 𝒊𝒏 𝒕𝒆𝒎𝒑 – 𝑾𝑩 𝒕𝒆𝒎𝒑)

d) Cooling capacity. This is the heat rejected in kCal/hr or TR, given as product of mass flow rate of water, specific heat and temperature difference.

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

Page 17 of 51 e) Evaporation loss. This is the water quantity evaporated for cooling duty. Theoretically the evaporation quantity works out to 1.8 m3 for every 1,000,000 kCal heat rejected. The following formula can be used (Perry): Equation 4 Evaporation Loss 𝟑 𝟑 𝑬𝒗𝒂𝒑𝒐𝒓𝒂𝒕𝒊𝒐𝒏 𝒍𝒐𝒔𝒔 (𝒎 𝒉𝒓) = 𝟎. 𝟎𝟎𝟎𝟖𝟓 × 𝟏. 𝟖 𝒙 𝒄𝒊𝒓𝒄𝒖𝒍𝒂𝒕𝒊𝒐𝒏 𝒓𝒂𝒕𝒆 (𝒎 𝒉𝒓) × (𝑻𝟏 − 𝑻𝟐 )

T1 - T2 = temperature difference between inlet and outlet water f) Cycles of concentration (C.O.C). This is the ratio of dissolved solids in circulating water to the dissolved solids in makeup water. g) Blow down losses depend upon cycles of concentration and the evaporation losses and is given by formula: Equation 5 Blow down 𝑩𝒍𝒐𝒘 𝒅𝒐𝒘𝒏 =

𝑬𝒗𝒂𝒑𝒐𝒓𝒂𝒕𝒊𝒐𝒏 𝑳𝒐𝒔𝒔 𝑪. 𝑶. 𝑪. − 𝟏

h) Liquid/Gas (L/G) ratio. The L/G ratio of a cooling tower is the ratio between the water and the air mass flow rates. Cooling towers have certain design values, but seasonal variations require adjustment and tuning of water and air flow rates to get the best cooling tower effectiveness. Adjustments can be made by water box loading changes or blade angle adjustments. Thermodynamic rules also dictate that the heat removed from the water must be equal to the heat absorbed by the surrounding air. Therefore the following formulae can be used: 𝐿 (𝑻𝟏 − 𝑻 𝟐 ) = 𝑮 (𝒉𝟐 − 𝒉𝟏 ) Equation 6 Liquid/Gas ratio 𝑳 (𝒉𝟐 − 𝒉𝟏 ) = 𝑮 (𝑻𝟏 − 𝑻 𝟐 )

Where: L/G = liquid to gas mass flow ratio (kg/kg) T1 = hot water temperature (°C) T2 = cold-water temperature (°C)

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

Page 18 of 51 h2 = enthalpy of air-water vapour mixture at exhaust wet-bulb temperature h1 = enthalpy of air-water vapour mixture at inlet wet-bulb temperature

9.5 Assessment The performance of cooling towers is evaluated to assess present levels of approach and range against their design values, identify areas of energy wastage and to suggest improvements. During the performance evaluation, portable monitoring instruments are used to measure the following parameters:        

Wet bulb temperature of air Dry bulb temperature of air Cooling tower inlet water temperature Cooling tower outlet water temperature Exhaust air temperature Electrical readings of pump and fan motors Water flow rate Air flow rate

9.6 Factors Affecting Performance 9.6.1 Design 9.6.1.1 Capacity Heat dissipation (in kCal/hour) and circulated flow rate (m3/hr) are not sufficient to understand cooling tower performance. Other factors, which we will see, must be stated along with flow rate m3/hr. For example, a cooling tower sized to cool 4540 m3/hr through a 13.9°C range might be larger than a cooling tower to cool 4540 m3/hr through 19.5°C range. 9.6.1.2 Range Range is determined not by the cooling tower, but by the process it is serving. The range at the exchanger is determined entirely by the heat load and the water circulation rate through the exchanger and on to the cooling water. Equation 7 CT Range Def. 2 𝑹𝒂𝒏𝒈𝒆 °𝑪 =

𝑯𝒆𝒂𝒕 𝑳𝒐𝒂𝒅 (𝒌𝑪𝒂𝒍/𝒉𝒓) 𝑾𝒂𝒕𝒆𝒓 𝑪𝒊𝒓𝒄𝒖𝒍𝒂𝒕𝒊𝒐𝒏 𝑹𝒂𝒕𝒆 (𝑳𝑷𝑯)

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

Page 19 of 51 Thus, Range is a function of the heat load and the flow circulated through the system. Cooling towers are usually specified to cool a certain flow rate from one temperature to another temperature at a certain wet bulb temperature. For example, the cooling tower might be specified to cool 48000 m3/hr from 44°C to 34°C at 26.7°C wet bulb temperature. 𝑪𝑻 𝑨𝒑𝒑𝒓𝒐𝒂𝒄𝒉 (𝟓°𝑪) = 𝑪𝑾 𝒐𝒖𝒕𝒍𝒆𝒕 𝒕𝒆𝒎𝒑 (𝟑𝟒°𝑪) − 𝑾𝒆𝒕 𝒃𝒖𝒍𝒃 𝒕𝒆𝒎𝒑 (𝟐𝟗°𝑪)

As a generalization, the closer the approach to the wet bulb, the more expensive the cooling tower due to increased size. Usually a 2.8°C approach to the design wet bulb is the coldest water temperature that cooling tower manufacturers will guarantee. If flow rate, range, approach and wet bulb had to be ranked in the order of their importance in sizing a tower, approach would be first with flow rate closely following the range and wet bulb would be of lesser importance. The range increases when the quantity of circulated water and heat load increase. This means that increasing the range as a result of added heat load requires a larger tower. There are two possible causes for the increased range: 

The inlet water temperature is increased (and the cold-water temperature at the exit remains the same). In this case it is economical to invest in removing the additional heat.



The exit water temperature is decreased (and the hot water temperature at the inlet remains the same). In this case the tower size would have to be increased considerably because the approach is also reduced, and this is not always economical.

9.6.1.3 Heat Load The heat load imposed on a cooling tower is determined by the process being served. The degree of cooling required is controlled by the desired operating temperature level of the process. In most cases, a low operating temperature is desirable to increase process efficiency or to improve the quality or quantity of the product. In some applications (e.g. internal combustion engines), however, high operating temperatures are desirable. The size and cost of the cooling tower is proportional to the heat load. If heat load calculations are low undersized

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

Page 20 of 51 equipment will be purchased. If the calculated load is high, oversize and more costly, equipment will result. Process heat loads may vary considerably depending upon the process involved. Determination of accurate process heat loads can become very complex but proper consideration can produce satisfactory results. On the other hand, air conditioning and refrigeration heat loads can be determined with greater accuracy. 9.6.1.4 Wet Bulb Temperature Wet bulb temperature is an important factor in performance of evaporative water cooling equipment. It is a controlling factor from the aspect of minimum cold water temperature to which water can be cooled by the evaporative method. Thus, the wet bulb temperature of the air entering the cooling tower determines operating temperature levels throughout the plant, process, or system. Theoretically, a cooling tower will cool water to the entering wet bulb temperature, when operating without a heat load. However, a thermal potential is required to reject heat, so it is not possible to cool water to the entering air wet bulb temperature, when a heat load is applied. The approach obtained is a function of thermal conditions and tower capability. Initial selection of towers with respect to design wet bulb temperature must be made on the basis of conditions existing at the tower site. The temperature selected is generally close to the average maximum wet bulb for the summer months. An important aspect of wet bulb selection is whether it is specified as ambient or inlet. The ambient wet bulb is the temperature, which exists generally in the cooling tower area, whereas inlet wet bulb is the wet bulb temperature of the air entering the tower. The later can be, and often is, affected by discharge vapours being re-circulated into the tower. Recirculation raises the effective wet bulb temperature of the air entering the tower with corresponding increase in the cold water temperature. Since there is no initial knowledge or control over the recirculation factor, the ambient wet bulb should be specified. The cooling tower supplier is required to furnish a tower of sufficient capability to absorb the effects of the increased wet bulb temperature peculiar to his own equipment.

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

Page 21 of 51 It is very important to have the cold water temperature low enough to exchange heat or to condense vapours at the optimum temperature level. By evaluating the cost and size of heat exchangers versus the cost and size of the cooling tower, the quantity and temperature of the cooling tower water can be selected to get the maximum economy for the particular process. The Table 7.1 illustrates the effect of approach on the size and cost of a cooling tower. The towers included were sized to cool 4540 m3/hr through a 16.67°C range at a 26.7°C design wet bulb. The overall width of all towers is 21.65 meters; the overall height, 15.25 meters, and the pump head, 10.6 m approximately. The design wet bulb temperature is determined by the geographical location. For a certain approach value (and at a constant range and flow range), the higher the wet bulb temperature, the smaller the tower required. For example, a 4540 m3/hr cooling tower selected for a16.67°C range and a 4.45°C approach to 21.11°C wet bulb would be larger than the same tower to a 26.67°C wet bulb. The reason is that air at the higher wet bulb temperature is capable of picking up more heat. This is explained for the two different wet bulb temperatures: 

Each kg of air entering the tower at a wet bulb temperature of 21.1°C contains 18.86 kCal. If the air leaves the tower at 32.2°C wet bulb temperature, each kg of air contains 24.17 kCal. At an increase of 11.1°C, the air picks up 12.1 kCal per kg of air.



Each kg of air entering the tower at a wet bulb temperature of 26.67°C contains 24.17 kCals. If the air leaves at 37.8°C wet bulb temperature, each kg of air contains 39.67 kCal. At an increase of 11.1°C, the air picks up 15.5 kCal per kg of air, which is much more than the first scenario.

9.6.1.5 Tower Size If heat load, range, approach and wet-bulb temperature are held constant, changing the fourth will affect the tower size as follows: a) Tower size varies inversely with approach. A longer approach requires a smaller tower. Conversely, a smaller approach requires an increasingly larger tower and, at 5°F approach,

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

Page 22 of 51 the effect upon tower size begins to become asymptotic. For that reason, it is not customary in the cooling tower industry to guarantee any approach of less than 5°F.

Figure 4 Tower size v/s approach

b) Tower size varies inversely with wet bulb temperature. When heat load, range, and approach values are fixed, reducing the design wet-bulb temperature increases the size of the tower. This is because most of the heat transfer in a cooling tower occurs by virtue of evaporation (which extracts approximately 1000 Btu’s for every pound of water evaporated), and air’s ability to absorb moisture reduces with temperature.

Figure 5 Tower size v/s wet-bulb

c) Tower size varies directly and linearly with heat load.

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Figure 6 Tower size v/s head load

d) Tower size varies inversely with range. Two primary factors account for this. First; increasing the range—also increases the ITD (driving force) between the incoming hot water temperature and the entering wet-bulb temperature. Second, increasing the range (at a constant heat load) requires that the water flow rate be decreased—which reduces the static pressure opposing the flow of air.

Figure 7 Tower size v/s range variance

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9.6.2 Fill media effects In a cooling tower, hot water is distributed above fill media and is cooled down through evaporation as it flows down the tower and gets in contact with air. The fill media impacts energy consumption in two ways: 

Electricity is used for pumping above the fill and for fans that create the air draft. An efficiently designed fill media with appropriate water distribution, drift eliminator, fan, gearbox and motor with therefore lead to lower electricity consumption.



Heat exchange between air and water is influenced by surface area of heat exchange, duration of heat exchange (interaction) and turbulence in water effecting thoroughness of intermixing. The fill media determines all of these and therefore influences the heat exchange. The greater the heat exchange, the more effective the cooling tower becomes.

There are three types of fills: a) Splash fill media. Splash fill media generates the required heat exchange area by splashing water over the fill media into smaller water droplets. The surface area of the water droplets is the surface area for heat exchange with the air. b) Film fill media. In a film fill, water forms a thin film on either side of fill sheets. The surface area of the fill sheets is the area for heat exchange with the surrounding air. Film fill can result in significant electricity savings due to fewer air and pumping head requirements. c) Low-clog film fills. Low-clog film fills with higher flute sizes were recently developed to handle high turbid waters. Low clog film fills are considered as the best choice for sea water in terms of power savings and performance compared to conventional splash type fills. Table 2 Design Values of Different Fills Possible L/G ratio Effective heat exchange area Fill height required Pumping head required Quantity of air required

Splash fill 1.1 – 1.5 30 – 45 m2/m3 5 – 10 m 9 – 12 m High

Film fill 1.5 – 2.0 150 m2/m3 1.2 – 1.5 m 5–8m Lowest

Low clog film fill 1.4 – 1.8 85 - 100 m2/m3 1.5 – 1.8 m 6–9m Low

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9.6.3 Water Distribution 9.6.3.1 Optimize cooling water treatment Cooling water treatment (e.g. to control suspended solids, algae growth) is mandatory for any cooling tower independent of what fill media is used. With increasing costs of water, efforts to increase Cycles of Concentration (COC), by cooling water treatment would help to reduce make up water requirements significantly. In large industries and power plants improving the COC is often considered a key area for water conservation. 9.6.3.2 Install drift eliminators It is very difficult to ignore drift problems in cooling towers. Nowadays most of the end user specifications assume a 0.02% drift loss. But thanks to technological developments and the production of PVC, manufacturers have improved drift eliminator designs. As a result drift losses can now be as low as 0.003 –0.001%.

9.6.4 Fans The purpose of a cooling tower fan is to move a specified quantity of air through the system. The fan has to overcome the system resistance, which is defined as the pressure loss, to move the air. The fan output or work done by the fan is the product of air flow and the pressure loss. The fan output and kW input determines the fan efficiency. The fan efficiency in turn is greatly dependent on the profile of the blade. Blades include: a) Metallic blades, which are manufactured by extrusion or casting processes and therefore it is difficult to produce ideal aerodynamic profiles b) Fibre reinforced plastic (FRP) blades, are normally hand moulded which makes it easier to produce an optimum aerodynamic profile tailored to specific duty conditions. Because FRP fans are light, they need a low starting torque requiring a lower HP motor, the lives of the gear box, motor and bearing is increased, and maintenance is easier. A 85-92% efficiency can be achieved with blades with an aerodynamic profile, optimum twist, taper and a high coefficient of lift to coefficient of drop ratio. However, this efficiency is drastically affected by factors such as tip clearance, obstacles to airflow and inlet shape, etc.

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Page 26 of 51 Cases reported where metallic or glass fibber reinforced plastic fan blades have been replaced by efficient hollow FRP blades. The resulting fan energy savings were in the order of 20-30%and with simple payback period of 6 to 7 months (NPC).

9.7 General Improvement Procedures The following could be fruitful options to improve energy efficiency of cooling towers: i.

Follow manufacturer’s recommended clearances around cooling towers and relocate or modify structures that interfere with the air intake or exhaust

ii.

Optimize cooling tower fan blade angle on a seasonal and/or load basis

iii.

Correct excessive and/or uneven fan blade tip clearance and poor fan balance

iv.

In old counter-flow cooling towers, replace old spray type nozzles with new square spray nozzles that do not clog

v.

Replace splash bars with self-extinguishing PVC cellular film fill

vi.

Install nozzles that spray in a more uniform water pattern

vii.

Clean plugged cooling tower distribution nozzles regularly

viii.

Balance flow to cooling tower hot water basins

ix.

Cover hot water basins to minimize algae growth that contributes to fouling

x.

Optimize the blow down flow rate, taking into account the cycles of concentration (COC)limit

xi.

Replace slat type drift eliminators with low-pressure drop, self-extinguishing PVC cellular units

xii.

Restrict flows through large loads to design values

xiii.

Keep the cooling water temperature to a minimum level by (a) segregating high heat loads like furnaces, air compressors, DG sets and (b) isolating cooling towers from sensitive applications like A/C plants, condensers of captive power plant etc. Note: A 1°Ccooling water temperature increase may increase the A/C compressor electricity consumption by 2.7%. A 1oC drop in cooling water temperature can give a heat rate saving of 5 kCal/kWh in a thermal power plant

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Page 27 of 51 xiv.

Monitor approach, effectiveness and cooling capacity to continuously optimize the cooling tower performance, but consider seasonal variations and side variations

xv.

Monitor liquid to gas ratio and cooling water flow rates and amend these depending on the design values and seasonal variations. For example: increase water loads during summer and times when approach is high and increase air flow during monsoon times and when approach is low.

xvi.

Consider COC improvement measures for water savings

xvii.

Consider energy efficient fibre reinforced plastic blade adoption for fan energy savings

xviii.

Control cooling tower fans based on exit water temperatures especially in small units

xix.

Check cooling water pumps regularly to maximize their efficiency

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10 Cooling Water Chemistry Cooling towers are dynamic systems because of the nature of their operation and the environment they function within. Tower systems sit outside, open to the elements, which makes them susceptible to dirt and debris carried by the wind. Their structure is also popular for birds and bugs to live in or around, because of the warm, wet environment. These factors present a wide range of operational concerns that must be understood and managed to ensure optimal thermal performance and asset reliability. Below is a brief discussion on the four primary cooling system treatment concerns encountered in most open re-circulating cooling systems.

10.1 Corrosion Corrosion is an electrochemical or chemical process that leads to the destruction of the system metallurgy. Figure illustrates the nature of a corrosion cell that may be encountered throughout the cooling system metallurgy. Metal is lost at the anode and deposited at the cathode. The process is enhanced by elevated dissolved mineral content in the water and the presence of oxygen, both of which are typical of most cooling tower systems.

Figure 8 Corrosion cell

There are different types of corrosion encountered in cooling tower systems including pitting, galvanic, microbiologically influenced and erosion corrosion Loss of system metallurgy, if pervasive enough, can result in failed heat exchangers, piping, or portions of the cooling tower itself. Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

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10.1.1

Corrosion Control

10.1.1.1 Cathodic Polarization Process of changing the anodic or cathodic potential or both to reduce the driving force of the corrosion reaction is called “polarization”. Polarization reduces the driving force of the corrosion reaction and minimizes metal loss by changing the potential of either the anode or the cathode or both so that the difference in potential between them is reduced to a minimum. If the amount of oxygen diffusion to the metal surface can be controlled, the corrosion reaction can be polarized. This is achieved by cathodic corrosion inhibitors. They form a film, which prevents the diffusion of oxygen to the cathode side. 10.1.1.2 Anodic Polarization Anodic surfaces can be polarized by formation of an oxide layer. This film formation is accomplished by a mechanism known as chemisorption. Stainless steel naturally forms such films. This unfortunately is not always the case with all metals. Most metals must be aided by the addition of such anodic corrosion inhibitors as chromate, nitrite, etc. 10.1.1.3 Passivation When corrosion reactions are completely polarized, the metal is said to be at “passive state” At this point there is no difference in potential between the anode and cathode areas, and corrosion ceases. When polarization is disrupted in a passive metal at a given point, a very active anodic site is set up, with resultant accelerated local corrosion, particularly if the metal was strongly anodically polarized.

10.1.2

Corrosion Inhibitors

The principal method of controlling corrosion in cooling water system is by means of chemical corrosion inhibitors. Their function in preventing corrosion lies in their ability to insulate the electric current between the cathode and anode. If the insulation effect occurs at the anodic site, then the inhibitor is classified as an anodic inhibitor and if the cathodic site is insulated then the inhibitor is classified as a cathodic inhibitor. Corrosion inhibitors are classified as anodic, cathodic or both depending upon the corrosion reaction each controls. Inhibition usually results from one or more of three general

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Page 30 of 51 mechanisms. In the first, the inhibitor molecule is adsorbed on the metal surface by the process of chemisorption, forming a thin protective film either by itself or in conjunction with metallic ions. In second mechanism inhibitors however merely cause a metal to form its own protective film of metal oxides, by increasing its resistance. In the third type inhibitor reacts with a potentially corrosive substance in the water. Anodic inhibitors build a thin protective film along the anode increasing the potential at the anode and slowing the corrosion reaction, the film is initiated at the anode although it may eventually cover the entire metal surface. Because this film is not visible to the naked eye so the appearance of the metal will be left unchanged. Cathodic inhibitors are generally less effective than the anodic type. But they often form a visible film along the cathode surface, which polarizes the metal by restricting the access of dissolved oxygen to the metal substrate. The film also acts to block hydrogen evolution sites and prevent the resultant depolarizing effect. Examples include:     

10.1.3

Chromates Orthophosphates Zinc Polyphosphates Synergic Blends like o zinc-chromates o chromate-polyphosphates o chromate-orthophosphate

Inhibitor Selection

It is often difficult to make a proper choice between the many cooling water corrosion inhibitors unless there is some understanding of their properties. Choice of the proper inhibitor is determined by:   

Design parameters Water composition Metals in the system

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Page 31 of 51     

Stress conditions Treatment level required pH Dissolved oxygen content Salts and SS composition

10.2 Scaling Scaling is the precipitation of dissolved minerals components that have become saturated in solution. Factors that contribute to scaling tendencies include water quality, pH, and temperature. Scale formation reduces the heat exchange ability of the system because of the insulating properties of scale, making the entire system work harder to meet the cooling demand. Deposits typically consist of mineral scales (i.e.CaCO3. CaSO4, Ca3(PO4)2, CaF2, etc), corrosion products (i.e. Fe2O3, Fe3O4, CuO etc), particular matter (i.e. clay, slit), and microbiological mass.

10.2.1

Types

10.2.1.1 Waterborne salts Precipitated salts of calcium and magnesium often form dense scales and sludge’s which are usually quite adherent and therefore difficult to remove. In addition they are effective heat insulators, which reduce process efficiency. Calcium carbonate, calcium sulphate, calcium and magnesium silicates and calcium phosphate are some of the more prevalent compounds found in cooling water systems. 10.2.1.2 Waterborne foulants A variety of such materials as suspended mud, sand, silt, clay, biological matter or even oil may enter a cooling water system through its make up supply. They usually accumulate in low flow areas, or in locations at which an abrupt change in flow velocity occurs. Therefore the most sedimentation is found in such places as cooling tower basins and heat exchangers. To control sedimentation it is necessary to control the suspended particulate matter. The control of particle size and density is accomplished by use of modern deposit control materials. To a certain degree mud, sand, slit, dirt and clay are suspended in most make up supplies. However the amount of these constituents is usually much greater for surface waters.

Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

Page 32 of 51 Microbiological growth may be a particularly troublesome foulant in the makeup supply. The microbiological population in a towers make up supply often approaches or exceeds the control limit for proper tower operation. Oil often adheres to metal; surfaces and acts as a deposit binder. Oil films serve as insulators and can seriously retard heat transfer. In addition oil acts as a nutrient for microbes, therefore increasing microbiological activity, fouling and slime binding. Also oil films prevent corrosion inhibitors from reaching and passivating metal surfaces. 10.2.1.3 Airborne foulants The air in contact with open cooling water systems contains many of the same suspended materials found in the makeup water. Sand, slit, clay, dirt, bacteria etc. entering with the air add to the overall fouling of the system. Airborne contamination by gases also helps in deposition. Oxygen and carbon dioxide accelerate corrosion, leading to deposition and further corrosion by the under-deposit mechanism. Since pick up of both gases occur continuously, near saturation levels of these dissolved gasses are present in the water. Gaseous contaminants such as sulphur dioxide, hydrogen sulphide and ammonia may also be absorbed from the air. The first two reduce oxidizing corrosion inhibitors (e.g. chromates) to insoluble foulants. Hydrogen sulphide is very corrosive and quickly forms iron sulphide deposits, which lead to further corrosion. Ammonia selectively corrodes copper and its alloys leading to the deposition of copper corrosion products.

10.2.2

Deposit Control Methods

10.2.2.1 Conventional treatments  Softening (sodium or hydrogen zeolite exchange, lime softening and demineralization all remove the ions that cause scale formation) 

Acid feed (acid neutralizes alkalinity in the water, thereby preventing carbonate formation)



Side stream filtration (Side stream filters are used in some cooling tower applications, with 1 to 5 % of the cooling water flow passing through the filter. Several type of media are used but sand is the most common, operating at a 10 % to 20 % efficiency level. For greater efficiency, anthracite or mixed media can be substituted. If the suspended solids are in the range of 10 to 30 ppm, 50~75 % removal can be achieved, and in highly turbid waters, 90 % removal is possible. In general a side stream filter allows cooling water turbidity to

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Page 33 of 51 approach the turbidity of the filter effluent. With oil contamination side stream filters are impractical because of rapid fouling of the filter medium.) 10.2.2.2 Use of Polymeric Deposit Control Agents A polymer is defined as macromolecule consisting of a number of repeating units of “building blocks”. These units are referred to as monomers. Modern technology has made it possible to build chains of various lengths and compositions by varying the polymerization conditions and the monomer groups incorporated into the structure. The behaviour of a polymer results primarily from two factors: its chain length or molecular weight and its functional group. These polymeric deposit control agents include, Scale inhibitors, Dispersants, Flocculants 10.2.2.3 Scale Inhibitors Scale inhibitors are important to the performance of many treatment programs. Scale inhibitors function by adsorbing on to suspended solids/scaling particles and adsorbing on to solids/ surfaces in the system, thereby acting to prevent growth of scale/deposits and enhancing performance of corrosion inhibitors. These polymers have the ability of adsorbing on active sites of the crystal to prevent any further growth of crystal. Some of the functional groups of the scale inhibitor adsorbed on the crystals but the rest of them are free from the adsorption and give electrical charge to the crystals. Thus, the static electrical repelling force of the crystals is increased and the crystals are kept in a dispersed condition. Certain polymers can distort scale crystals by disrupting their lattice structure and normal growth patterns. The inclusion of a relatively large irregularly shaped polymer in the scale lattice tends to prevent the deposition of a dense uniformly structured crystalline mass on the metal surface. These crystals can develop internal stresses which increase as the crystal grows, with the result that deposit breaks away from the metal surface. Anionic polymers such as polyacrylates, polymethacrylates and maleic anhydride derivatives are excellent scale control agents. Also polyphosphate, phosphate esters and phosphonates can control scale.

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Page 34 of 51 10.2.2.4 Dispersants “The principal role of a dispersant is to reduce the tendency for small particles to agglomerate”. Dispersants are polymers, which control particles by increasing charge on the particle surface, thereby keeping the particles repelled and suspended. A polymer can be adsorbed on foulant surface imparting a like charge to them and thereby causing the particles to remain in suspension because of charge repulsion. Dispersant polymer is a common component of cooling water treatment programs. These polymers prevent deposit because they keep suspended particles from adhering to pipes, tubes, or other surfaces in the cooling systems and are removed with the water by blow down. In order to be effective the polymers must strongly adhere to the particle surfaces so that the polymer’s fate is the same as the particle it is bound to. The amount of polymer necessary is a complex function of hardness, temperature, pH, and many other factors. Much of this is due to the increased thermodynamic “driving force” for precipitation of calcium carbonate or calcium phosphate. At high bulk water temperatures (>60 °C), high calcium concentrations (>750mg/lit as CaCO3), or low flow rates (