S. No.

Plant

EnergyInput for1 kWh,

kJ

Total heatenergywasted

kJ

Waste heatto stack

kJ

Waste heatcoolingwater

kJl. Ideal: All input heat

converted to electric energy

3600000

2. Modern efficient fossil fueled plant

9075548513694115

3. Modern atomic power plant

1213485465288018

4. Predicted future fossil fueled plant.

84414852.751264.53588

5. Predicted future nuclear breeder reactor plant.

86555066~184748

Heat Utilization in Thermal Station

• approximately more than 100 kg of water per kWh is required for condenser cooling.

• about 5% additional quantity is needed for other plant services, e.g., quenching of ash, boiler make up water and bearing cooling etc.

• For this purpose, the power plant can be located near a source of natural water e.g., rivers, lakes and coastal water for once through condenser cooling.

Once through cooling water system

• The modem trend is to locate the steam power stations as near as possible to the centre of gravi-ty of the electrical load instead of near a source of natural water to avoid huge transmission costs.

• There must be some arrangement to recool the circulating water and for this purpose cooling ponds and cooling towers are needed.

• This type of cooling water system is known as closed circuit or circulating cooling water system.

COOLING PONDS

Spray Cooling Pond

Dissipation of heat is influenced by the following factors :

1.Initial temperature of water entering the pond. 2.Atmospheric temperature. 3.Relative humidity. 4.Air velocity. 5.Solar radiation.6.Earth temperature.7.Atmospheric pressure.8.Area of pond.9.Depth of the pond.

• The disadvantages of cooling ponds are that considerable quantity of water may be carried away in suspension in air when its velocity is high and loss due to evaporation and also space consideration is there.

• This system is used only in low capacity electric power stations e.g., diesel engine power stations.

COOLING TOWERS

Arrangement of a cooling tower

 

• The heat exchange between air and water through direct contact is from two types of heat transfer.

1. The evaporative cooling of water. 2. The convective heating of air.

• Make up water must be added to the tower basin to replenish the water lost through evaporation.

• Another drawback is that the addition of large quantities of water vapours to the atmosphere can adversely affect the weather downward of the tower, causing higher than average precipitation.

 

• Amount of water evaporated is approximately 0.346 kg to 0.368 kg per 1000 kJ of heat load from the condenser.

TYPES OF COOLING TOWERS  

 

• According to the material, the cooling towers are of the following types :

1. Timber 2. Ferro-concrete 3. Multi-deck concrete towers 4. Metallic

 

• According to the nature of air draught, the cooling towers are of the following types:

1.Atmospheric 2.Natural draught cooling towers 3.Mechanical draught cooling towers

(a) Induced draught cooling towers (b) Forced draught cooling towers (c) Combined induced and forced draught cooling

towers

The natural and mechanical draught cooling towers are further classified as:

(a)counter-flow and (b) cross-flow towers.

ATMOSPHERIC COOLING TOWERS

Atmospheric cooling tower

NATURAL DRAUGHT COOLING TOWERS

Natural draught cooling tower

Natural Draft (N.D.) cooling towers are mostly selected:

•In cool and humid atmospheres, i.e, low wet bulb temperature and high relative humidity. •Where there is a combination of low wet bulb temperature and high condenser water inlet and outlet temperatures.

•In cases of heavy winter loads.

Hyperbolic N.D. cooling tower

MECHANICAL DRAUGHT COOLING TOWERS

Forced draught cooling tower

Induced draught cooling tower

• These towers require a smaller land area and can be built at most locations.

• The fans give good control over the air-flow and thus the water temperature.

• Also they cost less to install than natural draught towers.

However they have drawbacks also:

1. Local fogging and icing may occur in winter season.

2. Fan power requirements and maintenance costs make them over expensive to operate.

DRY COOLING TOWERS

Direct system

Indirect system