Cooling Tower Project Report

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Engro Fertilizers Limited | Daharki

Internship 2011 | Project Report

Cooling Tower

Chemistry and

Chemistry and Performanc

Performance Improvement

e 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

School of Chemical and Materials Engineering (SCME)

Materials Engineering (SCME)

National University of Sciences

National University of Sciences and Technology (NUST)

and Technology (NUST)

Email:

Email: osama_hasan@hot

[email protected]

mail.com

Contact:

Contact: 03453034

03453034516

516 August 2011

August 2011

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1 1 Transmittal

Transmittal

August 24, 2011 August 24, 2011 Mr. Jehangir Alam Khan

Mr. Jehangir Alam Khan Internship Coordinator Internship Coordinator Training Department Training Department

Engro Fertilizers Daharki Limited Engro Fertilizers Daharki Limited

Dear Sir Dear Sir

Please find enclosed the internship report due August 24, 2011.

Please find enclosed the internship report due August 24, 2011. The report as requisite by yourThe report as requisite by your office has been drafted on the assigned project

office has been drafted on the assigned project “Study the Cooling Tower Chemistry and“Study the Cooling Tower Chemistry and Identify Key

Identify Key ParameterParameters for s for Improving Performance”Improving Performance”. The report discusses the cooling tower. The report discusses the cooling tower design, chemistry and performance parameters along with

design, chemistry and performance parameters along with the suitable recommendations forthe suitable recommendations for the assigned project. Feedback will be

the assigned project. Feedback will be most appreciated.most appreciated.

Kind Regards Kind Regards

Osama Hasan Osama Hasan

Intern Operation (URUT III) Intern Operation (URUT III) Undergraduate Student at Undergraduate Student at

School of Chemical and Materials

School of Chemical and Materials Engineering (SCME)Engineering (SCME) National University of Sciences

National University of Sciences and Technology (NUST)and Technology (NUST) H

H – – 12 Islamabad12 Islamabad – – 4400044000 2008

2008 – – NUSTNUST – – BEBE – – ChemChem – – 2727 Email:

Email: [email protected]@hotmail.com Mobile: 03453034516 Mobile: 03453034516

Countersigned

Amer Ahmed Amer Ahmed (Mentor) (Mentor)

Shift Supervisor URUT III Shift Supervisor URUT III

Asim Rasheed Qureshi Asim Rasheed Qureshi

(Group Leader) (Group Leader) Unit Manager URUT III Unit Manager URUT III

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2 2 Acknowledgement

Acknowledgement

Author is than

Author is thankful tokful to

Almighty Allah, Almighty Allah,

For His unlimited blessings and bounties, For His unlimited blessings and bounties,

And for keeping h

And for keeping him sane, sound aim sane, sound and successful;nd successful;

His parents and friends, His parents and friends,

For all their support and trust in him and his aims; For all their support and trust in him and his aims;

His teachers and guides, His teachers and guides,

For teaching him things he knew not; For teaching him things he knew not;

NUST Career Development Centre, NUST Career Development Centre,

For bringing the opportunity of this

For bringing the opportunity of this excellent learning and exposure;excellent learning and exposure;

And last and th

And last and the most importae most important nt

Management and Employees of Engro Fertilizers Limited Management and Employees of Engro Fertilizers Limited

Especially his mentor

Especially his mentor Mr. Amer Ahmed Mr. Amer Ahmed and Unit Manager and Unit Manager Mr. Asim Rasheed Qureshi Mr. Asim Rasheed Qureshi

And all the shif

And all the shift coordinators, sut coordinators, supervisors, trainee engpervisors, trainee engineers, boardmen aineers, boardmen and area operators at Plnd area operators at Plant IIant II

For their utmost help,

For their utmost help, guidance and timeguidance and time

Which made author make most of his internship at plant site; Which made author make most of his internship at plant site;

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3 3 Table of Contents

4 4 List of Figures

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 5 List of Tables

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 6 List of Equations

List of Equations

Equation 1 CT Range ... 15 Equation 1 CT Range ... 15 Equation 2 CT Approach ... 16 Equation 2 CT Approach ... 16 Equation 3 CT Effectiveness ... 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 7 Abstract

Abstract

Cooling towers are one of the most

Cooling towers are one of the most important industrial utilities used to dissipate the unwantedimportant industrial utilities used to dissipate the unwanted

process heat to the atmosphere through the cooling water in the heat exchangers across the process heat to the atmosphere through the cooling water in the heat exchangers across the

plant site. Cooling tower is one of

plant site. Cooling tower is one of the most expensive utility in terms of the most expensive utility in terms of power consumptionpower consumption

and water circulation. Maintaining water quality in the circulation loops is one of the major 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 challenges in process optimization for most efficient performance. To identify the keykey

performance parameters with respect to perspective of

performance parameters with respect to perspective of thethe operations’ team, the wateroperations’ team, the water

chemistry is the

chemistry is the most crucial level and most crucial level and demands proper understanding to maintain completedemands proper understanding to maintain complete

control over the variations. control over the variations.

Latest technological developments have made the water conservation more

Latest technological developments have made the water conservation more efficient and use of efficient and use of

chemicals more limited by introducing

chemicals more limited by introducing“Recycling / reusing water“Recycling / reusing water practicespractices””andand“Chemical“Chemical free platforms”

free platforms”. With limited options available to the designed . With limited options available to the designed and operating cooling tower,and operating cooling tower, these areas could be explored for better and cost effective performance and environment these areas could be explored for better and cost effective performance and environment

friendly impact. friendly impact.

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8 8 Introduction

Introduction

“You cannot create experience, you must undergo it”

Industrial internships are incomparable experience for an undergraduate student. With Industrial internships are incomparable experience for an undergraduate student. With

fertilizer industry holding the maximum learning potential for a chemical engineer,

fertilizer industry holding the maximum learning potential for a chemical engineer, Engro leavesEngro leaves

an impact of its own. The six

an impact of its own. The six week internship experience is unique in every week internship experience is unique in every sense of the word.sense of the word.

The learning opportunities and industrial exposure at the EFERT made not just possible to relate The learning opportunities and industrial exposure at the EFERT made not just possible to relate

the book knowledge to field application but also in

the book knowledge to field application but also in developing a thorough understanddeveloping a thorough understanding of ing of

industrial practices and

industrial practices and operating concepts.operating concepts.

Enven 1.3

Enven 1.3 – – the world largest single train ammonia urea complex was an amazing experiencethe world largest single train ammonia urea complex was an amazing experience

for the author. From the up to date urea

for the author. From the up to date urea complex technology to world’s tallest prcomplex technology to world’s tallest prilling tower, itilling tower, it

added many landmarks in list of experience. With internship project over cooling water added many landmarks in list of experience. With internship project over cooling water

chemistry and performance improvement parameters, the author has

chemistry and performance improvement parameters, the author has compiled the informationcompiled the information

on cooling water design, chemistry and operation; which could serve as a

on cooling water design, chemistry and operation; which could serve as a comprehensive studycomprehensive study

aid on the subject. The recommendations generated are but most effective to date, which aid on the subject. The recommendations generated are but most effective to date, which

should be considered with economical feasibility. should be considered with economical feasibility.

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9 9 Cooling Tower

Cooling Tower

Cooling towers are a very important part of many chemical plants. The p

Cooling towers are a very important part of many chemical plants. The p rimary task of a coolingrimary task of a cooling tower is to reject heat into the

tower is to reject heat into the atmosphere. They represent a relatively inexpensive andatmosphere. They represent a relatively inexpensive and

dependable means of removing low-grade heat from cooling water. The make-up water source dependable means of removing low-grade heat from cooling water. The make-up water source is used to replenish water lost to

is used to replenish water lost to evaporation. Hot water from heat exchangers is sent to theevaporation. Hot water from heat exchangers is sent to the cooling tower. The water exits the cooling tower and is sent back to

cooling tower. The water exits the cooling tower and is sent back to the exchangers or to otherthe exchangers or to other units for further cooling.Cooling towers are able to lower the water temperatures more than units for further cooling.Cooling towers are able to lower the water temperatures more than devices that use only air to

devices that use only air to reject heat, like the radiator in a reject heat, like the radiator in a car, and are therefore more cost-car, and are therefore more cost-effective and energy efficient.

effective and energy efficient.

Figure 1 Schematic of an

Figure 1 Schematic of an Induced Draft Cooling TowerInduced Draft Cooling Tower

9.1 9.1 Components

Components

The basic components of a cooling tower include the frame and

The basic components of a cooling tower include the frame and casing, fill, cold-water basin,casing, fill, cold-water basin, drift eliminators, air inlet, louvers,

drift eliminators, air inlet, louvers, nozzles and fans. These are described below.nozzles and fans. These are described below.

a)

a) Frame and casing:Frame and casing: Most towers have structural frames that support the Most towers have structural frames that support the exterior enclosuresexterior enclosures (casings), motors, fans, and other components. With some

(casings), motors, fans, and other components. With some smaller designs, such as somesmaller designs, such as some glass fibre units, the casing may essentially be

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b)

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

maximizing water and air contact. There are two types of fill:



 Splash fill:Splash fill:Water falls over successive lWater falls over successive layers of horizontal splash bars, continuouslyayers of horizontal splash bars, continuously

breaking into smaller droplets, while also wetting the fill

breaking into smaller droplets, while also wetting the fill surface. Plastic splash fillssurface. Plastic splash fills promote better heat transfer than wood

promote better heat transfer than wood splash fills.splash fills.



 Film fill:Film fill:consists of thin, closely spaced plastic consists of thin, closely spaced plastic surfaces over which the water surfaces over which the water spreads,spreads,

forming a thin film in

forming a thin film in contact with the air. These surfaces may be flat, cocontact with the air. These surfaces may be flat, co rrugated,rrugated, honeycombed, or other patterns. The film type of fill

honeycombed, or other patterns. The film type of fill is the more efficient and providesis the more efficient and provides same heat transfer in a smaller volume than the

same heat transfer in a smaller volume than the splash fill.splash fill. c)

c) Cold-water basin:Cold-water basin: The cold-water basin is located at or near the bottom of the The cold-water basin is located at or near the bottom of the tower, and ittower, and it receives the cooled water that flows down through the tower and fill. The basin

receives the cooled water that flows down through the tower and fill. The basin usually hasusually has a sump or low point for the

a sump or low point for the cold-water discharge connection. Icold-water discharge connection. In many tower designs, then many tower designs, the coldwater basin is beneath the entire fill. In some forced draft counter flow d

coldwater basin is beneath the entire fill. In some forced draft counter flow d esign,esign, however, the water at the bottom of the fil

however, the water at the bottom of the fill is channelled to a l is channelled to a perimeter trough thatperimeter trough that functions as the coldwater basin. Propeller fans are mounted beneath the fill to blow the functions as the coldwater basin. Propeller fans are mounted beneath the fill to blow the airair up through the tower. With this design, the tower is

up through the tower. With this design, the tower is mounted on legs, providing easy accessmounted on legs, providing easy access to the fans and their motors.

to the fans and their motors. d)

d) Drift eliminators:Drift eliminators: These capture water droplets entrapped in the These capture water droplets entrapped in the air stream that otherwiseair stream that otherwise would be lost to the atmosphere.

would be lost to the atmosphere. e)

e) Air inlet:Air inlet: This is the point of entry for the air entering a tower. The inlet may take up anThis 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)

entire side of a tower (cross-flow design) or be located low on the or be located low on the side or the bottom of theside or the bottom of the tower (counter-flow design).

tower (counter-flow design). f)

f) Louvers:Louvers:Generally, cross-flow towers have inlet louvers. The purpose of louvers is Generally, cross-flow towers have inlet louvers. The purpose of louvers is toto equalize air flow into the fil

equalize air flow into the fill and retain the water within the tower. Many counter flowl and retain the water within the tower. Many counter flow tower designs do not require louvers.

tower designs do not require louvers. g)

g) Nozzles:Nozzles: These spray water to wet the fill. These spray water to wet the fill. Uniform water distribution at the top of the fill Uniform water distribution at the top of the fill isis essential to achieve proper wetting of the entire fill

essential to achieve proper wetting of the entire fill surface. Nozzles can either be fixed andsurface. Nozzles can either be fixed and spray in a round or square patterns, or they

spray in a round or square patterns, or they can be part of a rotating assembly as found incan be part of a rotating assembly as found in some circular cross-section towers.

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h)

h) Fans:Fans:Both axial (propeller type) and centrifugal fans are used in towers. Generally,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 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

found in forced draft towers. Depending upon their size, the type of propeller fans used isfans used is either fixed or variable pitch. A fan

either fixed or variable pitch. A fan with non-automatic adjustawith non-automatic adjustable pitch blades can be usedble pitch blades can be used over a wide kW range because the fan

over a wide kW range because the fan can be adjusted to deliver the desired air fcan be adjusted to deliver the desired air flow at thelow at the lowest power consumption. Automatic variable pitch blades can vary air

lowest power consumption. Automatic variable pitch blades can vary air flow in response toflow in response to changing load

changing load conditions.conditions.

9.2 9.2 Materials

Materials

Originally, cooling towers were constructed primarily with wood, including the frame, casing, 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

louvers, fill and cold-water basin. Sometimes the cold-water basin was made of concrete.concrete. Today, manufacturers use a variety

Today, manufacturers use a variety of materials to construct cooling towers.of materials to construct cooling towers.

Materials are chosen to

Materials are chosen to enhance corrosion resistance, reduce maintenance, and promoteenhance corrosion resistance, reduce maintenance, and promote reliability and long service life.

reliability and long service life. Galvanized steel, various grades of stainless steel, glass Galvanized steel, various grades of stainless steel, glass fibre,fibre, and concrete are widely used in tower construction, as well as

and concrete are widely used in tower construction, as well as aluminium and plastics for somealuminium and plastics for some components.

components.

a)

a) Frame and casing.Frame and casing. Wooden towers are still available, but many components are made of 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 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 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 towers (casings and basins) are constructed of galvanized steel or, where a corrosive atmosphere is a problem, the tower and/or the basis are

atmosphere is a problem, the tower and/or the basis are made of stainless steel. Largermade of stainless steel. Larger towers sometimes are made of concrete. Glass fibre i

towers sometimes are made of concrete. Glass fibre i s also widely used for cooling towers also widely used for cooling tower casings and basins, because they extend the life of the cooling tower and provide protection casings and basins, because they extend the life of the cooling tower and provide protection against harmful chemicals.

against harmful chemicals. b)

b) Fill.Fill.Plastics are widely used for fill, Plastics are widely used for fill, including PVC, polypropylene, and other polymers.including PVC, polypropylene, and other polymers. When water conditions require the use of splash fill,

When water conditions require the use of splash fill, treated wood splash fill is still used treated wood splash fill is still used inin wooden towers, but plastic splash fill is also widely used. Because of

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efficiency, film fill

efficiency, film fill is chosen for applications where the circulating water is generally fis chosen for applications where the circulating water is generally f ree of ree of debris that could block the

debris that could block the fill passageways.fill passageways. c)

c) Nozzles.Nozzles. Plastics are also widely used for nozzles. Many nozzles are Plastics are also widely used for nozzles. Many nozzles are made of PVC, ABS,made of PVC, ABS, polypropylene, and glass-filled nylon.

polypropylene, and glass-filled nylon. d)

d) Fans.Fans.Aluminium, glass fibre and hot-dipped galvanized steel are commonly used fanAluminium, glass fibre and hot-dipped galvanized steel are commonly used fan materials. Centrifugal fans are often fabricated from galvanized steel. Propeller fans are materials. Centrifugal fans are often fabricated from galvanized steel. Propeller fans are made from galvanized steel, aluminium, or moulded

made from galvanized steel, aluminium, or moulded glass fibre reinforced plastic.glass fibre reinforced plastic.

9.3 9.3 Types

Types

9.3.1

Natural draft cooling tower

The natural draft or hyperbolic cooling tower makes use of the

The natural draft or hyperbolic cooling tower makes use of the difference in temperaturedifference in temperature between the ambient air and the hotter air inside the

between the ambient air and the hotter air inside the tower. As hot air moves upwards throughtower. As hot air moves upwards through the tower (because hot air rises), fresh cool

the tower (because hot air rises), fresh cool air is drawn into the tower through an air inlet air is drawn into the tower through an air inlet atat the bottom. Due to the layout of the tower,

the bottom. Due to the layout of the tower, no fan is required and there is no fan is required and there is almost no circulationalmost no circulation of hot air that could affect the performance. Concrete is used

of hot air that could affect the performance. Concrete is used for the tower shell with a for the tower shell with a heightheight of up to 200

of up to 200 m. These cooling towers are mostly only for m. These cooling towers are mostly only for large heat duties because largelarge heat duties because large concrete structures are expensive. There are two main types of natural draft towers: concrete structures are expensive. There are two main types of natural draft towers:



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

tower tower



 Counter flow tower:Counter flow tower:air is drawn up through the falling water and the air is drawn up through the falling water and the fill is thereforefill is therefore

located inside the tower, although design

located inside the tower, although design depends on specific site depends on specific site conditionsconditions

9.3.2 9.3.2 Mechanical draft cooling tower

Mechanical draft cooling tower

Mechanical draft towers have large fans to force or draw air through circulated water. The 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 falls downwards over fill surfaces, which help increase the contact time between the water and the air - thi

water and the air - thi s helps maximize heat transfer between the two. Cooling rates of s helps maximize heat transfer between the two. Cooling rates of

mechanical draft towers depend upon various parameters such as fan diameter and speed 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 9.3.3 Open vs. Closed-Circuit Towers

Open vs. Closed-Circuit Towers

One of the primary differentiations between cooling towers is whether it is

One of the primary differentiations between cooling towers is whether it is an open or closed-an open or closed-circuit tower. In open towers, the cooling water is pumped through the equipment where circuit tower. In open towers, the cooling water is pumped through the equipment where itit picks up thermal energy and then flows directly to the

picks up thermal energy and then flows directly to the cooling tower where it is dispersedcooling tower where it is dispersed through spray nozzles over the fill,

through spray nozzles over the fill, where heat transfer occurs. Then, this same water iswhere heat transfer occurs. Then, this same water is collected in the tower sump and is

collected in the tower sump and is sent back to the equipment to begin the sent back to the equipment to begin the process again. In anprocess again. In an open tower any contaminants in the

open tower any contaminants in the water are circulated through the equipment being water are circulated through the equipment being cooled.cooled.

In a closed-circuit tower, sometimes referred to as a fluid

In a closed-circuit tower, sometimes referred to as a fluid cooler, the cooling water flowscooler, the cooling water flows through the equipment as in the open tower. The difference

through the equipment as in the open tower. The difference is when the water is pumped tois when the water is pumped to the cooling tower, it is pumped through a closed loop heat

the cooling tower, it is pumped through a closed loop heat exchanger that is internal to theexchanger that is internal to the cooling tower, then returned to the equipment. In this application, water in the closed loop is 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

not in direct contact with the evaporative water in the tower, which means contaminanmeans contaminants arets are not circulated through the equipment. In a closed-circuit tower, a small pump, known as a 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 “spray pump” circulates a separate body of evaporative water from the tower sump, through the sp

the spray nozzles and over the internal heat exchanger piping. This “open” evaporative body of ray nozzles and over the internal heat exchanger piping. This “open” evaporative body of water is contained within the tower and needs to be regularly

water is contained within the tower and needs to be regularly made up to replenishmade up to replenish evaporative and other losses. However, once water treatment in the closed cooling loop i evaporative and other losses. However, once water treatment in the closed cooling loop i ss stabilized, the only time it

stabilized, the only time it needs to be made up or adjusted is needs to be made up or adjusted is if there is a leak.if there is a leak.

9.3.4 9.3.4 Hybrid Towers

Hybrid Towers

Hybrid towers are closed towers which can operate either in the sensible heat transfer mode 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

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 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

stopped and heat is sensibly transferred to the flow of air across the fins of the coils containingacross the fins of the coils containing the cooling fluid. During periods when this is

the cooling fluid. During periods when this is not enough, a latent heat transfer system isnot enough, a latent heat transfer system is activated by switching on an evaporative cooler or water is sprayed across the dry coils

activated by switching on an evaporative cooler or water is sprayed across the dry coils to allowto allow for increased heat transfer through evaporation. These processes offer

for increased heat transfer through evaporation. These processes offer substantial savings insubstantial savings in water.

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Figure 2 Cooling Tower Types Figure 2 Cooling Tower Types

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Mechanical draft towers are available in a large range of

Mechanical draft towers are available in a large range of capacities. Towers can be eithercapacities. Towers can be either factory built or field erected

factory built or field erected – – for example concrete towers are only 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 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 capacity. Thus, many cooling towers are assemblies of two or more individual cooling towers oror “cells.” The number of cells they

“cells.” The number of cells they have, e.g.,have, e.g., an eight-cell tower, often refers to such tan eight-cell tower, often refers to such towers.owers. Multiple-cell towers can be lineal, square, or round depending upon the shape of the

Multiple-cell towers can be lineal, square, or round depending upon the shape of the individualindividual cells and whether the air inlets are

cells and whether the air inlets are located on the sides or bottoms of the cells.located on the sides or bottoms of the cells.

Table 1 Types of Cooling Towers Table 1 Types of Cooling Towers Type

Type Advantages Advantages DisadvantagesDisadvantages Forced draft

Forced draft

Air is blown through the tower by a fan located Air is blown through the tower by a fan located in the air inlet

in the air inlet



 Suited for high airSuited for high air

resistance due to resistance due to centrifugal blower fans centrifugal blower fans



 Fans are relatively quietFans are relatively quiet 

 Recirculation due to high air-Recirculation due to high

air-entry and low air-exit entry and low air-exit

velocities, which can be solved velocities, which can be solved by locating towers in plant by locating towers in plant rooms combined with rooms combined with discharge ducts discharge ducts Induced draft cross flow

Induced draft cross flow



 Water enters at top and passes Water enters at top and passes over fillover fill 

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

opposite sides (double-flow tower) opposite sides (double-flow tower)



 An induced draft fan draws An induced draft fan draws air across fillair across fill

towards exit at top of towards exit at top of towertower



 Less recirculatiLess recirculation ton thanhan

forced draft towers forced draft towers because the speed of because the speed of exit air is 3-4 times exit air is 3-4 times higher than entering air higher than entering air



 Fans and the motor driveFans and the motor drive

mechanism require mechanism require weather-proofing against moisture and proofing against moisture and corrosion because they are in corrosion because they are in the path of humid exit air the path of humid exit air Induced draft counter flow

Induced draft counter flow



 Hot water enters at the topHot water enters at the top 

 Air enters bottom and exits at the Air enters bottom and exits at the toptop 

 Uses forced and induced draft fansUses forced and induced draft fans

9.4 9.4 Performance

Performance

These measured parameters and then used to determine the cooling tower performance in These measured parameters and then used to determine the cooling tower performance in several ways.

several ways.

a)

a) Range.Range. This is the difference between the This is the difference between the cooling tower water inlet and outlet temperature.cooling tower water inlet and outlet temperature. A high CT Range means that the cooling tower has been able to

A high CT Range means that the cooling tower has been able to reduce the waterreduce the water temperature effectively, and is thus performing well. The formula i

temperature effectively, and is thus performing well. The formula i s:s:

Equation 1 CT Range Equation 1 CT Range



 



(°(°



) ) ==



 

 



(°(°



))

−

− 

 

 



(°(°



)) b)

b) Approach. Approach.This is the difference between the cooling tower outlet coldwater temperatureThis is the difference between the cooling tower outlet coldwater temperature and ambient wet bulb temperature. The lower the approach the better the cooling tower and ambient wet bulb temperature. The lower the approach the better the cooling tower

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performance; although

performance; although, both range and approach should be , both range and approach should be monitored, the `Approach’ is amonitored, the `Approach’ is a better indicator of cooling

better indicator of cooling tower performance.tower performance.

Equation 2 CT Approach Equation 2 CT Approach



 



(°(°



) ) ==



 

 



(°(°



))

−

− 

 

 



(°(°



))

Figure 3 Range and approach schematic Figure 3 Range and approach schematic

c)

c) EffectivenessEffectiveness. This is the ratio . This is the ratio between the range and the ideal range (in percentage), i.e.between the range and the ideal range (in percentage), i.e. difference between cooling water inlet

difference between cooling water inlet temperature and ambient wet bulb temperature, ortemperature and ambient wet bulb temperature, or in other words it is

in other words it is = Range / (Range + Approach). The higher this ratio, the = Range / (Range + Approach). The higher this ratio, the higher thehigher the cooling tower effectiveness.

cooling tower effectiveness.

Equation 3 CT Effectiveness Equation 3 CT Effectiveness   ((%%)) ==((  ––   )) ((   ––  )) ××  d)

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

(17)

e)

e) Evaporation lossEvaporation loss. This is the water quantity evaporated for cooling duty. Theoretically the. This is the water quantity evaporated for cooling duty. Theoretically the evaporation quantity works out to 1.8 m

evaporation quantity works out to 1.8 m33for every 1,000,000 kCal heat rejected. Thefor every 1,000,000 kCal heat rejected. The following formula can be used (Perry):

following formula can be used (Perry):

Equation 4 Evaporation Loss Equation 4 Evaporation Loss



 ((__)) == .. ×× ..   ((__) × ) × ((__−− __))

T

T11- T- T22= temperature difference between inlet = temperature difference between inlet and outlet waterand outlet water

f)

f) Cycles of Cycles of concentrationconcentration(C.O.C). This is the ratio of dissolved solids in (C.O.C). This is the ratio of dissolved solids in circulating water tocirculating water to the dissolved solids in makeup water.

the dissolved solids in makeup water. g)

g) Blow down lossesBlow down losses depend upon cycles of concentration and the evaporation losses and isdepend upon cycles of concentration and the evaporation losses and is given by formula:

given by formula:

Equation 5 Blow down Equation 5 Blow down



  ==  



......−−  h)

h) Liquid/Gas (L/G) ratioLiquid/Gas (L/G) ratio. The L/G ratio of . The L/G ratio of a cooling tower is the ratio between the water a cooling tower is the ratio between the water andand the air mass flow rates. Cooling towers have certain design values, but seasonal variations 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

require adjustment and tuning of water and air flow rates to get the best cooling towerbest cooling tower effectiveness. Adjustments can be made by water box loading changes or blade angle effectiveness. Adjustments can be made by water box loading changes or blade angle adjustments. Thermodynam

adjustments. Thermodynamic rules also ic rules also dictate that the heat removed dictate that the heat removed from the water mustfrom the water must be equal to the heat absorbed by the surrounding air. Therefore the following formulae can be equal to the heat absorbed by the surrounding air. Therefore the following formulae can be used:

be used:

((__−− __) ) == ((__−− __)) Equation 6 Liquid/Gas ratio Equation 6 Liquid/Gas ratio

  == ((__−− __)) ((__−− __)) Where: Where:

L/G = liquid to gas mass flow

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

T11= hot water temperature (= hot water temperature (°°C)C)

T

(18)

h

h22= enthalpy of air-water = enthalpy of air-water vapour mixture at exhaust wet-bulb temperaturevapour mixture at exhaust wet-bulb temperature

h

h11= enthalpy of air-water vapour mixture at inlet wet-bulb temperature= enthalpy of air-water vapour mixture at inlet wet-bulb temperature

9.5 9.5 Assessment

Assessment

The performance of cooling towers is evaluated to assess present levels of

The performance of cooling towers is evaluated to assess present levels of approach and rangeapproach and range against their design values, identify areas of

against their design values, identify areas of energy wastage and to suggest improvements.energy wastage and to suggest improvements. During the performance evaluation, portable monitoring instruments are used

During the performance evaluation, portable monitoring instruments are used to measure theto measure the following parameters:

following parameters:



 Wet bulb temperature of airWet bulb temperature of air 

 Dry bulb temperature of airDry bulb temperature of air 

 Cooling tower inlet water Cooling tower inlet water temperaturetemperature 

 Cooling tower outlet water temperatureCooling tower outlet water temperature 

 Exhaust air temperatureExhaust air temperature 

 Electrical readings of pump and fan motorsElectrical readings of pump and fan motors 

 Water flow rateWater flow rate 

 Air flow rateAir flow rate

9.6 9.6 Factors Affecting Performance

Factors Affecting Performance

9.6.1 9.6.1 Design

Design

9.6.1.1

9.6.1.1 Capacity Capacity

Heat dissipation (in kCal/hour) and circulated flow rate (m

Heat dissipation (in kCal/hour) and circulated flow rate (m33/hr) are not /hr) are not sufficient to understandsufficient to understand cooling tower performance. Other factors, which we will see, must be stated along with flow cooling tower performance. Other factors, which we will see, must be stated along with flow rate m

rate m33/hr. For example, a cooling tower sized to cool 4540 /hr. For example, a cooling tower sized to cool 4540 mm33/hr through a 13.9/hr through a 13.9°°_{C range might}_{C range might}

be larger than a cooling tower to cool 4540 m

be larger than a cooling tower to cool 4540 m33/hr through 19.5/hr through 19.5°°_{C range.}_{C range.}

9.6.1.2

9.6.1.2 RangeRange

Range is determined not by the cooling tower, but by

Range is determined not by the cooling tower, but by the process it is serving. The range the process it is serving. The range at theat the exchanger is determined entirely by the heat load and the

exchanger is determined entirely by the heat load and the water circulation rate through thewater circulation rate through the exchanger and on to the cooling water.

exchanger and on to the cooling water.

Equation 7 CT Range Def. 2 Equation 7 CT Range Def. 2



°° ==   ((//))



(19)

Thus, Range is a function of the heat

Thus, Range is a function of the heat load and the flow circulated through the system.load and the flow circulated through the system.

Cooling towers are usually specified to cool a

Cooling towers are usually specified to cool a certain flow rate from one temperature tocertain flow rate from one temperature to

another temperature at a certain wet bulb temperature. For example, the cooling tower might another temperature at a certain wet bulb temperature. For example, the cooling tower might be specified to cool 48000 m

be specified to cool 48000 m33

/hr from 44

/hr from 44°°_{C to 34}_{C to 34}°°_{C at 26.7}_{C at 26.7}°°_{C wet bulb temperature.}_{C wet bulb temperature.}



  ((°°) ) ==   ((°°))−−  ((°°))

As a generalization, the closer the approach to the wet bulb, the more

As a generalization, the closer the approach to the wet bulb, the more expensive the coolingexpensive the cooling tower due to increased size. Usually a 2.8

tower due to increased size. Usually a 2.8°°_{C approach to the design wet bulb is the coldest}_{C approach to the design wet bulb is the coldest}

water temperature that cooling tower manufacturers will guarantee. If

water temperature that cooling tower manufacturers will guarantee. If flow rate, range,flow rate, range, approach and wet bulb had to be ranked in the order

approach and wet bulb had to be ranked in the order of their importance in sizing a tower,of their importance in sizing a tower, approach would be first with flow rate closely following the

approach would be first with flow rate closely following the range and wet bulb would be of range and wet bulb would be of lesser importance.

lesser importance.

The range increases when the quantity of circulated water and heat load increase. This means 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

that increasing the range as a result of added heat load requires a larger tower. There are larger tower. There are twotwo possible causes for the increased range:

possible causes for the increased range:



 The inlet water temperature is increased (and the cold-water temperature at the exitThe inlet water temperature is increased (and the cold-water temperature at the exit

remains the same). In this case it is

remains the same). In this case it is economical to invest in removing the additional heat.economical to invest in removing the additional heat.



 The exit water temperature is decreased (and the hot water temperature at the inletThe exit water temperature is decreased (and the hot water temperature at the inlet

remains the same). In this case the tower size

remains the same). In this case the tower size would have to be increased considerablywould have to be increased considerably because the approach is also reduced, and this is not always economical.

because the approach is also reduced, and this is not always economical.

9.6.1.3

9.6.1.3 Heat Load Heat Load

The heat load imposed on a

The heat load imposed on a cooling tower is determined by the process being served. Thecooling tower is determined by the process being served. The degree of cooling required is

degree of cooling required is controlled by the desired operating temperature level of thecontrolled by the desired operating temperature level of the process. In most cases, a low operating temperature is desirable to increase

process. In most cases, a low operating temperature is desirable to increase process efficiencyprocess efficiency or to improve the quality or

or to improve the quality or quantity of the product. In some applications (e.g. internalquantity of the product. In some applications (e.g. internal combustion engines), however, high operating temperatures are desirable. The

combustion engines), however, high operating temperatures are desirable. The size and cost of size and cost of the cooling tower is proportional to the heat load. If

(20)

equipment will be purchased. If the calculated load is high, oversize

equipment will be purchased. If the calculated load is high, oversize and more costly,and more costly, equipment will result.

equipment will result.

Process heat loads may

Process heat loads may vary considerably depending upon the process involved. Determinationvary considerably depending upon the process involved. Determination of accurate process heat loads

of accurate process heat loads can become very complex but can become very complex but proper consideration can produceproper consideration can produce satisfactory results. On the other hand, air conditioning and refrigeration heat loads can be satisfactory results. On the other hand, air conditioning and refrigeration heat loads can be determined with greater accuracy.

determined with greater accuracy.

9.6.1.4

9.6.1.4 Wet Bulb TemperatureWet Bulb Temperature

Wet bulb temperature is an important factor in performance of evaporative water cooling 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

equipment. It is a controlling factor from the aspect of minimum cold water temperature towater temperature to which water can be cooled by the evaporative method. Thus, the wet bulb temperature of the which water can be cooled by the evaporative method. Thus, the wet bulb temperature of the air entering the

air entering the cooling tower determines operating temperature levels cooling tower determines operating temperature levels throughout the plant,throughout the plant, process, or system. Theoretically, a cooling tower will cool water to the

process, or system. Theoretically, a cooling tower will cool water to the entering wet bulbentering wet bulb temperature, when operating without a heat load. However, a thermal potential is required to temperature, when operating without a heat load. However, a thermal potential is required to reject heat, so it is

reject heat, so it is not possible to cool water to the entering air wet not possible to cool water to the entering air wet bulb temperature, when abulb temperature, when a heat load is applied. The approach obtained is a function of thermal conditions and tower heat load is applied. The approach obtained is a function of thermal conditions and tower capability.

capability.

Initial selection of towers with respect to design wet

Initial selection of towers with respect to design wet bulb temperature must be made on thebulb temperature must be made on the basis of conditions existing at the tower site. The temperature selected is generally close

basis of conditions existing at the tower site. The temperature selected is generally close to theto the average maximum wet bulb for the summer months. An important aspect of wet bulb selection average maximum wet bulb for the summer months. An important aspect of wet bulb selection is whether it is spe

is whether it is specified as ambient or inlet. The ambient wet cified as ambient or inlet. The ambient wet bulb is the temperature, whichbulb is the temperature, which exists generally in the cooling tower

exists generally in the cooling tower area, whereas inlet wet bulb is the wet bulb temperaturearea, whereas inlet wet bulb is the wet bulb temperature of the air entering the tower. The

of the air entering the tower. The later can be, and often is, affected by dilater can be, and often is, affected by discharge vapours beingscharge vapours being re-circulated into the tower. Recirculation raises the effective wet bulb temperature of the air 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 entering the tower with corresponding increase in the cold water temperature. Since there is no initial knowledge or control over the recirculation factor, the

no initial knowledge or control over the recirculation factor, the ambient wet bulb should beambient wet bulb should be specified. The cooling tower supplier is required to furnish a

specified. The cooling tower supplier is required to furnish a tower of sufficient capability totower of sufficient capability to absorb the effects of the increased wet bulb temperature peculiar to his own equipment. absorb the effects of the increased wet bulb temperature peculiar to his own equipment.

(21)

It is very

It is very important to have the cold water temperature low enough to exchange heat or toimportant 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 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 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

cooling tower water can be selected to get the maximum economy for the particular process.maximum economy for the particular process.

The Table 7.1 illustrates the effect

The Table 7.1 illustrates the effect of approach on the size and cost of a cooling tower. Theof approach on the size and cost of a cooling tower. The towers included were sized to cool 4540 m

towers included were sized to cool 4540 m33/hr through a 16.67/hr through a 16.67°°_{C range at a 26.7}_{C range at a 26.7}°°_{C design wet}_{C design wet}

bulb. The overall width of all

bulb. The overall width of all towers is 21.65 meters; the overall height, 15.25 meters, and towers is 21.65 meters; the overall height, 15.25 meters, and thethe pump head, 10.6 m

pump head, 10.6 m approximately.approximately.

The design wet bulb temperature is determined by the

The design wet bulb temperature is determined by the geographical location. For a certaingeographical location. For a certain approach value (and at a constant range and flow range), the higher the wet bulb temperature, 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

the smaller the tower required. For example, a 4540 mm33/hr cooling tower selected for a16.67/hr cooling tower selected for a16.67°°_C_C

range and a 4.45

range and a 4.45°°_{C approach to 21.11}_{C approach to 21.11}°°_{C wet bulb would be larger than the same}_{C wet bulb would be larger than the same tower to a}_{tower to a}

26.67

26.67°°_{C wet bulb. The reason is that air}_{C wet bulb. The reason is that air at the higher wet bulb temperature is}_{at the higher wet bulb temperature is capable of}_{capable of}

picking up more heat. This is explained for the

picking up more heat. This is explained for the two different wet bulb temperatures:two different wet bulb temperatures:



 Each kg of air entering the tower at a Each kg of air entering the tower at a wet bulb temperature of 21.1wet bulb temperature of 21.1°°C contains 18.86 kCal. If C contains 18.86 kCal. If

the air leaves the tower at

the air leaves the tower at 32.232.2°°C wet bulb temperature, each kg of air contains 24.17 kCal.C wet bulb temperature, each kg of air contains 24.17 kCal. At an increase of 11.1

At an increase of 11.1°°_{C, the air picks up 12.1 kCal per}_{C, the air picks up 12.1 kCal per kg of air.}_{kg of air.}



 Each kg of air entering the tower at a Each kg of air entering the tower at a wet bulb temperature of 26.67wet bulb temperature of 26.67°°C contains 24.17 kCals.C contains 24.17 kCals.

If the air leaves at 37.8

If the air leaves at 37.8°°_{C wet bulb temperature, each kg of air contains 39.67 kCal. At an}_{C wet bulb temperature, each kg of air contains 39.67 kCal. At an}

increase of 11.1

increase of 11.1°°_{C, the air picks up 15.5 kCal per kg}_{C, the air picks up 15.5 kCal per kg of air, which is much more than the}_{of air, which is much more than the first}_first

scenario. scenario.

9.6.1.5

9.6.1.5 Tower SizeTower Size If heat load,

If heat load, range, approach and wet-bulb temperature are held range, approach and wet-bulb temperature are held constant, changing the fourthconstant, changing the fourth will affect the tower size as

will affect the tower size as follows:follows:

a)

a) Tower size varies inversely with Tower size varies inversely with approach. A longer approach requirapproach. A longer approach requires a smaller tower.es a smaller tower. Conversely, a smaller approach requires an increasingly larger tower and, at 5°F

(22)

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

Figure 4 Tower size v/s approach Figure 4 Tower size v/s approach

b)

b) Tower size varies inversely with Tower size varies inversely with wet bulb temperature. When heat load, range, andwet bulb temperature. When heat load, range, and

approach values are fixed, reducing the design wet-bulb temperature increases the size of 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

the tower. This is because most of the heat transfer in a cooling tower occurs by virtue of a cooling tower occurs by virtue of ev

evaporation (which extracts approximately 1000 Btu’s for every aporation (which extracts approximately 1000 Btu’s for every pound of waterpound of water evaporated), and air’s ability to

evaporated), and air’s ability to absorb moisture reduces with temperature.absorb moisture reduces with temperature.

Figure 5 Tower size v/s wet-bulb Figure 5 Tower size v/s wet-bulb

c)

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Figure 6 Tower size v/s head load Figure 6 Tower size v/s head load

d)

d) Tower size varies inversely with Tower size varies inversely with range. Two primary factors account for this. First; increasingrange. Two primary factors account for this. First; increasing the range

the range——also increases the ITD (driving force) between the incoming hot wateralso increases the ITD (driving force) between the incoming hot water temperature and the entering wet-bulb temperature. Second, increasing the

temperature and the entering wet-bulb temperature. Second, increasing the range (at arange (at a constant heat load) requires that the water flow rate be decreased

constant heat load) requires that the water flow rate be decreased——which reduces thewhich reduces the static pressure opposing the flow of air.

static pressure opposing the flow of air.

Figure 7 Tower size v/s range variance Figure 7 Tower size v/s range variance

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9.6.2 9.6.2 Fill media effects

Fill media effects

In a cooling tower, hot water is di

In a cooling tower, hot water is distributed above fill media and is cooled down stributed above fill media and is cooled down throughthrough evaporation as it flows down the tower and gets in contact with air. The f

evaporation as it flows down the tower and gets in contact with air. The f ill media impactsill media impacts energy consumption in two ways:

energy consumption in two ways:



 Electricity is used for pumping above the fill Electricity is used for pumping above the fill and for fans that create the air draft. Anand for fans that create the air draft. An

efficiently designed fill media with appropriate water distribution, drift eliminator, fan, efficiently designed fill media with appropriate water distribution, drift eliminator, fan, gearbox and motor with therefore

gearbox and motor with therefore lead to lower lead to lower electricity consumption.electricity consumption.



 Heat exchange between air and water is influenced by surface area of heat exchange,Heat exchange between air and water is influenced by surface area of heat exchange,

duration of heat exchange (interaction) and turbulence in

duration of heat exchange (interaction) and turbulence in water effecting thoroughness of water effecting thoroughness of intermixing. The fill media determines all

intermixing. The fill media determines all of these and therefore influences the heatof these and therefore influences the heat exchange. The greater the heat exchange, the more effective the cooling tower

exchange. The greater the heat exchange, the more effective the cooling tower becomes.becomes.

There are three types of fills: There are three types of fills: a)

a) Splash fill media.Splash fill media.Splash fill media generates the required heat exchange area by splashingSplash fill media generates the required heat exchange area by splashing water over the fill

water over the fill media into smaller water droplets. The surface area of the media into smaller water droplets. The surface area of the water dropletswater droplets is the surface area for heat exchange with the air.

is the surface area for heat exchange with the air. b)

b) Film fill media.Film fill media. In a film fill, water forms a thin film on either side of fill sheets. The surfaceIn a film fill, water forms a thin film on either side of fill sheets. The surface area of the fill

area of the fill sheets is the area for heat exchange with the sheets is the area for heat exchange with the surrounding air. Film fill cansurrounding air. Film fill can result in significant electricity savings due to fewer air

result in significant electricity savings due to fewer air and pumping head requirements.and pumping head requirements. c)

c) Low-clog film fills.Low-clog film fills. Low-clog film fills with higher flute sizes were Low-clog film fills with higher flute sizes were recently developed torecently developed to handle high turbid waters. Low clog film fills

handle high turbid waters. Low clog film fills are considered as the best choice for sea waterare considered as the best choice for sea water in terms of power savings and performance compared to conventional splash type fills. in terms of power savings and performance compared to conventional splash type fills.

Table 2 Design Values of Differe

Table 2 Design Values of Differe nt Fillsnt Fills Splash fill

Splash fill Film fillFilm fill Low clog film fillLow clog film fill Possible

Possible L/G L/G ratio ratio 1.11.1_–_–1.5 1.5 1.51.5_–_–2.0 2.0 1.41.4_–_–1.81.8 Effective

Effective heat heat exchange exchange area area 3030_–_–45 m2/m3 45 m2/m3 150 150 m2/m3 m2/m3 85 85 - - 100 100 m2/m3m2/m3 Fill

Fill height height required required 55_–_–10 10 m m 1.21.2_–_–1.5 1.5 m m 1.51.5_–_–1.8 m1.8 m Pumping

Pumping head head required required 99_–_–12 12 m m 55_–_–8 m 8 m 66_–_–9 m9 m Quantity

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9.6.3 9.6.3 Water Distribution

Water Distribution

9.6.3.1

9.6.3.1 Optimize cooling water treatment Optimize cooling water treatment Cooling water treatment (e.g. to

Cooling water treatment (e.g. to control suspended solids, algae growth) is mandatory for control suspended solids, algae growth) is mandatory for anyany cooling tower independent of what fill media is used. Wi

cooling tower independent of what fill media is used. Wi th increasing costs of water, efforts toth increasing costs of water, efforts to increase Cycles of Concentration (COC), by cooling water treatment would help to reduce make 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 up water requirements significantly. In large industries and power plants improving the COC is often considered a key area for water conservation.

often considered a key area for water conservation.

9.6.3.2

9.6.3.2 Install drift eliminatorsInstall drift eliminators It is very difficult

It is very difficult to ignore drift problems in cooling towers. Nowadays most of the end userto ignore drift problems in cooling towers. Nowadays most of the end user specifications assume a 0.02% drift l

specifications assume a 0.02% drift loss. But thanks to technological developments and theoss. But thanks to technological developments and the production of PVC, manufacturers have improved drift eliminator designs. As a result drift production of PVC, manufacturers have improved drift eliminator designs. As a result drift losses can now be as low as

losses can now be as low as 0.0030.003 – –0.001%.0.001%.

9.6.4 9.6.4 Fans

Fans

The purpose of a cooling tower fan is

The purpose of a cooling tower fan is to move a specified quantity of air through the system.to move a specified quantity of air through the system. The fan has to overcome the system resistance, which is defined

The fan has to overcome the system resistance, which is defined as the pressure loss, to as the pressure loss, to movemove the air. The fan output or work done by

the air. The fan output or work done by the fan is the product of air fthe fan is the product of air flow and the pressure llow and the pressure loss.oss. The fan output and kW input determines the fan efficiency.

The fan output and kW input determines the fan efficiency. The fan efficiency in turn is

The fan efficiency in turn is greatly dependent on the profile of the blade. Blades greatly dependent on the profile of the blade. Blades include:include:

a)

a) Metallic bladesMetallic blades, which are manufactured by extrusion or casting processes and therefore it, which are manufactured by extrusion or casting processes and therefore it is difficult to produce ideal aerodynamic profiles

is difficult to produce ideal aerodynamic profiles b)

b) Fibre reinforced plastic (FRP) bladesFibre reinforced plastic (FRP) blades, are normally hand moulded which makes it easier to, are normally hand moulded which makes it easier to produce an optimum aerodynamic profile tailored to

produce an optimum aerodynamic profile tailored to specific duty conditions. Because FRPspecific duty conditions. Because FRP fans are light, they need a l

fans are light, they need a low starting torque requiring a lower HP motor, the lives ow starting torque requiring a lower HP motor, the lives of theof the gear box, motor and bearing is increased, and maintenance is easier.

gear box, motor and bearing is increased, and maintenance is easier.

A 85-92% efficiency can be achieved with blades with an

A 85-92% efficiency can be achieved with blades with an aerodynamic profile, optimum twist,aerodynamic profile, optimum twist, taper and a high coefficient of lift

taper and a high coefficient of lift to coefficient of drop ratio. However, this efficiency to coefficient of drop ratio. However, this efficiency isis drastically affected by factors such as tip clearance, obstacles to airflow and inlet shape, drastically affected by factors such as tip clearance, obstacles to airflow and inlet shape, etc.etc.