Dalhousie University Mechanical Engineeringpoisson.me.dal.ca/site2/courses/mech4340/MECH4830 - Cooling Tow… · 3. Cooling Tower Types ... include wet cooling versus dry cooling,

Dalhousie University – Mechanical Engineering

MECH 4830 – Energy Management

Cooling Towers

February 13, 2015

Greg Donovan

Meagan Buchanan

MECH 4340 – Energy Management M. Buchanan Cooling Towers G. Donovan

Dalhousie University Department of Mechanical Engineering Page 2 of 32

Table of Contents

List of Figures ................................................................................................................................................ 3

Abstract ......................................................................................................................................................... 4

1. Introduction ........................................................................................................................................ 5

2. Operating Conditions .......................................................................................................................... 8

3. Cooling Tower Types ......................................................................................................................... 10

3.1. Natural Draft Cooling Towers ................................................................................................... 11

3.2. Mechanical Draft Cooling Tower .............................................................................................. 12

3.3. Cooling fluids ............................................................................................................................. 13

4. Cooling Tower Components ............................................................................................................. 15

4.1. Drive Systems ............................................................................................................................ 17

4.2. Driven Systems .......................................................................................................................... 20

4.3. Fixed Equipment ....................................................................................................................... 25

5. Summary ........................................................................................................................................... 28

References .................................................................................................................................................. 32



List of Figures

Figure 1: Cooling Tower Characteristics Curve ............................................................................................. 6

Figure 2 : Cooling tower cell with multiple fans [2] ...................................................................................... 9

Figure 3 – Wet cooling tower representation [5] ....................................................................................... 15

Figure 4 : Air Cooled Heat Exchanger Representation [9] .......................................................................... 16

Figure 5: Two rotor turbine with cover removed [12] ................................................................................ 19

Figure 6 : Cutaway of centrifugal pump [13] .............................................................................................. 21

Figure 7 - Simplified pump and heat exchanger network of a cooling water system [12] ......................... 22



Abstract

Cooling Towers, are used in different applications worldwide for process cooling purposes. These

applications include power plants, oil refineries, chemical plants and even building cooling. This

report will delve into how cooling towers function, different types of cooling towers including wet

cooling and dry cooling and different airflow types of the coolers including cross and counter flow

towers. This report discusses the different components of the cooling towers including the fixed

equipment, the drive and driven equipment.

A discussion of fixed equipment including the piping network of wet cooling towers, including

problems with corrosion in the pipes due to the microbial content of the water is performed. The air

cooled heat exchanger tube bank fin types and fouling are also discussed.

Drive systems of motors and turbines are discussed. The types of motors used in cooling towers are

explained and the gains of motor efficiency increases are explored. Turbine drives are also discussed

with the fundamental operation and benefits to turbine use outlined.

Driven systems of fan drives and pumps are considered. Efficiency gains from different fan blades,

installation and alignment and belt types are shown. The different types of pumps used in cooling

towers are outlined and the importance of pump network configuration discussed.



1. Introduction

Cooling towers are used to reject heat from industrial and commercial processes as required. This is

performed by transferring heat from a cooling fluid, typically water, but occasionally air or another

fluid, by means of an interfacial film. This heat is then transferred from the film to air by typical heat

transfer, or evaporative methods. [1] The amount of heat transferred from the cooing fluid must be

equal to that of the heat absorbed by the air flowing through the tower. This heat transfer can be

represented by the following equation:

𝐿

𝐺=

ℎ2 − ℎ1

𝑇1 − 𝑇2

Where L/G represents the liquid to gas ratio of water to dry air, h represents the enthalpy at the

exhaust and inlet wet bulb temperatures and the temperatures of the hot and cold waters. [1] This

equation is typically used to express the cooling tower characteristics. This equation can also be

related to the following equation:

𝐾𝑎𝑉

𝐿= ∫

𝑑𝑇

ℎ𝑤 − ℎ𝑎

𝑇1

𝑇2

Which represents the heat transfer obtained from the transfer of sensible heat and the evaporation

of water. KaV/L represents the cooling tower characteristics. [1]With these two equations known it

is possible, in a similar manner to that of steam tables, to predict the cooling tower performances by

use of Figure 1, from the Standard Handbook of Plant Engineering book below.



Figure 1: Cooling Tower Characteristics Curve

This allows a proper tower with appropriate cooling capabilities to be chosen. It is worth noting that

the most important design characteristic is the ratio of liquid to gas for cooling tower choice. As

cooling towers will inherently not operate at their design point it is impertinent to view the

characteristic curve before a cooling tower is chosen for implementation. [1]



Cooling tower operation has certain loss factors that should be accounted for. This includes losses

such as evaporation loss due to the increase of heat into the cooling water. Another loss that should

be considered is drift loss from the water vapour that will be present in the exhaust airflow from the

tower, and blowdown losses which is from water that is discharged to prevent mineral or solid

buildup in the cooling water. To account for these losses make-up water must be added to the system.

The amount of make-up water required can be determined by using rules of thumbs to determine

the amount of water from the above losses.



2. Operating Conditions

Cooling towers typically run well under what would be considered normal ambient conditions, which

would include average temperatures above zero and little precipitation. These conditions represent

the design conditions of the cooling towers and are when the tower would experience maximum

efficiency. However, these ideal conditions do not exist for some regions of the world for at least half

of the year. Canada would be an excellent example of this an atypical climate from the original design

climate.

The atypical climate conditions require that special precautions be taken into account to operate

cooling towers in regions with colder weather. The threshold for sustained freezing conditions is

more than 24 hours below zero, and no freeze thaw cycle in effect. [2] During these conditions it is

required to maintain the proper heat load, this will prevent wet cooling towers from having the water

freeze when it passes through the exchanger. If this heat load cannot be maintained it is important to

bypass the tower to prevent any damage to the equipment. It is vitally important to inspect the

towers during colder months to ensure that there is no ice buildup. [2]

It is also important to run the cooling towers under the intended manufacturer operating conditions.

This includes ensuring that the minimum or more water flow rate is obtained over the cooling tower.

This may decrease efficiency of the tower but will prevent damage to the equipment. [2]

For towers with multiple cells it is necessary to ensure that during operation the fans are not run in

such a manner that promotes the water to be cooled too much. A visual representation of this is

available in Figure 2.



Figure 2 : Cooling tower cell with multiple fans [2]

As shown above, if only one fan is run in a cell there is the potential that the water in the third cell

could be freezing, as it is close to the freezing point. To counteract this effect, often variable speed

drives are placed on each of the fan cells and are all run at the same speed. This enables all the fans

to be running and prevents the temperature gradient difference. [2]

To increase general fan efficiency during winter months fan blade pitch can be adjusted to account

for the difference in the air density between warm and cold weather application. Re-pitching the fan

depending on the weather conditions can add unwanted maintenance costs that can counteract the

efficiency gains. Adjusting fan pitches that have never been re-pitched can also cause cracking along

the hub or blades and lead to failures. This is why most users will leave the fan pitched at the design

maximum, which is less efficient for atypical climate, and use variable speed drives to mitigate the

freezing conditions. [3]



3. Cooling Tower Types

There are different types of cooling towers currently in application, mostly depending on the location

and resources available. There are natural draft cooling devices such as pond cooling, spray and wood

filled atmospheric cooling and chimney towers. Predominately in oil refineries mechanical draft

cooling towers are employed as they allow the amount of air supply to be controlled using a fan.

There are different configurations of the mechanical draft cooling towers that can be implemented,

once again depending on the types of resources available. [4] The main differences in system designs

include wet cooling versus dry cooling, the manner in which air is induced into the system, either

forced or induced draft and the difference in the manner the air flows through the cooling system

either in a counter or cross flow. [4]



3.1. Natural Draft Cooling Towers

Natural draft cooling towers do not use any mechanical device such as a fan to achieve airflow

throughout the tower. These devices can be configured in manners such as an atmospheric spray

tower, which has the airflow travel through a spray of water, or a draft tower where water is added

to a natural flow of air for cooling. [5] Another type of natural draft cooling towers is a dry manner

in which a hot fluid flows through radiators where airflow flows over the radiators to cool the fluid.

Natural draft cooling towers are typically employed at steam power plants for cooling purposes. [6]

Natural draft cooling towers loose cooling efficiency as crosswind velocities increase. Traditional

natural draft cooling towers are design to run most efficiently at wind speeds of approximately 3m/s

(or 7 mph), wind breakers either in a radiator or solid fashion can be implemented to decelerate

airflow, increasing efficiencies up to 9%. [6] A study performed by M. Goodarzi and R. Ramezanpour

have explored alternative geometries of the natural draft cooling towers to improve cooling

efficiencies when the towers are subjected to a crosswind. It was determined that for wind speeds of

approximately 10m/s (or 22 mph) the cooling efficiency of round cooling towers increased from

80.7% to 84.4% (+3.7%) for an elliptical cooling towers with a diameter ratio of 0.75, and 91.8%

(+11.1%) for a cooling tower with an elliptical cross section of 0.50. [6]



3.2. Mechanical Draft Cooling Tower

Mechanical draft cooling towers employ fans to regulate the volume of air that is passed over the

fluid requiring cooling. This means that mechanical tower cooling efficiencies are considered to have

much more stable values as they do not depend on wind speed like the natural draft cooling towers.

The fan cooling present in mechanical draft towers means the cooling system will be subject from

less change due to atmospheric conditions. Mechanical cooling towers can be forced or induced draft,

and can be either wet or dry cooling systems. [5]

3.2.1. Mechanical Draft Variations

There are different types of mechanical draft coolers. There are induced and forced draft coolers, this

indicates how the airflow is drawn over the cooler. Induced draft coolers use a fan at the exit of a

cooler to create airflow. Forced draft coolers use a fan at the inlet of the cooler to force airflow

through the cooler. [4]

Mechanical draft coolers also vary in the manner of which the air is airflow is directed over the

cooling exchange. The coolers can either have a counter or cross flow airflow in the cooler. A counter

flow cooling tower has airflow in the vertical direction to the main flow of fluid needing to be cooled.

A cross flow cooling tower has airflow in the horizontal direction to the main flow of fluid needing to

be cooled. Cross flow cooling towers can also be considered either single or double flow towers. A

single flow tower consists of only a single inlet, while double flow will have two inlets. [5]



3.3. Cooling fluids

Cooling towers require a medium to extract heat from the process fluid. This can be achieved with

either liquids such as water or dry fluids such as air depending on which is more accessible in the

needed application. Air cooling towers are typically referred to as air cooled heat exchangers.

Water used in wet cooling towers can have issues of fouling due to the hardness of the water often

used. Water in cooling towers are often treated with corrosion and biological growth inhibitors,

meaning that this mineral buildup tends to have the highest effect on water losses of the towers, by

requiring either a water treatment plant to be put in place or more makeup water to be added to the

systems. A study that was conducted concluded that for a small scale 3 ton cooling tower the

mineralized calcium levels increased by six times over a two month period. [7]

Water cooled systems will use less overall energy than air cooled heat exchangers, and there are

many energy efficiency gains that can be attributed for both systems from more efficient

technologies. [8]

Air cooled heat exchangers will have a higher capital cost than a water cooling tower alternative, the

cost to provide water to the wet system along with increased maintenance costs can make the dry

system have a lower lifetime running cost. Depending on the availability of water it may be more

desirable to implement a dry system to conserve water [9]

It is important to note that air cooled heat exchangers should not have water added to increase the

cooling capabilities as the systems are not designed for this. The action of spraying water on the

exchangers is commonly referred to as “California Cooling”. It can cause the finned tubes of the

exchanger to separate, due to being rapidly cooled by the water. This expansion allows water to seep

in between the fins and will decrease the heat exchange between the air and the tubes by created an



insulated region around the tubes. The water may also create a build-up of calcium carbonate and

other deposits which will also decrease the cooling capabilities of the tube bank exchanger. This will

lead to the need of the replacement of the entire tube bank, which can be very costly and is not

justified by any short term efficiency gains obtained from the water spray. [3]



4. Cooling Tower Components

Systems such as cooling towers require different components to run. The equipment required can be

broken down into the following main systems: fixed equipment, such as piping, mechanical systems

such as drives and air circulation systems.

A mechanical draft wet cooling tower system will typically have a fan drive that directs air either in

a counter or cross flow through a spray of water to induce cooling. This fan drive will either be direct

driven or belt driven through means of an electric motor. The water used for cooling is distributed

by a piping network that will spray water typically at the top, and a basin that collects used water

below. The piping network will require a pump to move the flow of liquid through it. An excellent

representation of a system is available in Figure 3.

Figure 3 – Wet cooling tower representation [5]



Mechanical draft air cooled heat exchangers will typically consist of a fan drive, a plenum and a heat

exchanger. The heat exchange will have two headers and a tube bank of finned tubes to promote heat

exchange. This is as shown in Figure 4.

Figure 4 : Air Cooled Heat Exchanger Representation [9]



4.1. Drive Systems

Drive systems are used to run different parts of the cooling towers. This can include drives used to

power the pumps required to circulate the water in wet cooling tower processes, or the motors used

to drive fans in forced draft mechanical cooling towers.

4.1.1. Motors

Motors are typically used to drive the fans that are part of the mechanical draft cooling tower

systems. Motors can also be used to power the pumps that are used to circulate water throughout

the wet cooling tower systems.

Electric motors function by using electromagnetism through a stator to rotate a rotor at a certain

speed. In industry, typically squirrel cage induction motors are used. These motors generally operate

at a consistent constant speed, and depending on what type of design of the motor resistance will

determine what sort of starting torque the motor has. These types of motors tend to have issues with

slip as the rotor will not spin at the exact synchronous speed. [10]

Motors that are used in cooling tower applications require a certain level of environmental protection

as they are often exposed to a lot of water either through process water or rain. The motor enclosure

used is a totally enclosed motor, which prevents the exchange of air between the inside and outside

of the motor. [10]

Motors used for fan drive applications without variable speed drives should be equipped with an

anti-rotation device to prevent wind milling of the fan blades. This is caused when a draft turns a

blade that is not in use. This effect can cause damage to the motors not equipped with variable speed

drives as when the motor is started up it is unable to overcome the backwards spin of the fan and

will fail. This leads to unnecessary replacement costs. The wind milling effect also poses a safety risk



to workers as the blades must be stopped before any maintenance can be performed. The anti-

rotation device is a simple one way clutch device that prevents the fan from the wind mill effect when

not in use. [3]

It is important to consider motor efficiency when purchasing motors as in the United States they

represent over half of all the energy consumption. The National Electrical Manufacturers association

have put forth standards of minimum standards and electric motor must meet to be considered

energy efficient. This is based off of the output a motor can output over the input requirements of the

motor, and are typically above 90% efficiency for motors larger than 10 HP. Energy efficient motors

represent large energy savings as upgrading a motor one percent can typically save approximately

$200 a year for a 50 HP motor. This can amount to a large sum of energy savings for industries that

require mass amounts of motors for operation. [11]

High efficiency motors are up to 10% more efficient than motors from 20 years ago. Increased

efficiency can enable more powerful motors to be placed into cooling tower applications where more

cooling capacity can be obtained without increasing power costs. [3]

4.1.2. Steam Turbines

Steam turbines may be used as an alternative to electric motors as drivers for pumps [12]. A steam

turbine uses the heat energy in steam to generate rotational power [13]. The type of turbine generally

used for mechanical drive is the impulse turbine. Rotational power is generated by redirecting the

steam into high speed jets using nozzles and diaphragms [13]. These jets impact blades mounted on

the rotor, causing it to rotate. During this process, the steam expands and is reduced in pressure. The

pressure differential created results in a net moment on the rotor that supplements the impact force

of the steam [13]. The number of sets of moving blades, or stages, varies for different turbines. More



stages increases the turbine efficiency, at the expense of increased cost. [12] Figure 5 depicts a two

rotor turbine.

Figure 5: Two rotor turbine with cover removed [12]

The use of steam turbines is highly dependent on steam availability in the plant [12]. This provides a

way to use waste heat generated within a plant. Turbines can also replace throttling valves in a plant

and allow energy to be used when line pressures are lowered [12]. The electrical power requirement

for a plant is reduced when steam turbines are employed [12]. This can be advantageous if electricity

costs are high or if the reliability of the plant electrical system is subpar. Steam turbines are also

desirable for flexibility as they can often be operated at up to 20% reduced load [12]. Long term

flexibility also exists, as rotors can be replaced and reconfigured during turbine refurbishments to

better match current power requirements [13].

The efficiency for a typical single stage turbine is up to 60% when compared to the available steam

energy [12]. This is in contrast to the electric motor efficiencies discussed in Section 4.1.1 of over

90%. Energy efficiency advantages associated with the use of turbines in a plant are dependent on

utilizing waste heat. Modern technologies that maximize efficiency of turbines include computer

designed blades and new types of seals that reduce steam leakage [13].



4.2. Driven Systems

Driven systems in cooling towers are represented by the pumps used to move cooling fluids around,

or the fan drives used to create cooling in either induced or forced draft mechanical cooling towers.

These systems require either an electric motor or turbine unit to be driven.

4.2.1. Pumps

Pumps are required to circulate the cooling water through the plant piping network, heat exchangers,

and to elevate the water to the height of equipment and through the cooling tower itself [14]. The

initial water source is the basin at the bottom of the cooling tower, which will result in near or slightly

above atmospheric pressure at the pump suction. Pumps used in cooling towers are typically of the

centrifugal or axial flow variety [4].

Axial flow pumps are pumps that act like a propeller that is enclosed in a housing. The propeller

element, or impeller, creates a pressure change between the upstream and downstream flow of the

pump. Axial flow pumps are most suitable to deliver low head pressures and high flow rates. [15]

Axial pumps are subject to overloading issues when the pump flow is throttled below maximum

efficiency conditions, this should be accounted for with cooling tower applications and be considered

for any design implementations.

Centrifugal pumps are considered radial flow machines. They are considered better suited to larger

head pressures and lower flow rates than axial flow pumps. A centrifugal pump has fluid enter

through the eye of the impeller and will travel outwards from the impeller vanes to the edge where

it enters the pump case. From this point the fluid then travels through the case towards the discharge

pipe. A pressure increase due to the high velocity of flow from the over the impeller reducing to the

pump casing has the potential to cause internal failures if the system is not designed correctly. The



power required for shutoff flow is far less than the flow for maximum efficiency. This means that the

flow can be better throttled between shut off and maximum efficiency without overloading an electric

driver. [15] This aspect of radial pumps would be very useful for cooling tower applications where

more or less water may be used.

Figure 6 : Cutaway of centrifugal pump [15]

The complexity and size of the piping and heat exchanger network of the plant dictates the pump

requirements. Pumps must be specified for both the required pressure increase of the system and

the flow required [14]. The specific configuration of pumps in a cooling water piping network has a

significant effect on the energy required to circulate the water. The typical configuration is for a

number of parallel pumps being used to increase pressure in the main cooling tower supply header

[14]. The pressure of the header is fixed. As process conditions require additional flow, more pumps

are brought online. From the cooling water supply header, a number of parallel branches are made



to direct cooling water to heat exchangers. Once the cooling water has passed through the heat

exchanger, the water is routed to the main water return header [14].

The problem with this configuration arises from the requirement for the pressure delta across each

parallel cooling water branch to be the same for proper flow distribution [14]. In reality, different

heat exchangers result in different pressure drops and equipment is not all fixed at the same height.

To work in this configuration, the discharge pressure of the main header pumps is dictated by the

maximum pressure required for any single branch [14]. For branches with a lower pressure

requirement, the water must be throttled. In order to reduce energy consumption, a pump

configuration such as the one proposed by Sun et al in Figure 7 can be used. In this configuration, the

header pumps operate at a lower pressure and are supplemented with pumps on branch lines to raise

pressure as required by each branch [14]. Proper pump sizing in this configuration can minimize the

requirement for throttling.

Figure 7 - Simplified pump and heat exchanger network of a cooling water system [14]



4.2.2. Fan Drives

Mechanical cooling towers can use fans to promote the flow of air to improve cooling efficiencies.

There are different types of fans that could be used to perform this. Fan blades can be constructed

from wood, plastic or metal. It is important to note that for cooling tower applications that blades

that are constructed from aluminum will not achieve their design rated airflow, due to inefficient

blade design. These blades which are typically straight-cord will only achieve 35% - 55% efficiency.

More modern fans which have better aerodynamic properties will have higher efficiencies, ranging

from 75% - 85% efficiency. [3]

This amount of increased air flow is desirable for air cooled heat exchangers, but will not be desirable

for wet cooling towers as more airflow can drag the cooling water stream too far into the tower.

Water in the tower can create corrosion of metal parts in both the driving equipment and the fixed,

and create non desirable working conditions for workers depending on the water quality. An airflow

increase of approximately 10% will allow a cooling duty increase of 5%-7% for condenser coolers,

3%-6% for liquid coolers and 2%-5% for vapor coolers, Blades can be constructed in such a way that

they can be noise reducing. [3]

The blades are connected to a central hub where they are attached with screws that allow the blades

to be adjustable. To run properly the blades need to be properly installed and aligned. Ways to

improve efficiencies of the fan blades include ensuring that the fan blades are pitched properly. This

is adjusting the blades to be set to ±0.5⁰ of each other, performing this will reduce vibration. [3]

Blades also must be installed in such a manner that the blades are spaced equally to reduce overall

fan vibration is reduced. Fan blades need to be adjusted in such a manner that the tip of the blade

offers enough clearance to run properly, but not have enough to promote air recirculation around

the tip. By either readjusting the blades, or adding a honeycomb aluminum seal fan performance can



be increased by up to 2%. [3] Fans will typically have a metal or plastic seal around the hub. If this

seal is missing or damaged it can decrease fan efficiency by causing recirculation. This seal disk

should typically be a quarter of the total fan diameter and if in place can increase fan efficiency by

2%. Air cooled heat exchanger applications can obtain a 3% efficiency increase by adding inlet bells

to the fan. [3]

Fans are either direct driven or driven by a belt system. Direct drive systems can only gain efficiencies

through the driver, while belt drives have different efficiency opportunities. There are two main

types of belts, v-belts and cog style. V-belts are a tapered belt that usually consists of a rubber jacket

and tension cord on the inside used to transmit power. V-belts generally use grooved pulleys as

sheaves and are usually grouped in sets to meet capacity. [17] V-belts slip more in the summer and

need to be re-tensioned throughout their lifetimes. [3] Cog belts are classified as cog style or timing

belts. The belts have teeth to aid with slippage and will use toothed sheaves. [17] Timing belts when

aligned and tensioned properly are considered 98% efficient and after re-tensioning shortly into

installation should not require and further maintenance in the life of the belt. The use of v-belts

should be minimized where possible as they increase bearing load of motors due to the tensioning

required for efficient running. If improper tensioning is used it will either lead to premature bearing

failures of the motors, or slippage which will require a shut down for the belt to be re-tensioned. [3]



4.3. Fixed Equipment

Equipment such as piping networks and heat exchangers are integral to cooling tower operation.

Given that cooling towers are often employed in refineries and other type of petrochemical plants,

the cooling capacity is generally distributed between a variety of processes [14]. Heat exchangers

allow the cooling tower water to exchange heat with other fluids.

The commonly used shell and tube heat exchanger consists of a series of tubes inside of a large

container. One fluid flows through the tubes and the other fluid flows in the container, over the

exterior of the tubes. Heat transfer takes place though the tube walls. A variety of sizes and

configurations for shell and tube heat exchangers exist, depending on the amount of cooling required

and fluid properties. [18]

Air cooled heat exchangers use a tube bank heat exchanger to cool the fluid. These tube bank bundles

consist of a nozzle and return header which entrain a system of finned tubes. These finned tubes can

be constructed of extruded fins, embedded fins or footed fins. Footed fins are the cheapest option as

they are constructed and welded to the outside of the tubes in the exchanger. These fins are not overly

effective in staying attached to the tubes over time and will lose cooling efficiency. Embedded fins are

fins that are integrated into the tube bank. These fins are disadvantageous because they require

thicker tube walls to be implemented. Extruded fins offer the most reliability as they are a solid piece

of fin. The efficiency of these fins should not degrade over time. Extruded fins are best suited for

corrosive environments because of this. If any of the fins on the tube bank are damaged, which could

include bent or missing, it will affect the thermal capabilities of the heat exchanger. This damage will

decrease the overall cooling capacity of the tube bank. Damage can also occur due to inadequate

thermal restraint from the headers. This occurs from rapid cooling of the fluid without any restraint,

bending the tubes and distorting their forms. This will also decrease thermal efficiency [18]



The tube banks present in air cooled heat exchangers are often subject to fouling around the fin area.

This fouling can be attributed to dirt and debris build up from the process area, or corrosion from

weather. This debris can completely coat the fins and reduce their cooling capabilities to a minimum.

Often, to return to the original cooling capabilities of the exchanger it is necessary to clean all of the

debris from the fins. [3]

When integrated into a plant, a cooling tower will interface with what may be a complex piping

network in order to distribute the cooling water and return it to the cooling tower. This piping

network will interface with any heat exchangers or pumps in the system. [14]

Corrosion is a general concern for piping networks, but for cooling towers the primary challenge with

fixed equipment is microbial growth [18]. Microbial growth can be in the form of algae, bacteria, or

fungi and can have a variety of negative effects on cooling tower equipment. The cooling water is

conductive to microbial growth due to high oxygen content and the concentrating effect of

evaporative cooling [18]. Additionally, the presence of stagnant water anywhere in the system

encourages growth [18].

Process water entering a cooling tower will contain some amount of microbes and other dissolved

material. As water evaporates in the cooling tower, the dissolved and suspended material remain in

the cooling circuit. [18]

Corrosion and fouling are the two primary mechanisms by which microbes affect cooling tower

equipment, though other damage mechanisms exist such as accelerated rotting of wood structures

[18]. The corrosion is a result of the presence of microbes such as certain bacteria that have the ability

to break down the metal, typically carbon steel, which makes up the pipe wall [19]. A biofilm will

form that covers these microbes and provides protection from the water flow [19]. Fouling is a term

used to describe a reduction in heat exchanger performance as a result of a contaminated heat



transfer surface [17]. As the inner or outer surface of the tubes in a shell and tube heat exchanger

become covered with a biofilm, the resistance to heat transfer increases. This has a negative effect on

the heat exchanger efficiency and additional cooling water flow would be required to cool a given

amount of process fluid.

If the cooling water was released to the environment when the microbes were of high concentration,

this would be a form of pollution as many of these microbes can cause human harm. As an example,

the bacteria that cause legionaries disease are often found in cooling water systems [18]. A variety of

other methods are employed to manage the microbes present in cooling towers. Biocides are

chemicals such as chlorine and ozone that are capable of killing the microbes. The use of biocides is

being reduced due to environmental concerns [18] [19].

Bio dispersants are substances that prevent microbes from attaching to and biofilms from forming

on equipment without killing the microbes [18]. Increasing the use of plastics can reduce corrosion

concerns. Heat exchanger fouling can be managed through the introduction of sponge balls to the

cooling water steam when tube side. These balls are forced through the tubes of the heat exchanger

and physically wipe the biofilm away [19]. This can be done when equipment is online, using a

strainer to recover the balls. Similarly, brushes can be used that are integrated into the heat

exchanger and can be slid back and forth inside the tubes by reversing flow [19]. An additional

method is to use flexible tubes in the heat exchanger. These tubes vibrate with the flow to prevent

the formation of biofilms [19]. Basic changes to cooling tower operation can also reduce the impact

of microbes. Since stagnant water encourages the growth of biofilms, stagnant water should be

avoided wherever possible. This includes minimizing basin and sump levels and recirculating or

draining the water when the cooling tower is shut down or on standby. Configuring heat exchangers

so that cooling water is present on the tube side also reduces cooling water exposure to stagnant

flow. [19]



5. Summary

Cooling Towers, both wet and dry are used in different applications worldwide for cooling purposes.

These applications include power plants, oil refineries, chemical plants and even building cooling.

Cooling towers are used to reject heat from industrial and commercial processes as required. This is

performed by transferring heat from a cooling fluid, typically water, but occasionally air or another

fluid by means of an interfacial film. This heat is then transferred from the film to air by typical heat

transfer, or evaporative methods. [1] The amount of heat transferred from the cooing fluid must be

equal to that of the heat absorbed by the air flowing through the tower. This heat transfer can be

difficult in colder temperatures as the water used in wet cooling towers can freeze when the flow of

water through the cooling tower systems is unequal. [2]

The towers can be natural of mechanical draft. This is the difference of allow natural airflow through

the tower or mechanically inducing airflow through the tower. There are different types of

mechanical draft coolers. There are induced and forced draft coolers which indicate how the airflow

is drawn over the cooler. Induced draft coolers use a fan at the exit of a cooler to create airflow. Forced

draft coolers use a fan at the inlet of the cooler to force airflow through the cooler. [4]

These towers can also have different airflow through. The coolers can either have a counter or cross

flow airflow in the cooler. A counter flow cooling tower has airflow in the vertical direction to the

main flow of fluid needing to be cooled. A cross flow cooling tower has airflow in the horizontal

direction to the main flow of fluid needing to be cooled. Cross flow cooling towers can also be

considered either single or double flow towers. A single flow tower consists of only a single inlet,

while double flow will have two inlets. [4]



Cooling towers can either be wet or air cooled as well. Water used in wet cooling towers can have

issues of fouling due to the hardness of the water often used. Water in cooling towers are often

treated with corrosion and biological growth inhibitors, meaning that this mineral buildup tends to

have the highest effect on water losses of the towers, by requiring either a water treatment plant to

be put in place or more makeup water to be added to the systems. Water cooled systems will use less

overall energy than air cooled heat exchangers, and there are many energy efficiency gains that can

be attributed for both systems from more efficient technologies. [8]

Air cooled heat exchangers will have a higher capital cost than a water cooling tower alternative, the

cost to provide water to the wet system along with increased maintenance costs can make the dry

system have a lower lifetime running cost. Depending on the availability of water it may be more

desirable to implement a dry system to conserve water [9]

The components of the cooling towers are readily broken down into fixed equipment, drive

equipment and driven equipment. The fixed equipment consists of the piping network used to

transport the water and the tube banks used in air cooled heat exchangers as other heat exchangers.

Drive equipment can be either electric motors or turbines and driven equipment consists of fan

drives and pumps used to transport either cooling or cooled fluids.

One of the biggest threats to fixed equipment is microbial contamination. This can result in

accelerated corrosion of piping components as well as the fouling of heat exchangers [18]. Typical

methods to manage microbes in cooling water have traditionally been biocides such as chlorine, but

more environmentally friendly methods such as bio dispersants and the use of alternative materials

and operation techniques are increasingly employed to reduce the impact of these organisms. [19]

Motors are typically used to drive the fans that are part of the mechanical draft cooling tower

systems. Motors can also be used to power the pumps that are used to circulate water throughout



the wet cooling tower systems. It is important to consider motor efficiency when purchasing motors

as in the United States they represent over half of all the energy consumption. The National Electrical

Manufacturers association have standards that determine the minimum requirements for a motor to

be considered energy efficient. These motors are typically above 90% efficiency for motors larger

than 10 HP. Energy efficient motors represent large energy savings as upgrading a motor one percent

can typically save approximately $200 a year for a 50 HP motor. [11]

As an alternative to electric motors, turbines may also be used as drivers for pumps used in cooling

tower systems [13]. Although less efficient when directly compared to electric motors, turbines can

be used to harness waste heat generated in neighbouring processes and are more flexible in

operation than electric motors [12].

Modern fans which have better aerodynamic properties will have higher efficiencies, ranging from

75% - 85% efficiency. An airflow increase of approximately 10% will allow a cooling duty increase of

5%-7% for condenser coolers, 3%-6% for liquid coolers and 2%-5% for vapor coolers. [3]

Fan blade efficiency can be improved by ensuring that the fan blades are pitched to ±0.5⁰ of each

other, performing this will reduce vibration. [3] Fan blades need to be adjusted in such a manner that

the tip of the blade offers enough clearance to run properly, but not have enough to promote air

recirculation around the tip a snug fit will increase fan performance can by up to 2%. [3] Proper

installation of a good condition fan hub will increase fan efficiency by 2%. Air cooled heat exchanger

applications can obtain a 3% efficiency increase by adding inlet bells to the fan. [3]

Fans are either direct driven or driven by a belt system. Direct drive systems can only gain efficiencies

through the driver, while belt drives have different efficiency opportunities. Timing belts when

aligned and tensioned properly are considered 98% efficient and after re-tensioning shortly into

installation should not require and further maintenance in the life of the belt. The use of v-belts



should be minimized where possible as they increase bearing load of motors due to the tensioning

required for efficient running. If improper tensioning is used it will either lead to premature bearing

failures of the motors, or slippage which will require a shut down for the belt to be re-tensioned. [3]

Pumps are essential to the operation of a cooling tower, as the water is a motive fluid that requires

energy input in order to be distributed throughout the network of piping and plant equipment.

Centrifugal and axial flow pumps are typically employed, depending on pressure and flow

requirements [4]. Networks of pumps in parallel and series are constructed in order to allow the

system to handle varying demand in pressure and flow according to plant operation. The flexibility

and configuration of this network is important for overall energy usage and to minimize the use of

throttling devices [14].



References

[1] R. Rosaler, Standard Handbook of Plant Engineering, New York: McGRAW-HILL, 2002.

[2] P. Lindahl, "Cold Weather Operation Cooling Towers," ASHRAE, pp. 26-37, 2014.

[3] N. Agius, "Optimize air-cooled heat exchanger performance," Hydrocarbon Processing, pp. 89-94, 2006.

[4] K. McKelvey and M. Brooke, The Industrial Cooling Tower, New York: Elsevier Publishing Company, 1959.

[5] J. Hensley, Cooling Tower Fundamentals, Kansas: The Marley Cooling Tower Company, 1985.

[6] M. Goodarzi and R. Ramezanpour, "Alternative geometry for cylindrical natural draft cooling tower with higher cooling efficiency under crosswind conditions," Energy Conversion and Management, vol. 77, pp. 243-249, 2014.

[7] C. Cremaschi and X. Wu, "Effect of Fouling on the Thermal Performance of Condensers and on the Water Consumption in Cooling Tower Systems," Heat Transfer Engineering, vol. 36, no. 7-8, 2013.

[8] F. Morrison, "Saving Energy With Cooling Towers," ASHRAE, pp. 34-40, 2014.

[9] D. Kroger, Air-cooled Heat Exchangers and Cooling Towers, Oklahoma: PenWell, 2004.

[10] F. Petruzella, Electric Motors and Control Systems, New York: McGraw Hill, 2010.

[11] US Department of Energy, "Buying an Energy-Efficient Electric Motor," [Online]. Available: https://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/mc-0382.pdf. [Accessed 24 January 2015].

[12] M. A. Cerce and V. P. Patel, "Selecting Steam Turbines for Pump Drives," 2003. [Online]. Available: http://turbolab.tamu.edu/proc/pumpproc/P20/12.pdf. [Accessed 10 February 2015].

[13] H. P. Bloch and M. P. Singh, Steam Turbines: Design, Applications, and Rerating, vol. II, McGraw-Hill, 2009.

[14] K. H. Chu, C. Deng, Y. Wang, X. Feng and J. Sun, "Pump network optimization for a cooling water system," Energy, vol. 67, pp. 506-512, 2014.

[15] C. Crowe, D. Elger, B. Williams and J. Roberson, Engineering Fluid Mechanics, John Wiley and Sons, 2009.

[16] R. Juvinall and K. Marshek, Fundamentals of Machine Component Design, 2006.

[17] F. Incropera, D. Dewitt, T. Bergman and A. Lavine, Funamentals of Heat and Mass Transfer, John Wiley and Sons, 2205.

[18] S. G. Choudhary, "Emerging microbial control issues in cooling water systems," Hydrocarbon Processing, vol. 77, no. 5, pp. 91-103, May 1998.

[19] T. R. Bott, "Techniques for reducing the amount of biocide necessary to counteract the effects of biofilm growth in cooling water systems," Applied Thermal Engineering, vol. 18, pp. 1059-1066, 1998.

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Dalhousie University Mechanical Engineeringpoisson.me.dal.ca/site2/courses/mech4340/MECH4830 - Cooling Tow… · 3. Cooling Tower Types ... include wet cooling versus dry cooling,