COOLING TOWERS

A report of the seminar presented in

partial fulfillment for the award of the degree of

Bachelor of Technology

by the university of Calicut

Submitted by

DIVYA R NAIR

Seventh semester

Department of Civil EngineeringN.S.S College of Engineering

Palakkad, 678008Kerala

COOLING TOWERS

DIVYA R NAIR

Most industrial production processes need cooling water to operate efficiently and safely. Power plants, petrochemical industries, refrigeration and air conditioning industries, refiners, steel mills all rely heavily on equipment or processes that require efficient temperature control. Cooling water systems control these temperature and cooling towers are one of such cooling water system. So cooling towers are one of the most important components of such industries that reject a large quantity of heat to the surroundings to cool the water, being circulated around machinery, to the desired level. Today, cooling tower serves as prominent landmark on the skyline of a city. This paper deals with the development, importance, function, types, merits, demerits and other details about cooling towers.Keywords - Cooling tower, natural draught, mechanical draught, cooling fills

1.0 INTRODUCTION

Most industrial production processes need cooling water to operate efficiently and

safely. Refiners, steel mills, petrochemical manufacturing plants, electric utilities and paper

mills all rely heavily on equipment or processes that require efficient temperature control.

Cooling water systems control these temperatures by transferring heat from hot process fluids

into cooling water. As this happens the cooling water itself get hot, before it can be used

again it must either be cooled or replaced by a fresh supply of cool water. This makeup water

contains dissolved minerals, suspended solids, debris, bacteria and other impurities.

As the water continues to circulate throughout the system, other contaminants begin to

concentrate. As the temperature rises, cooling equipment efficiency is threatened and a total

plant shutdown can result. Effective cooling water operation and treatment can prevent such

occurrence. Different types of cooling water systems are now used.

Under graduate student ,Department of civil Engineering, NSS.college of

engineering.Palakkad-08,Kerala

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Cooling water systems are either evaporative or non-evaporative. Non-evaporative systems

include once through cooling and closed loop system. Evaporative cooling system include

open recirculating system in which heat rejection is accomplished in cooling towers,

evaporative condensers etc.Once through cooling water system is used to cool processes or

equipment and then is discharged to waste. Characteristically, it includes large volumes of

water and small increase in water temperature. Once through cooling is usually employed

when water is readily available in large volume at low cost. Common sources are rivers,

lakes and wells where the only cost involved is that of pumping.

Once through cooling is currently prevalent in utilities, steel mills and paper mills.

Scale, corrosion, fouling and biological fouling are all problems for once through cooling

system.

A closed cooling water system is a recirculating water system that does not cool by

evaporation and has very little water loss. Closed systems offer the advantages of precise

temperature control, which is critical in many process applications, and low treatment cost.

Closed systems can be reliably operated at very high temperature and under subfreezing

conditions using ethylene glycol, alcohol or brenis. But a secondary cooling system and heat

exchanger are needed to cool the closed system. So higher capital and operating costs are the

disadvantages of this type.

As recently as 20 years ago, cooling towers were more exception than the rule in the

industry because of their severely high operating cost and the large amount of capital required

for construction. But with today’s need for water conservation and minimal environmental

impact, industry is turning more and more to recycling water.

A cooling tower is a heat exchanger. It transfers heat from circulating water to the

atmosphere. It accomplishes this by providing intimate mixing of water and air, which results

in cooling primarily by evaporating approximately 1% of the flow for each 100Fdrop in

temperature.Thus cooling towers are one of the most important components of such

industries, that reject a large quantity of heat to the surroundings to cool the water being

circulated around machinery to the desired level.

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FIG.1COOLING TOWERS

2.0 IMPORTANCE OF COOLING TOWERS

For the last two decades, electricity boards and power production authorities have

become more inclined to opt for thermal power stations, the major reasons behind this being

the acute power shortage and a short gestation period of Thermal plants. Though hydel power

projects constitute the most preferred category because of cheaper power and pollution free

generation yet their long gestation period and uncertainty in their commissioning forces them

to take a backseat. The result is that more and more thermal plants are springing up around

the country

Sky scrapping chimneys and majestic super structures called cooling towers having

half a kilometer long circumferences constitute proud and essential features of every thermal

plant. Serving as prominent land marks on the skyline of a city, these two structures remain

integral part of a thermal power station except in rare cases where a cooling tower can be

avoided due to availability of cooling water required for the power station from a canal

running in the vicinity of the thermal power station. Ropar Thermal plant is one such case.

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Otherwise most of the thermal plants have cooling towers as part of their cooling water

systems.

3.0 COMPONENTS OF COOLING TOWER

Each cooling tower, may be a Natural draught, induced draught of forced draught

consists of an air circulation system, a hot water distribution system, a heat transfer system, a

heat transfer system and a cold-water basin. However, it is the transfer system provided

inside the cooling towers that ascertains their efficiency. This, the heat transfer system is

known as the heart of the cooling towers.

4.0 PROCESS

Cooling towers are very important part of chemical plants. They represent a relatively

inexpensive and dependable means of removing low-grade heat from cooling water. The

make up water source is used to replenish water lost to evaporation. Hot water from heat

exchanger is send to the cooling tower . This water is then cooled and sent back to the plant.

Heat is transferred by two mechanisms. A portion of the cooling water generally 1 to

3% actually evaporates as it mixes with air, latent heat is given up in this phase change.

Sensible heat transfer, in which heat is exchanged without a phase change makes up the

balance.

Cooling fills are the contact surfaces that transfer heat from water to air. These fills are

selected in such a way that they provide maximum contact area between water and air for

easier transfer of heat but with minimum cost. This fills are provided at a lower level in

comparison to the overall height of the cooling towers. The fill is supported on a suitable and

well designed supporting structure provided inside the cooling tower. The fill is so designed

that the stream of water, which is to be cooled, falling over it trickles down the fills while

the air is supply from the bottom through the air inlet area provided between diagonal

columns. The heat is transferred from water to cooling air by convection and water gets

cooled. The magnitude of this job can be judged from the fact that for a 210 MW power

plant, 30 million litters of water is to be cooled by a cooling tower every hour. The fill

provided in the heat transfer system has therefore to be extremely efficient.

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5.0 TYPES OF COOLING TOWER

Cooling towers may be classified as natural draft or mechanical draft

The natural draft or hyperbolic cooling tower is designed to take advantage of the

temperature differences between the ambient air and the hotter air inside the tower. The

design creates a chimney effect that causes the cold air at the bottom of the tower to push the

warmer air out the top. Natural draft towers are divided into two basic types: cross flow and

counterflow. In a cross flow tower, air is drawn across the falling water. In this design the

fill is located outside the tower. The fill is contained within a counterflow tower since the air

is drawn up and through the falling water. Design selection depends upon conditions at the

particular site.

In mechanical draft cooling tower, water is distributed as evenly as possible at the top

of the tower and allowed to drop through the air. Fans are used to increase the airflow.

Packing or fill inside the tower keeps the water evenly distributed and increases the water

surface area. The greater the surface area, the greater the air contact and therefore, the greater

the cooling efficiency. The water falls downward over fill surface, which help increase the

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contact time between the water and the air. This helps to maximize heat transfer between the

two.

Mechanical draft towers are divided into two basic designs: forced draft or induced

draft. They are easily distinguished because a forced draft tower has fans on the side, and an

induced draft has fans on the top. Induced draft towers ate also divided into two basic

designs: counter flow and cross flow. As in the natural draft towers, a cross flow tower draws

the air across the falling water droplets and out the stack. They are identifiable by their open

decks and the louvers that go all the way from top to bottom of each tower cell. Counter flow

mechanical draft towers are identified by their perpendicular sidewalls and closed decks, i.e.

mechanical draft towers offer control of cooling rates in their fan diameter and speed of

operation. These towers often contain several areas (each with their own fan) called cells.

Regardless of whether the air is pulled or pushed through the tower and whether or

not the air is fan-assisted, the principle, the problems and the solutions are the same.

6.0 SELECTION OF COOLING TOWER

Due to the tremendous size of natural draft towers (500 ft high and 400 ft in diameter

at the base)they are generally used for water flow rates above 200000gal/ min. Usually they

are used for thermal power station. For thermal power stations, Natural draught cooling

towers are preferred over induced draught cooling towers in view if the huge discharge

operation and maintenance of induced draught cooling tower is a constant headache and a big

dent in the revenue generated by the power plants.

Over and above this the danger of a shutdown always keeps lurking and one such

shutdown may cause a loss of electricity worth crores of rupees. By now it has become a

world wide accepted view that natural draught cooling towers should be preferred to induced

draught cooling towers because of their maintenance free service throughout their designed

life. Thus natural draught cooling towers remain the most preferred ones because of their

long lives, long maintenance, low cost of operation and high dependability even in areas of

high humidity. Thickness of their hyperbolic shaped shell proves lesser than that of an egg if

developed to an equal volume.

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Cooling towers need high operating cost and large amount of capital required for

construction. But with today’s need for water conservation and minimal environmental

impact industry in turning more and more to recycling water.

7.0 SIZING OF COOLING TOWER

Many choices and decisions are required to properly size a cooling tower. At

minimum, be sure the following specification stipulates:

1. Flow rate.

2. Total heat rejection.

3. Cold water temperature.

4. Hot water temperature

5. Elevation above sea level

6. Tower type (cross flow or counter flow)

7. Materials of construction

8. Fill media choice

9. Water quality

10. Noise limitation

11. Scope of supply(who is responsible for basin, external piping etc)

8.0 DEVELOPMENT OF COOLING TOWER

Now, cooling tower technology was come up a long way. With the increase in power

requirement, the height and diameter of cooling tower also increased. Initially, the overall

height was below 20 meters and now the 200-meter mark strands crossed in many countries.

The discharge to be handled by cooling towers has also risen from a mere 100 m3/hr to an

incredible figure of 200000 m3/hr.

Development of a chimney cooler has proved a milestone in cooling technology. The

internal fill has been under study for a long time and now PVC fill is fast replacing the RCC

louvers. A few latest developments in cooling tower technology are explained here.

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8.1 Cooling Fills

The cooling fills can be termed as the heart of a cooling tower. These are the contact

surfaces that transfer heat from water to air. Earliest of the fills used in cooling towers built in

pre-independence era consisted of timber splash bars which used to be crossed stacked over

one another in multi-layers inside the cooling towers and water used to fall over them and

spilt into droplets. These splash bars numbered in lacs and the fill height ranged from 7 to 10

meters depending upon the quantity of water to be cooled and degree of cooling.

Later on, concrete was brought into use and lacs of triangular shaped concrete bars

replaced the timber splash bars. Most of the cooling towers in India have been provided with

RCC fills. These fills are lacs in number and placed in many layers one across the other to

allow the water to trickle down one across them and get cooled. These used to be precast at

cooling tower sites and then erected into position. These being heavier their erection and

casting constricted a gigantic task. Precasting of 4 to 5 lacks splash bars for a 210MW plant

cooling tower and then placing them in multilayers constitutes a huge job.

In early nineties, PVC splash bars were brought under use in India with the advantage

of their being lighter in weight and easier to handle. The weight problem was solved but that

of large height of fill and consequently higher pumping head still persisted. So the research

for a better alternative continued.

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8.1.1 Latest Development

Now, the latest development in this direction is the film fill. Whereas a fill height of 7

to10m is required for a heavy duty cooling tower (cooling the water by more than 100c) when

splash bars are used, same result can be obtained by using just 1.5 to 2m height of the newly

developed film fill. This discovery further results in lowering the pumping head as water is

now to be pumped to a lesser height, thus bringing great savings in pumping cost.

Film fills consist of extremely thin PVC or plastic sheets, diagonally corrugated and

joined together by using a special type of glue to form film packs that are strong enough to

bear the weight of a man standing over them. Thickness of sheet is as less as 0.25mm. The

sheets, generally of size 1.2mx0.3m, are so designed that the the corrugations don’t settle in

one another but act as fillets to allow the water film to follow through them. The film packs

are stacked together in the cooling tower over the supporting structure so as to cover the

whole of water distribution area. Keeping in view the cooling requirement, twi or four such

layers each having 0.3m is laid. For a heavy duty cooling tower such as in power plants a

maximum of 4 layers having a total depth of 1.2m are sufficient. The height of fill thus gets

reduced to mere one sixth or one seventh of that required for concrete or timber splash bars.

So efficient are the film fills.

The idea behind creating diagonal corrugations in the fill packs is to establish contact

between the hot water and the air inflow for maximum possible period to effect required heat

transfer between them without obstructing the flow of water. The size of corrugations is

selected with respect to the quality of water to be cooled. Cooler the water lesser is the size of

the flutes and more efficient the cooling tower is. If water is not clean enough smaller size

flutes are adopted.

The size of corrugations is selected with respect to the quality of water to be cooled.

Cleaner the water, lesser is the size of flutes and more efficient the cooling towers can be. For

power plant cooling towers, 19mm size flutes are found to be mosty suitable. If water is not

clean enough smaller size flutes are adopted, clogging of fill packs occur obstructing the flow

of water 12mm size flutes are suitable for conditioning industries where the wateer to be used

has to be very clean.

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PVC is the most preferred material to produce fill packs. This material being lighter in

weight, fire resistant, having good tensile and impact strength and resistant to chemical attack

has the ability to mould itself to the shape it is given.It can be of white or black colour

depending upon the colour of compound used in its manufacture. It has been successfully

used in Europe and America where even black colour PVC has not posed any problem. Its

only drawback is that it can get softened at high temperature. This aspect can be taken care of

by making it UV resistant and by selecting a non heat-absorbing colour for it.

In a tropical country like India, the fill packs may acquire high temperature if film

thickness is lesser and its colour is black. It has been observed that when the air temperature is

400c and the fill packs are stacked in sunlight, these can acquire a temperature of 550c. White

fills are found to a temperature of just 400c under similar conditions. Black film fills may

touch a temperature of 900c under Indian weather conditions and are therefore not

recommended, as this figure is higher than the vicat softening temperature. In case black fills

are essential to be used, these must be UV stabilized by adding suitable stabilizers to the PVC

compound during the manufacture of fills. Furthermore black film fills should never be

stacked in direct sunlight. Once erected inside the cooling towers these fills will not pose any

problem as the inside temperature is never so high as to endanger the stability of fill packs.

These precautions are not much to be observed if multiple benefits of film fills are to be

availe.

8.2Cooling Chimneys

A new concept has been developed abroad under which a cooling tower and a

chimney have been combined into a single structure known as “cooling chimney”. A cooling

chimney effectively serves both the purposes and helps saving about 40% of total cost of both

the structures. This is a significant achievement.

Under this new concept, the flue gases are conveyed into the cooling tower through

one of two duets and then discharged into the atmosphere with the cooling air plume. Even

the old cooling towers are being retrofitted with flue gas discharge systems thus avoiding the

need of a chimney.

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A cooling chimney cools down the water by 10c thus acting as a cooling tower and

also discharges the flue gases into atmosphere thus acting as a chimney. The concept has

been fully established and as on date. More than a score of such cooling chimney are under

construction around the world.

The prerequisite to use a cooling chimney is to cool down the flue gases to be

discharged into atmosphere by passing them through a desulpherisation plant has been

declared compulsory for all thermal power stations to come up in future. Space is earmarked

for them in the power plants presently under construction

8.3 Water Distribution

Uniform water distribution constitutes a major yardstick for cooling tower

efficiency. To achieve this, the spray units have constantly been under study. The latest

sprayer, made of plastic material and attached to the distribution pipes distributes water

uniformly over the cooling fill. These require only a low nozzle inlet pressure. The sprayers

are not susceptible to contamination and the pollutants contained in the cooling water are

constantly flushed from the system. These sprayers are now being used in India.

8.4 Arrival Of Streif Formwork

Arrival of streif hydraulic formwork in India has helped in considerable reduction in

time period for construction of cooling tower shell. This equipment consists of a five-deck

system, which keeps riding up along rails fixed continuously all around the structure. The

number of units can be varied depending upon the circumference of the structure by keeping

the center to center spacing of units between 3 to 8 metres. Each structural formwork carries

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two separate hydraulic jacks that can be operated independently or jointly. Streif formwork

can work with a fast speed. The daily output can be as high as 5 feet. The load of the

formwork, the live load and all working loads get transferred to concrete that is 3 to 5 days

old. A cooling tower shell of 90 meters base diameter and 120 m height can be constructed in

4.5 to 5 months with the equipment.

9.0 MERITS OF COOLING TOWERS

Cooling towers are the essential features of every thermal power plant. This serve as

a prominent landmark on the skyline of a city.

If the water used in the industries for cooling purposes is directly discharge to water

bodies it will affect the aquatic bodies. If cooling tower is used, it will reduce the

environmental impact.

As recently as 20 years ago, cooling towers were more the exception than the rule in

the industry because of their severely high operating cost and the large amount of capital for

construction. But today’s need for water conservation and minimal environmental impact,

industry is turning more and more to recycling water.

10.0 DEMERITS OF COOLING TOWERS

There are some problems in cooling tower. Both biological growth and dissolved

solids precipitation, if left unchecked will destroy the thermal capability of cooling tower.

Care must be taken in the selection of cooling tower fill media. In addition to biological

growth, dissolved solids, in favorable conditions of temperature and alkalinity may precipitate

within critical area of the fill media and cooling water piping. Both biological growth and

dissolved solids precipitation if left unchecked will destroy the thermal capacity of cooling

tower.

By the use of oxidizing biocides such as chlorine and excessive bicarbonate alkalinity;

biological growth which can clog the nozzle and foul the heat exchange equipment; corrosion

of the metal components; general fouling by silt, clay, oil, metal oxides, calcium and Mg salts,

organic and other chemical products which can cause reduced heat transfer; and scaling by the

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crystallization and precipitation of salts or oxides on surfaces are the major causes for

concern.

The following table gives the cooling tower problems and preventives.

Potential problem Factors Causative Agents Corrective

Treatments

Wood

deterioration

Microbiological

chemical

Cellulolyticfungi

Chlorine

Fungicides

Acid

Biological growths Temperature

Nutrients, pH

Bacteria

Fungi, Algae

Chlorine

Chlorine doners

General Fouling Suspended solids

Water

Velocity,

Temperature

Silt, Oil Polyelectrolytes

Polyacrylates

Corrosion Aeration

pH, Temperature

Oxygen

Carbon dioxide

Chlorine

Chromate

Zinc

Polyphoaphate

Scaling Calcium

Alkalinity

Temperature,pH

Calcium carbonate

Calcium sulphate

Phosphonates

Polyphosphates

Acid

Table1: cooling tower problems and preventives

Adequate maintenance is required for efficient cooling tower action. Another

disadvantage is that it will have high operating cost and large amount of capital required for

construction.

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11.0 CONCLUSION

Cooling tower is an essential piece of equipment in the plant, and the increasing cost

and decreasing availability of process water can no longer be ignored. It is possible to

upgrade the performance of cooling tower with simple repairs and upgrades, or replace it

with the latest technologies.

There are some new concepts for cooling towers. More efficient designs for cooling

towers are being sought. Space requirements for both hyperbolic and induced draft towers are

so great that combining the best features of both would reduce the space required, improve the

efficiency and reduce the variability of performance. Therefore mechanical draft had been

added to hyperbolic towers to achieve this higher efficiency. Designs for an assisted draft

tower raising the capacity of a natural draft tower from 250 to 660 MW have been completed.

Multiple stacks on single units for high discharge have also been suggested.

Eventhough cooling towers has some problems, today’s need for water conservation

and minimal environmental impact, it is an important structure.

ACKNOWLEDGEMENT

I take immense pleasure to express my sincere gratitude to Smt.K.Subha, Sr.Lecturer,

Department of Civil Engineering, NSS College of Engineering, Palakkad for the valuable

guidance and constant encouragement he has rendered as a seminar guide. I would also like to

thank Dr.A.K.Raji, Staff-in-charge of seminar for her assistance during my seminar. Last, but

not the least, I thank one and all my friends whose contribution lead to the completion of my

seminar.

Above all I thank God almighty for His blessings.

REFERENCES

1 Jagvir Goyal, A new heart for cooling towers, National Building Material and

Construction work- January 2002, pp.51-53

2 Jagvir Goyal, Latest trends in cooling tower construction, National Building

Material and Construction work-July 1998,pp 6-8

3 www.etdepotine.com

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4 www.aqma.gov

5 www.kellysearch.com

6 www.achrnews.com

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