Necessity of Cooling Ponds and Cooling Towers

1. The circulating water required for condensing the steam in the condenser depends on the extent of vacuum to be maintained in the condenser, hence, it depends on the extent of cooling desired and the temperature of cooling water available.
2. The heated water coming out of the condenser is required to be cooled so that it can be re-circulated in the condenser.
3. The cooling water required in an open system is about 50 times the mass flow of steam while in the closed loop using cooling towers it is about 5 to 7.5 kg/ kWh. Therefore in large steam power plants the cooling water required say for a 100 MW plant may be of the order of 15 x 10^x litres per day.

Depending upon the availability of source of water, the cooling systems can be classified as :

1. Open or once through system
2. Closed cooling system
3. Mixed cooling system

1. If the plant is located near the river or sea having availability of water throughout the year, open or once through cooling system can be used as shown in Figure A.
2. Cooling water from upstream side of the river is pumped and supplied to the condenser through a filter.
3. After circulation and absorption of heat by water in the condenser, the heated water is discharged back into the river.
4. The distance between uptake and downtake points of cooling and hot water respectively is kept large.

1. Schematic diagram of a closed cooling system is shown in Figure B.
2. This system is usually employed when the sufficient cooling water required for condensing the steam from lake or river is not available.
3. It employs a cooling tower in which the heated water from condenser is cooled by spraying it through nozzles and by transfer of heat to surrounding air.
4. The water evaporated during cooling is only drawn from river or any other source as make up water (2 to 5%) to be circulated into the condenser.

1. The mixed cooling system is the combination of both open and closed systems described above.
2. In this system the hot water received from condenser is divided in two parts, one part of it is directly discharged into the river as in case of open system and the remainder part is passed into the cooling tower for its cooling and then discharged into the river.
3. The cooling water is supplied from river directly into the condenser. The system is shown in Figure C.

The system has the advantage that it reduces the size of cooling tower and the cooling water requirement since it is not required to be recirculated. Another advantage is that the average temperature of hot water discharged into the river is reduced.