cooling towers 2003

8/3/2019 Cooling Towers 2003

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COOLING TOWERS

Definition:

# In power plants the hot water from condenser iscooled in cooling tower, so that it can be reused incondenser for condensation of steam.

# In a cooling tower water is made to trickle downdrop by drop so that it comes in contact with the airmoving in the opposite direction.

# As a result of this some water is evaporated andis taken away with air. In evaporation the heat istaken away from the bulk of water, which is thuscooled.


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Factors affecting cooling of water in

a cooling tower are:

1. Temperature of air.

2. Humidity of air.

3. Temperature of hot air.4. Size and height of tower.

5. Velocity of air entering tower.

6. Accessibility of air to all parts of tower.7. Degree of uniformity in descending water.

8. Arrangement of plates in tower.


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The cooling towers may be classified as

follows:

1. Natural draught cooling towers2. Mechanical draught cooling towers:

(i) Forced draught cooling towers

(ii) Induced draught cooling towers.


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NATURAL DRAUGHT COOLING

TOWER:

# In this type of tower, the hot water from thecondenser is pumped to the troughs and

nozzles situated near the bottom.# Troughs spray the water falls in the form of

droplets into a pond situated at the bottom ofthe tower.

# The air enters the cooling tower from airopenings provided near the base, rises upwardand takes up the heat of falling water.


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Advantages:

# Low operating and maintenance cost.

# It gives more or less trouble free operation.

# Considerable less ground area required.

# The towers may be as high as 125 m and 100 m indiameter at the base with the capability of withstandingwinds of very high speed. These structures are more or less

self-supported structures.# The enlarged top of the tower allows water to fall out of

suspension

Disadvantages:# High initial cost.

# Its performance varies with the seasonal changes in DBT(dry bulb temperature) and R.H. (relative humidity) of air.


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MECHANICAL DRAUGHT

COOLING TOWERS:

# In these towers the draught of air for cooling

the tower is produced mechanically by

means of propeller fans.

# These towers are usually built in cells or

units, the capacity depending upon thenumber of cells used.


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FORCED DRAUGHT COOLING

TOWER:

It is similar to natural draught tower as far asinterior construction is concerned, but the sides of

the tower are closed and form an air and water tight

structure, except for fan openings at the base for theinlet of fresh air, and the outlet at the top for the exit

of air and vapours.

There are hoods at the base projecting from themain portion of the tower where the fans are placed

for forcing the air, into the tower.


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ADVANTAGES OF FORCED

DRAUGHT COOLING TOWER:

1. More efficient (than induced

draught).

2. No problem of fan blade erosion (asit handles dry air only).

3. More safe.4. The vibration and noise are

minimum.


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DISADVANTAGES OF FORCED

DRAUGHT COOLING TOWER:

1. The fan size is limited to 4 meters.

2. Power requirement high (approximately

double that of induced draught system forthe same capacity).

3. In the cold weather, ice is formed on

nearby equipments and buildings or in thefan housing itself. The frost in the fan

outlet can break the fan blades.


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INDUCED DRAUGHT

COOLING TOWER: In these type of cooling towers ,the

fans are placed at the top of thetower and they draw the air in

through louvers extending all

around the tower at its base.


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Advantages of induced draught

cooling tower:

1. The coldest water comes in contact withthe driest air and warmest water comes incontact with the most humid air.

2. In this tower, the recirculation is seldom aproblem.

3. Lower first cost (due to the reduced pump

capacity and smaller length of waterpipes).

4. Less space required.

5. This tower is capable of cooling through awide range.


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Disadvantages of induced draught

cooling tower:

1. The air velocities through the packingare unevenly distributed and it has verylittle movement near the walls and centre

of the tower.

2. Higher H.P. motor is required to drivethe fan comparatively. This is due to the

fact that the static pressure loss is higheras restricted area at base tends to chokeoff the flow of higher velocity air.

ADVANTAGE OF MECHANICAL


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ADVANTAGE OF MECHANICAL

DRAUGHT TOWER OVER

NATURAL DRAUGHT TOWERS:1. These towers require a small land area

and can be built at most locations.

2. The fans give a good control over the air

flow and thus the water temperature.

3. Less costly to install than natural draughttowers.


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DISADVANTAGE OF MECHANICAL

DRAUGHT TOWER OVER NATURAL

DRAUGHT TOWERS:

1. Fan power requirements andmaintenance costs make them more

expensive to operate.

2. Local fogging and icing may occur inwinter season.


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DRY COOLING TOWERS:

The necessity of using dry cooling towers

has been felt due to the following reasons:

# Natural water resources can be used fordissipating heat in power plants but there

are some limitations to do so, as discussed

earlier. Moreover there is a potentialthermal pollution problem in this system.


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# In evaporative cooling system (cooling

towers) due to continuous evaporation of

water, the concentration of impurities goes onincreasing and so the disposal of tower blow

down may become a serious problem at some

sites. Besides this make up water for tower use

is limited in some areas.

# Thus the use of dry cooling towers in power

plants is the only alternative. The dry system

rejects the heat directly to the atmosphere

which is the largest heat sink available.

CLASSIFICATION OF DRY COOLING TOWERS


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CLASSIFICATION OF DRY COOLING TOWERS:

Direct system

Indirect system

DIRECT SYSTEM

# In direct system the exhaust steam from the turbine ispassed on to the cooling coils where it is condensed

directly by means of air.# Cooling coils constitute air cooled condenser. This

system has the disadvantage that large ducts are neededto convey the exhaust steam to the exchanger coils, to

minimize the pressure drop.# This places a practical limit on the size of the unit. This

system is limited to maximum generator unit sizes of200 MW to 300 MW.


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INDIRECT SYSTEM


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INDIRECT SYSTEM:

# In this case, the exhaust steam is condensed in a

spray condenser by means of circulating water.# The major part of the water from the condenser

flows back to the cooling coils and an amount

equal to the exhaust steam from the turbine isdirected back to the boiler feed water circuit.

# Some energy recovery is possible by using

hydraulic turbine (T) to drive the circulatingwater pumps. This system is applicable to all

large unit sizes.


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ADVANTAGES OF DRY

COOLING TOWERS:

1.It solves the problem of thermal pollution of waterand as the circulating water does not come intodirect contact with the cooling air, there is noevaporative loss of water as in wet type tower.

2. This system eliminates water supply as a significantplant site selection factor. Power plants can be sitedcloser to the load centre.

3. There is minimum air pollution.

4. From environmental view point, the dry towersoffer several desirable features e.g. no fog, noblowdown treatment, no windage loss of water, noevaporative loss of water and no thermal dischargeto water source.


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DISADVANTAGES OF DRY

COOLING TOWERS:

1. Their performance is limited by dry bulb

temperature and so turbine exhaust temperatures are

much higher resulting in a substantial loss of turbine

efficiency, most critical in warm climates.

2. Due to low heat transfer co-efficient, dry cooling

towers require enormous volumes of air large

surface areas and are less effective at high natural airtemperatures.


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MAINTENANCE OF COOLING

TOWERS:

The regular maintenance of cooling towers is

very essential to achieve the desired cooling

and to reduce the depreciation costs. Themaintenance of cooling towers includes the

following:

1. The fans, motors housings etc. should be

inspected from time to time.

2. Motor bearings should be greased and gear

boxes oiled. Any unusual noise or vibrations in

them should be corrected immediately.

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3. At least once in a year motors gear

boxes should be checked for structural

weakness.4. The circulating water should be tested

for hardness and should be kept free from

impurities to avoid scale formations and

to avoid corrosive action of water.

5. The water spraying nozzles should beinspected regularly for clogging.

cooling towers 2003

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