COOLING
TOWERS
By: Prof K. M.Joshi
Assi. 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.
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Cooling towers make use of evaporation whereby some of the water
is evaporated into a moving air stream and subsequently discharged
into the atmosphere.
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
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.
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Necessity of Cooling the Condenser Water
TYPES OF COOLING TOWERS
There are two main types of cooling towers:
The natural draft and mechanical draft cooling towers.
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Types of cooling system and Tower
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.
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.
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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.
Counter flow
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
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Cross flow
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.
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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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.
Forced Draft Cooling Tower
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Induced draft counter
flow cooling tower
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Forced Draft Towers:
The advantages of this type of towers are:
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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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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Induced Draft Tower:
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.
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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 and
smaller length of water pipes. The power consumption per kg of water
cooled is less compared with F.D. fan system.
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Cooling Ponds
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.
Types of Cooling Ponds:
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1) Natural and Directed Flow System.
In natural flow system, water coming out from the condenser is just allowed
to flow into the pond as shown in Fig.
The water gets more time and passes over a more surface, so the
cooling achieved is very effective.
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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.
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.
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.
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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.
Disadvantages of cooling ponds are:
The considerably large area for cooling is required. large.
Spray losses due to evaporation and windage run high.
There is no control over the temperature of cooled water.
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
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.
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