Cooling Tower | Chemistry and Performance Improvement

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Cooling Tower Chemistry and Performance Indicators | Internship Report 2011

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

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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

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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;

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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 General Improvement Procedures ................................................................................. 26

10 Cooling Water Chemistry ................................................................................................... 28

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

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

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

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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

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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

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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.

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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.

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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 cost-

effective 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.

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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.

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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

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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.

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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 closed-

circuit 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.

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Figure 2 Cooling Tower Types

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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

Forced draft Air is blown through the tower by a fan located

in the air inlet

Suited for high air

resistance due to

centrifugal blower fans

Fans are relatively quiet

Recirculation due to high air-

entry and low air-exit

velocities, which can be solved

by locating towers in plant

rooms combined with

discharge ducts

Induced draft cross flow

Water enters at top and passes over fill

Air enters on one side (single-flow tower) or

opposite sides (double-flow tower)

An induced draft fan draws air across fill

towards exit at top of tower

Less recirculation than

forced draft towers

because the speed of

exit air is 3-4 times

higher than entering air

Fans and the motor drive

mechanism require weather-

proofing against moisture and

corrosion because they are in

the path of humid exit air

Induced draft counter flow

Hot water enters at the top

Air enters bottom and exits at the top

Uses forced and induced draft fans

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

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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)

(𝑪𝑾 𝒊𝒏 𝒕𝒆𝒎𝒑 – 𝑾𝑩 𝒕𝒆𝒎𝒑) × 𝟏𝟎𝟎

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.

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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

𝑬𝒗𝒂𝒑𝒐𝒓𝒂𝒕𝒊𝒐𝒏 𝒍𝒐𝒔𝒔 (𝒎𝟑

𝒉𝒓 ) = 𝟎. 𝟎𝟎𝟎𝟖𝟓 × 𝟏. 𝟖 𝒙 𝒄𝒊𝒓𝒄𝒖𝒍𝒂𝒕𝒊𝒐𝒏 𝒓𝒂𝒕𝒆 (𝒎𝟑

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

𝑮=

(𝒉𝟐 − 𝒉𝟏)

(𝑻𝟏 − 𝑻 𝟐)

Where:

L/G = liquid to gas mass flow ratio (kg/kg)

T1 = hot water temperature (°C)

T2 = cold-water temperature (°C)

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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)

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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

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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.

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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,

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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

Splash fill Film fill Low clog film fill

Possible L/G ratio 1.1 – 1.5 1.5 – 2.0 1.4 – 1.8 Effective heat exchange area 30 – 45 m2/m3 150 m2/m3 85 - 100 m2/m3 Fill height required 5 – 10 m 1.2 – 1.5 m 1.5 – 1.8 m Pumping head required 9 – 12 m 5 – 8 m 6 – 9 m Quantity of air required High Lowest 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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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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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.

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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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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:

Chromates

Orthophosphates

Zinc

Polyphosphates

Synergic Blends like

o zinc-chromates

o chromate-polyphosphates

o chromate-orthophosphate

10.1.3 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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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.

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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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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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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 (<1 m/sec), the tendency for scale formation, even with cooling

water treatment programs, is greatly increased.

10.2.2.5 Flocculants

A high molecular weight polymer can attach itself to many foulant particles creating a low

density floc. With an increase in the overall size of suspended material, there is a corresponding

decrease in the surface area available for attachment, which reduces the extent of deposition

possible.

Much of suspended matter found in cooling water has a negative surface charge. This charge

keeps the suspended matter separated. If the surface charge of the particles can be reduced,

the particle will agglomerate into light, fluffy flocs with little tendency to adhere to metal

surfaces. This can be accomplished by adding a long chain oppositely charged (cationic)

polymer to the cooling water, which neutralizes the negative charge of the suspended material.

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10.3 Microbial Growth Microbiological activity is microorganisms that live and grow in the cooling tower and cooling

system. Cooling towers present the perfect environment for biological activity due to the warm,

moist environment. There are two distinct categories of biological activity in the tower system.

The first being plank tonic, which is bioactivity suspended, or floating in solution. The other is

sessile bio-growth, which is the category given to all biological activity, biofilms, or bio-fouling

that stick to a surface in the cooling system. Bio films are problematic for multiple reasons.

They have strong insulating properties, they contribute to fouling and corrosion, and the bi-

products they create that contribute to further micro-biological activity. They can be found in

and around the tower structure, or they can be found in chiller bundles, on heat exchangers

surfaces, and in the system piping. Additionally, bio films and algae mats are problematic

because they are difficult to kill. Careful monitoring of biocide treatments, along with routine

measurements of biological activity are important to ensure bio-activity is controlled and

limited throughout the cooling system. Cooling water microorganisms include: Algae, Fungi,

and Bacteria etc

Figure 9Biofouled Heat Exchanger

10.3.1 Problems Continued accumulation and growth of microorganisms in a cooling water system causes a

number of problems. Good corrosion and deposit control programs are incumbent upon a

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successful microbial control program. A plant unable to control microbial growth will

experience increased difficulty in controlling corrosion and deposition. Another problem

associated with microbial growth is the deterioration of cooling tower lumber this reduces the

efficiency of the cooling tower operation and increases operating cost of the plant.

Microbiological growth also causes environmental pollution.

10.3.1.1 Microbiological Induced Corrosion, (MIC)

Any corrosion initiated or propagated by the action of microorganisms either directly or

indirectly is called MIC.

Many microorganisms found in cooling water utilize hydrogen in their metabolic processes,

which often results in the cathodic depolarization of the corrosion reaction. Many microbial

species present special corrosion problems, in addition to those inherent in the basic nature of

their actions. Sulphate reducing bacteria produce extremely dangerous hydrogen sulphide gas,

which corrodes metals by low pH attack and by the formation of ferrous sulphide.

Sulphate oxidizing bacteria produce sulphuric acid and produce localized low pH areas in the

system. Corrosion proceeds very rapidly in these low pH areas. Nitrifying bacteria nullify the

effectiveness of nitrite corrosion inhibitors by oxidizing nitrite to nitrate. This is the most

serious in closed re-circulating systems which commonly use nitrite as a corrosion inhibitor in

the systems where NH3 is present in water.

10.3.1.2 Deposit Problems

Deposit of microbial matter may lead to physical problems in the system, culminating in loss of

efficiency, heat transfer and production. The accumulation of bio matter on the internal

sections of cooling towers can seriously reduce the units efficiency e.g. deposition on splash

plates will increase the water droplet size and will reduce the effective surface area. Algae can

plug the holes in the distribution deck of a cooling tower producing uneven distribution of

water over the tower packing, resulting in a serious loss in efficiency.

10.3.2 Selection of Micro Biocides A number of factors will determine the proper choice of micro biocide or combination of micro

biocides, oxidizing and non-oxidizing micro biocide. The selection of a micro biocide involves

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several factors. First it must be effective in inhibiting almost all -microbial activity. Second, it

must be economical in a treatment programme. This is often accomplished by combining a

small amount of an expensive but highly effective, micro biocide with another less expensive

one resulting in broad spectrum control at reasonable cost. Environmental discharge and

disposal considerations constitute another factor, which determines the choice of micro

biocides. Disposal problems caused by toxicity have limited the use of certain micro biocides in

many areas. The micro biocide chosen must be easily detoxified before cooling system bleed off

reaches receiving streams. The operating parameters of the cooling water system will also

affect the choice of a micro biocide. Temperature, pH and system design are fundamental

considerations in a decision involving oxidizing or non oxidizing toxicants. Other considerations

may include,

The nature of treatments being used for control of scale and corrosion

Whether to apply the biocide continuously, intermittent, or as a periodic shock dose.

The appropriate dose required

Location of point of addition.

10.3.3 Oxidizing Toxicants

10.3.3.1 Chlorination The most commonly used oxidizing micro biocide is Chlorine. It is the most effective of all

halogens. Chlorine is an excellent algaecide and sporicide. It is also an excellent bactericide in

most circumstances. Free residual chlorine at levels of 0.5 ppm and slightly above are usually

enough to control most microbial growth. A number of factors determine the amount of

chlorine required in an open cooling water system. These include chlorine demand, contact

time, pH, and temperature of the water.

When chlorine gas is fed to water, it hydrolyzes to form two acids, hypochlorous acid (biocide)

and hydrochloric acid, respectively.

Cl2 + H2O HOCl + HCl

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Hypochlorous acid is very weak acid but an extremely powerful oxidizing agent. It easily diffuses

through the cell walls of microorganisms, and reacts with the cytoplasm to produce chemically

stable nitrogen chlorine bonds with the cell proteins.

Some quantity of Hypochlorous acid will ionize into hypochlorite ions according to this

reversible reaction.

HOCl H+ + OCl-

The PH of the cooling water is directly responsible for the extent of ionization of hypochlorous

acid. The acid state is favoured by low pH .At pH 7.5 there will be approximately equal amounts

of acid and hypochlorite ion. Chlorine becomes ineffective as a micro biocide at pH 9.5 or

greater as a result of total ionization. A, pH range of 6.5~7 is considered practical for chlorine

based microbial control programme. Hypochlorous acid is estimated to be twenty times more

reactive (effective) as a micro biocide than the hypochlorite ions.

If ammonia is present in cooling water then chlorine reacts with ammonia to form chloramines

(NH2Cl etc) due to which there is a decrease in the residual chlorine within the system.

Chloramines are poor biocides and are more harmful environmentally than chlorine itself due

to very long half life.

Because the amount of chlorine added to the system is directly proportional to the alkalinity

reduction. Many plants find it necessary to suspend acid feed during chlorination periods in

order to avoid low PH excursions. Chlorine is destroyed by sunlight and by aeration so, its

dosing is preferred at night to prolong its effect.

Other oxidizing biocides include ozone, chlorine dioxide and hypochlorites.

Hypochlorites are salts of hypochlorous acid. They are composed of sodium hypochlorite

(NaOCl) and calcium hypochlorite (Ca (OCl) 2) when they are added to cooling water system

function in much the same way as chlorine gas.

Chlorine dioxide is a gas produced at site from sodium chlorite with chlorine gas. It does not

form hypochlorous acid in water like chlorine it exists as dissolved chlorine dioxide in

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solution and is generally less reactive as a micro biocide but more effective than chlorine at

higher PH ranges.

Ozone is a powerful and naturally unstable gas. As a micro biocide it reacts in much the

same manner as the other oxidizers” by combining with protein and inactivating enzymes

that are essential to cell respiration”.

10.3.3.2 Bromination

For systems, operating at above 7.0 pH i.e. alkaline media like Phosphate treatment system,

bromine is more efficient than chlorine as a biocide. Because 50 % of hypochlorous acid, HOCl

(biocide) formed due to chlorination, ionize into hypochlorite ions (OCl-) at pH 7.5. Hypochlorite

ions as a biocide are twenty times less effective than HOCl. At pH 8.0, Chlorination will yield

only 20 % HOCl& 80 % OCl ions. But at this pH bromination will yield 80 % HOBr (micro biocide)

& 20 % OBr ions-. That is why at alkaline pH bromination is more effective than chlorination in

the control of microbiological growth. At pH (8~9.3), only a small percentage of chlorine is

available as the active toxicant, hypochlorous acid.

Target bromination is one of the most effective oxidizing biocide treatments for cooling water

systems. This is achieved by feeding sodium bromide with a chlorine- based oxidant in a 1:1

molar ratio to achieve bromination.

Bulab-6040 used at FFC for bromination is a sodium bromide salt. It has no oxidizing capability

until it is activated by reacting it with hypochlorous acid to yield hypobromous acid.

Hypochlorous acid is generated through addition of sodium hypochlorite or chlorine in water

as,

Cl2 + H2 O HOCl + HCl

NaOCl + H2O HOCl + NaOH

This HOCl reacts with NaBr as,

HOCl + NaBr → HOBr + NaCl

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In the presence of NH3 bromamines are formed which are more effective than chloramines in

the control of bacteria. Also bromamines breakdown more quickly than chloramines in the

environment and has lower long- term environmental toxicity.

10.3.4 Non Oxidizing Biocides Non-oxidizing biocides can be more effective than oxidizing biocide because of their overall

control of algae, fungi, and bacteria. They have also greater persistence, as many of them are

PH independent. They are used in conjunction with oxidizing micro biocides for broad control.

Most of plants chlorinate intermittently and add a non -oxidizer once or twice a week or as per

requirement. Their mode of activity is to inhibit cell growth by preventing the transfer of

energy or life sustaining chemical reactions occurring within the cell.

Organic sulphur compounds include a wide variety of different biocides, Methylene

bisthiocynate` (MTB) is most common, which is effective in controlling algae, Fungi and

bacteria.

10.4 Chemical Dosing at CT – 4 The chemical dosing at cooling tower 4 are recommended and monitored by the service

provider Buckman. Keeping in view the basic medium of operation the company has

recommended the appropriate service dosing and their quantity at the operating C.O.C. The

tables here explain the dosing, their rates and applications.

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Table 3 Chemical Dosing Rate

Chemicals Product

(PPM)

Product

(Kg)

Product

(Lit)

Status Frequency Drop Rate

Bulab 9063 18 95 73 Neat 24 hrs 50 ml /59 Sec

Bulab 7024 20 106 94 Neat 24 hrs 50 ml /46 Sec

Bulab 9067 18 95 79 2 x

Diluted

24 hrs 110ml / min

Bulab 8006 5 26 24 Neat 24 hrs 17ml / min

Bulab 6041 * 200 149 With Hypo @ 70 Lit /hr 583ml / 30 Sec

Bulab 3847 82 1000 Neat Monthly N/A

Table 4 Chemical Dosing at CT 4

Chemicals Trade Name Intended Application

Zinc and inorganic

phosphate

BULAB 9063 Provides both anodic and cathodic corrosion

protection

Zinc BULAB 9050 Protection of mild steel piping and equipment in

cooling water systems

Bromine oxidizing biocide BULAB 6041 Used pH above 7.2

Surfactant

BULAB 3847 Prevents growth of Bacteria, Fungi, Algae and

Sulphate Reducing Bacteria (SRB); Proven

effectiveness in ammonia containing CW

Hypochlorite - Kills the bacteria

Bio-dispersant BULAB 8006 Designed to inhibit slime build up; enhances the

effectiveness of microorganism control

Polymer

BLS 9067 Controls the precipitation of calcium phosphate

& stabilize the Zinc

Phosphonate and Polymer BULAB 7024 Controls the precipitation of calcium carbonate

and deposition due to silt or other particulate

material

Page 42 of 51


11 Performance Improvement

11.1 Water Use

The hierarchy of opportunities approach can be used to identify and prioritise water efficiency

opportunities.

Figure 10 Hierarchy of opportunities

11.1.1 Reduce water loss

Reducing water losses reduces the quantity of make-up water required for the system.

Potential opportunities to reduce water loss include:

• Fixing leaks

• Reducing splash

• Optimising overflow

• Eliminating drift – drift losses should be maintained at less than 0.002% of cooling water

circulation rate. Repair or install new systems to achieve best practice.

Equation 8 Water losses

𝐖𝐚𝐭𝐞𝐫 𝐋𝐨𝐬𝐬𝐞𝐬 = 𝐐𝐮𝐚𝐧𝐭𝐢𝐭𝐲 𝐨𝐟 𝐦𝐚𝐤𝐞 − 𝐮𝐩 𝐰𝐚𝐭𝐞𝐫

𝐂𝐲𝐜𝐥𝐞 𝐨𝐟 𝐜𝐨𝐧𝐜𝐞𝐧𝐭𝐫𝐚𝐭𝐢𝐨𝐧 − 𝐐𝐮𝐚𝐧𝐭𝐢𝐭𝐲 𝐨𝐟 𝐛𝐥𝐨𝐰𝐝𝐨𝐰𝐧

1• Reduce water loss

2• Reduce blow down

3• Use alternative water supplies

4• Reuse blow down

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11.1.2 Reduce blow down

11.1.2.1 Increase cycles of concentration

As water evaporates from cooling towers the contaminants, salts and minerals measured as

total dissolved solids (TDS) that accumulate can cause biological growth, corrosion and scale

resulting in tower damage, poor heat transfer and possibly the growth of harmful bacteria such

as Legionella. The sources of contaminants include:

• Salts and minerals already in the make-up water

• Chemicals added to reduce corrosion, scale and biological growth

• Pollutants entering the water during the evaporation phase from the surrounding air such

as dust.

To reduce the build up of these contaminants, a portion of the water in the tower is bled off

(blow down). This water loss from the tower is then replaced with fresh incoming make-up

water. A conductivity probe or sensor in the tower basin initiates blow down when the levels of

dissolved solids exceed a set value. ‘Cycles of concentration’ (C.O.C.) compare the level of

dissolved solids in the tower’s make-up water to the level of dissolved solids in the tower’s

bleed water.

Equation 9 Cycle of Concentration C.O.C.

𝐂𝐲𝐜𝐥𝐞𝐬 𝐨𝐟 𝐂𝐨𝐧𝐜𝐞𝐧𝐭𝐫𝐚𝐭𝐢𝐨𝐧 𝐂.𝐎. 𝐂 = 𝐓𝐨𝐭𝐚𝐥 𝐝𝐢𝐬𝐬𝐨𝐥𝐯𝐞𝐝 𝐬𝐨𝐥𝐢𝐝𝐬 𝐢𝐧 𝐭𝐡𝐞 𝐦𝐚𝐤𝐞 − 𝐮𝐩 𝐰𝐚𝐭𝐞𝐫

𝐓𝐨𝐭𝐚𝐥 𝐝𝐢𝐬𝐨𝐥𝐯𝐞𝐝 𝐬𝐨𝐥𝐢𝐝𝐬 𝐢𝐧 𝐭𝐡𝐞 𝐛𝐥𝐞𝐞𝐝𝐞𝐝 𝐰𝐚𝐭𝐞𝐫

Increasing the number of C.O.C. will reduce the volume of blow down and consequently the

volume of make-up water required by the tower. The maximum C.O.C. for a tower will depend

on the quality of the make-up water and the corrosion resistance of the tower’s basin and

condenser. C.O.C. over 5 is considered to be efficient but this is not always achievable. Scale

forming ions such as calcium and magnesium can often be precipitated out (by water softeners)

or kept in solution (by acids) through effective water treatment enabling the tower to operate

at higher cycles of concentration.

According to the Queensland Water Commission, a cooling tower is considered inefficient if:

Page 44 of 51


• The system is operating at less than 5 COC or 1850 mg/L TDS/2750 μs/cm conductivity

(allowed only in documented instances of high-TDS make-up water); and/or

• System losses are greater than 8% of the make-up water.

11.1.3 Use alternative water supplies Alternative water supplies have the potential to reduce potable water requirements in cooling

towers, through direct substitution and by reducing the cycles of concentration. Alternative

water supply options include recycled water, process or rainwater. Note that health risks need

to be considered when assessing the viability of alternate water supplies. Additional water

treatment may also be required depending on the quality of water available.

11.1.4 Reuse blow down Potential opportunities to reuse cooling tower blow down include:

• Toilet and urinal flushing (treatment may be required)

• Landscape irrigation (may require dilution with potable or rainwater due to salt content or

treatment)

• Cleaning (health risk assessment may be required and the impacts of corrosion should Be

considered).

11.2 Water treatment Almost all well-managed cooling towers use a water treatment program. The goal of a water

treatment program is to maintain a clean heat transfer surface and preserve capital while

minimizing water consumption and meeting discharge limits. Critical water chemistry

parameters that require review and control include pH, alkalinity, conductivity, hardness,

microbial growth, biocide and corrosion inhibitor levels. Depending on the quality of the make-

up water, treatment programs may include corrosion and scaling inhibitors, such as organo-

phosphate types, along with biological fouling inhibitors. Historically, chemicals have been fed

into the system by automatic feeders on timers or actuated by conductivity meters. Automatic

chemical feeding tends to decrease chemical dosing requirements. Current technology allows

chemicals to be monitored and controlled online 24-7 in proportion to demand. This ensures

results and can allow cycles to be increased. Where overfeed is prevalent, it can reduce

Page 45 of 51


chemical feed, too. Water treatment is required in cooling towers to prevent corrosion of the

system, build up of scale and for microbiological control. Typically this is carried out through

one of the following:

• Direct chemical dosing (to prevent scale and prohibit corrosion)

• Acid dosing (to control ph and scale)

• Ozone dosing (or other microbial treatment to prevent microbial growth)

• Pre-treatment of make-up water (e.g. Water softening, reverse osmosis)

• Side stream filtration (to prevent solid build up)

• Cover exposed areas of cooling towers (to reduce algal growth).

11.2.1 Sulphuric “Acid” Treatment Sulphuric acid can be used in cooling tower water to help control scale build-up. When properly

applied, sulphuric acid will lower the water’s pH and help convert the calcium bicarbonate scale

to a more soluble calcium sulphate form. In central North Carolina, most plants will be able to

operate six to 10 cycles of concentration without acid feed. Along our coasts, acid can be used

to increase cycles as water tends to be harder and higher in alkalinity. The same can be said if

hard alkaline well water is used as tower make-up. Important precautions need to be taken

when using sulphuric acid treatment. Because sulphuric acid is an aggressive acid that will

corrode metal, it must be carefully dosed into the system and must be used in conjunction with

an appropriate corrosion inhibitor. Workers handling sulphuric acid must exercise caution to

prevent contact with eyes or skin. All personnel should receive training on proper handling,

management and accident response for sulphuric acid used at the facility.

11.2.2 Side Stream Filtration In cooling towers that use make-up water with high suspended solids, or in cases where

airborne contaminants such as dust can enter cooling tower water, side stream filtration can be

used to reduce solids build up in the system. Typically, five to 20 per-cent of the circulating flow

can be filtered using a rapid sand filter or a cartridge filter system.

Rapid sand filters can remove solids as small as 15 microns in diameter while cartridges are

effective to remove solids to 10 microns or less. High efficiency filters can remove particles

Page 46 of 51


down to 0.5 microns. Neither of these filters are effective at removing dissolved solids, but can

remove mobile mineral scale precipitants and other solid contaminants in the water. The

advantages of side stream filtration systems are reduced particle loading on the tower. This

ensures heat transfer efficiency and may reduce biocide or dispersant demands.

11.2.3 Ozone Ozone can be a very effective agent to treat nuisance organics in the cooling water. Ozone

treatment also is reported to control the scale by forming mineral oxides that will precipitate

out to the water in the form of sludge. This sludge collects on the cooling tower basin, in a

separation tank or other low-flow areas. Ozone treatment consists of an air compressor, an

ozone generator, a diffuser or contactor and a control system. The initial capital costs of such

systems are high but have been reported to provide payback in 18months.

Table 5 Treatment options comparison

Option Advantages Disadvantages

Operation improvements

to control blow down and

chemical additions

Low capital costs

Low operating costs

Low maintenance requirements

None

Sulphuric acid treatment Low capital cost

Low operating cost

Increased concentration ratio, when

alkalinity limited

Potential safety hazard

Potential for corrosion damage if

overdosed

Side stream filtration Low possibility of fouling

Improved operation efficiency

Moderately high capital cost

No effectiveness on dissolved solids

Additional maintenance

Ozonation Reduced chance for organic fouling

Reduced liquid chemical requirements

High capital investment

Complex system

Possible health issue

Magnet System Reduced or eliminated

chemical usage

Novel technology

Controversial performance claims

Reuse of water within the

facility

Reduces overall facility water

consumption

Potential for increased fouling, scale

or corrosion

Possible need for additional water

treatment

11.2.4 Magnets Some vendors offer special water-treating magnets that are reported to alter the surface

charge of suspended particles in cooling tower water. The particles help disrupt and break loose

Page 47 of 51


deposits on surfaces in the cooling tower system. The particles settle in a low-velocity area of

the cooling tower -- such as sumps --where they can be mechanically removed. Suppliers of

these magnetic treatment systems claim that magnets will remove scale without conventional

chemicals. Also, a similar novel treatment technology, called an electrostatic field generator, is

also reported.

11.2.5 Sonication An emerging technology is sonication or ultrasound which uses vibration to remove fats. This

technology can be used in wastewater systems to emulsify fats making them easier to remove

by methods such as DAF. Sonication has also been trialled in conjunction with anaerobic

treatment as a means of disrupting sludge production to yield a larger quantity of biogas.

11.2.6 Electro coagulation Electro coagulation can be used to remove suspended and colloidal solids, fats, oils and grease

and complex organics. The process involves passing an electrical current through water to

initiate a range of electrochemical reactions which destabilise, suspend, emulsify or dissolve

contaminants in the wastewater which forces them to precipitate.

11.2.7 Activated carbon Activated carbon is generally used after biological or physical-chemical treatment to polish

waste water for reuse. The carbon absorbs both organic and inorganic compounds including

heavy metals. Activated carbon is formed by heating carbon containing substances such as coal

or charcoal in the presence of steam to form highly porous carbon providing a large surface

area for contaminants to adsorb onto. Activated carbon can be regenerated on site by heating

carbon to a high temperature. Using activated carbon prior to a disinfection phase can reduce

the disinfection requirement. The use of activated carbon as part of the cooling tower or boiler

water treatment can lead to better water efficiencies through reduced bleed.

11.2.8 Ultraviolet radiation (UV) This chemical-free method of disinfecting water inactivates microorganisms such as protozoa,

bacteria, moulds and yeasts through the use of ultraviolet radiation. The effectiveness of the

system can be increased with the simultaneous use of ozone. However, water quality

Page 48 of 51


characteristics such as high turbidity, organic components and flow rate can reduce efficacy.

Like ozone, UV radiation does not provide any residual sanitisation compared with chlorine.

11.2.9 Hydrocavitation Hydrocavitation is a chemical free system of water treatment. Two streams of water are

accelerated to high velocities and collide which results in hydrodynamic cavitation and

mechanical shear forces, which are believed to kill bacteria and reduce corrosion activity. It

removes the need for chemicals and can increase the ability to reuse water.

It is generally applied to cooling tower water (refer to case study below) as it can control

corrosion and kill legionella. However, new studies are investigating the efficiency of removing

heavy metals, phosphorous and trichloroethylene (TCE) from wastewater with additional

reductions in BOD.

Figure 11 Hydrocavitation system

11.2.10 Radio frequencies Radio frequencies alter the water’s scaling tendencies by creating a “seeding” mechanism that

agglomerates scale-forming minerals in the water. This technology removes minerals before

they can be deposited on heat exchange surfaces.

Page 49 of 51


12 Recommendation

Based on the study on the assigned project, it is recommended to reduce the water leakages in

the tower by overcoming the construction flaws of the project. Further it also recommended to

ursue the options for water and chemical conservation opportunities in cooling tower

operation. The field will unleash the wide spectrum of cost effective and environmental friendly

operating practices which would be next to the international eco-efficiency standards.

Water conservation will not only reduce the load on environment and natural resources, but

would also enable the organization to claim for eco-efficiency indicator points – a new brand

image perspective.

The adoption of chemical free platforms completely or partially will reduce the cost of chemical

purchases, dependence of service provider and most important – regional leadership in

emerging the cooling water treatment technologies, since the application has only been

adopted in Western continents.

Page 50 of 51


13 References

Bonneville Power Administration. (1991, November). Optimizing Cooling Tower Performance.

Technology Update, pp. 1-4.

Clayton Technologies. (2011). Clayton Cooling Towers. Indore, India: Clayton Technologies India

Pvt. Ltd.

Daeil Aqua Co., Ltd. (2004, May 10). Cooling Tower Thermal Design Manual. Retrieved August

2011, from Cooling Tower Technical Site:

http://myhome.hanafos.com/~criok/english/publication/thermal/thermallisteng.html

Federal Energy Management Program. (2011). NASA Marshal Space Flight Center Improves

Cooling System Performance. Huntsville, Alabama: US Department of Energy.

General Services Administration. (2011). Water Management: A Comprehensive Approach for

Facility Managers. In Water Management Guide (pp. 1-140). Kansas City.

Ken Mortensen. (2003, May). How to Manage Cooling Tower Water Quality. RSE Journal, pp. 1-

4.

Muhammad Yousuf. (2010). Cooling Tower Treatment Manual. Mirpur Mathelo: Fauji Fertilizer

Company.

N.C. Department of Environment and Natural Resources. (2009). Water Efficiency Manual. North

Carolina: N.C. Department of Environment and Natural Resources.

Pacific Northwest National Laboratory. (2011). Cooling Towers: Understanding Key Components

of Cooling Towers and How to Improve Water Efficiency. US Department of Energy.

Ray Congdon, Rand Conger, Mike Groh, Roger van Gelder. (2011). Cooling Tower Efficiency

Manual. In R. C. Ray Congdon, Cool Tunes (pp. 1-26). Washington DC: Water Smart Technology

Program.

Saving Water. (2011, August). Improve Control of Cooling Tower Water. WATER SMART

TECHNOLOGY, pp. 1-2.

SPX Cooling Technologies. (1986). Cooling Tower Performance. USA: Cooling Tower Information

Index.

SPX Cooling Technologies. (2005). Corrosion Protection for Cooling Towers. Kansas: SPX Cooling

Technologies.

Sydney Waters. (2010). Water conservation. Sydney: Sydney Waters.

Page 51 of 51


UNEP. (2006). Cooling Tower. In Energy Efficiency Guide for Industry in Asia (pp. 1-17). New

York: United Nations Environment Programme.

Uniquest Pty Ltd. (2010). Cooling Tower Efficiency. St Lucia, Queensland: Working Group for

Cleaner Production.

Uniquest Pty Ltd. (2010). Cooling Tower - Chill your bill. St Lucia, Queensland: Working Group for

Cleaner Production.

Uniquest Pty Ltd. (2010). Other Treatment Options. St Luciam Quensland: Working Group or

Cleaner Production.

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