CCOOLING TTOWERS Tower.pdfThere are two main types of cooling towers: The natural draft and mechanical draft cooling towers. 11:48:10 . Naturaldraftcoolingtower ... Forced Draft Cooling

CCOOLINGOOLING

TTOWERSOWERSTTOWERSOWERS

By: Prof K. M.JoshiAssi. Professor, MED,

SSAS Institute of Technology, Surat.

What is a cooling tower?

A cooling tower is an equipment used to reduce the temperature of a

water stream by extracting heat from water coming-out from

condenser and emitting it to the atmosphere.

Cooling towers make use of evaporation whereby some of the water

is evaporated into a moving air stream and subsequently discharged

into the atmosphere.

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Necessity of Cooling the Condenser WaterNecessity of Cooling the Condenser Water

The high cost of water makes it necessary to use cooling towers for

water cooled condensers.

The cooling water requirement in an open system is about 50 times

the flow of steam to the condenser. Even with closed cooling system using

cooling towers, the requirement for cooling water is also considerably large as

5 to 8 kg/kW-hr.

This means a 1000 MW station will require about 100 thousand tons

of circulating water per day even with the use of cooling towers. This huge

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of circulating water per day even with the use of cooling towers. This huge

requirement of cooling water is equivalent to a requirement of big city like

Mubai.

As the cooling water takes the latent heat of steam in the condenser,

the temperature of the water increases. The hot water coming out of the

condenser cannot be used again in a closed system without pre-cooling.

This is because, the hot water coming out if used again will not be

able to absorb the heat as temp, reaches near to Ts saturation temperature of

steam at condenser pressure and the condenser vacuum cannot be

maintained. Therefore it is absolutely necessary to precool the water coming

out of condenser before using again.

Types of cooling system and TowerTypes of cooling system and Tower

TYPES OF COOLING TOWERS

There are two main types of cooling towers:

The natural draft and mechanical draft cooling towers.

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11:48:1011:48:1011:48:1011:48:10

Natural draft cooling tower

The natural draft or hyperbolic cooling tower makes use

of the difference in temperature between the ambient air and

the hotter air inside the tower.

As hot air moves upwards through the tower (because

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As hot air moves upwards through the tower (because

hot air rises), fresh cool air is drawn into the tower through an

air inlet at the bottom. Due to the layout of the tower, no fan is

required.

Concrete is used for the tower shell with a height of up to

200 m. These cooling towers are mostly only for large heat

duties because large concrete structures are expensive.

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

located outside the tower

Counter flow tower: air is drawn up through the falling water and

the fill is therefore located inside the tower, although design depends

on specific site conditions

Cross flow Counter flow

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Mechanical draft cooling tower

Mechanical draft towers have large fans to force or draw air

through circulated water. The water falls downwards over fill surfaces,

which help increase the contact time between the water and the air - this

helps maximize heat transfer between the two.

Cooling rates of mechanical draft towers depend upon various

parameters such as fan diameter and speed of operation, fills for system

resistance etc.

Mechanical draft towers are available in a large range of

capacities. Towers can be either factory built or field erected – for example

concrete towers are only field erected.

Many towers are constructed so that they can be grouped together

to achieve the desired capacity. Thus, many cooling towers are assemblies

of two or more individual cooling towers or called “cells.”

Multiple-cell towers can be lineal, square, or round depending upon

the shape of the individual cells and whether the air inlets are located on

the sides or bottoms of the cells.

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Forced Draft Cooling Tower

Induced draft counter

flow cooling tower

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11:48:1011:48:1011:48:1011:48:10

Forced Draft Towers:

The advantages of this type of towers are:

1. This is more efficient than induced draft as some of the air velocity is

converted into static pressure in the tower and recovered in the form of

useful work.

2. The vibration and noise are minimum as mechanical equipments are set

on a solid foundation.

3. As it handles dry air, problems of fan blade erosion are avoided.

4. It is more safe as it is located on the ground level.

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4. It is more safe as it is located on the ground level.

The major disadvantages of this tower are:

1. There is possibility of recirculation of hot, humid exhaust coming out from

the top of the tower through the low pressure air intake region. The

recirculation can cut the efficiency as much as 20%. [To avoid this, a

cross-flow tower as shown in Fig. 18.21 can be used.]

2. During cold weather, ice is formed on nearby equipments and buildings or

in the fan housing itself. The frost in the fan outlet can brake the fan

blades.

3. The fan size is limited to 4 metres.

4. The power requirement of FD. fan system is approximately double that of

I.D. system for the same capacity.

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11:48:1011:48:1011:48:1011:48:10

Induced Draft Tower:The advantages of this tower over forced draft are:

1. The main advantage is that coldest water comes in contact with the driest

air and warmest water comes in contact with the most humid air.

2. The recirculation is seldom a problem with this tower as outlet fan

discharges the heated and humid air directly away from the air-intakes

below the tower.

3. The size of 20 m in diameter can be used.

4. Claims are made that the I.D. fan tower has the advantages of lower first

cost, requires less space, is capable of cooling through a wide range.

5. The first cost is lower due to the reduction in pump capacity required and5. The first cost is lower due to the reduction in pump capacity required and

smaller length of water pipes. The power consumption per kg of water

cooled is less compared with F.D. fan system.

The disadvantages of this water cooling system are:

1. The static pressure loss is higher as restricted area at base tends to choke

off the flow of high velocity air. This requires higher power motor to drive

the fan compared with forced draft handling equivalent amount of air.

2. The air velocities through the packings are unevenly distributed and it has

very little movement near the walls and centre of the tower.

3. The forced draft cooling tower is competitive to induced draft type in terms

of initial cost for cooling capacity up to 15000 litres of water per minute. For

larger quantity, the economics shifts in favour of the induced draft tower. ww

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The spray cooling poud is one of the simplest methods of cooling the

condenser water although it is not efficient. Thousands of spray ponds were put

into operation until cooling towers came into picture in 1920s.

Cooling PondsCooling Ponds

1) Natural and Directed Flow System.

In natural flow system, water coming out from the condenser is just allowed

Types of Cooling Ponds:

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In natural flow system, water coming out from the condenser is just allowed

to flow into the pond as shown in Fig.

In directed flow system, the hot water coming out of condenser enters

the middle channel as shown in Figure and on reaching the far end

divides into two currents, being directed by the baffle walls so as to

traverse the pond several times before uniting at the intake point.

The water gets more time and passes over a more surface, so the

cooling achieved is very effective.

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2) Single Deck and Double Deck Systems:

In a single deck system, spray nozzles are arranged at the same elevation as

shown in Figure 1. Its effective cooling effect is less than double deck system.shown in Figure 1. Its effective cooling effect is less than double deck system.

In double deck system, spray nozzles are arranged at different elevations as

shown in Figure 2. Its cooling effect is more than single deck system as

water comes in contact with air at lower temperature.

3) Open and Louvre Fence: In case of open pond, the drift

losses will be more if the wind velocity is high. This can be

avoided by providing Louvre fence as shown in Figure.

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Disadvantages of cooling ponds are:

1. The considerably large area for cooling is required. large.

2. Spray losses due to evaporation and windage run high.

3. There is no control over the temperature of cooled water.

4. The cooling effect is reduced with the decrease in wind

velocity and if the load on the plant increases, the pond

does not respond to the requirement. When the maximum

cooling is required during increased load, it provides

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cooling is required during increased load, it provides

minimum cooling in absence of wind flow.

5. The cooling efficiency is low compared with cooling tower.

Spray cooling pond system is rarely used nowadays in

modern thermal power plants of high capacity as it

requires large surface area which is rarely available.