Types of Cooling Towers

Types of Cooling Towers

Various types of cooling towers are :

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

These are further classified as :

(a) Forced draught cooling towers.

(b) Induced draught cooling towers.

1. Natural Draught Cooling Towers :

1. A natural draught cooling tower is shown in Figure A. Hot water from the condenser is supplied to troughs and nozzles with the help of a pump at about 8-10 m from bottom.
2. Water falls in the form of spray and the air entering from bottom rises upwards and it takes up the heat from falling water.
3. Some of water during its fall is evaporated which is carried along by the heated water.
4. The cooled water is collected at the bottom of cooling tower and returned to the condenser.
5. The circulation of air is natural due to the density difference between the heated air in the cooling tower and the surrounding air like natural draught in chimney.
6. Cooling towers are made of either concrete, timer or steel.

Figure A

Advantages of Natural Draught Cooling Towers :

1. Low operating and maintenance cost since no fan is needed.
2. Less ground space is needed.
3. Provides trouble free operation.

Disadvantages of Natural Draught Cooling Towers :

1. Capital cost is high since the size of cooling tower may be upto 100 m diameter base and 120 m height.
2. Performance of cooling tower varies with season since it is dependent of dry bulb temperature and relative humidity of air.

2. Forced Draught Cooling Tower :

1. Figure C shows the schematic diagram of a forced draught cooling tower. 
2. In this system, a fan is installed at the bottom of the cooling tower. 
3. The hot water from condenser is supplied at the top of the cooling tower which is sprayed through the nozzles. 
4. Water droplets fall on the packings. 
5. The rising air cools the incoming water. The entrained water in air is eliminated by the spray eliminators.

Figure B

Advantages over natural cooling towers :

1. Size of cooling tower and its piping is reduced, therefore capital cost is reduced.
2. It is more efficient.
3. It also increases the efficiency of plant.

Disadvantages of Forced Draught Cooling Tower :

1. The running cost is high since power is needed to operate the fan.
2. Fan size is limited.
3. In cold weather, ice formation may take place on fan blades, thus the fan blades may break. Maintenance cost is high.

3. Induced Draught Cooling Tower :

• Induced draught cooling tower is similar to forced draught cooling tower except that the fan is installed at the top of the tower instead at the bottom.
• Air enters at the bottom due to the vacuum created in the cooling tower as shown in Figure C.

Figure C

Advantages of Induced Draught Cooling Tower :

1. The cooling water can be re-circulated without any problem.
2. Hot water comes in contact with dry air, thus its efficiency is high.
3. Capital cost is low compared to natural cooling tower.
4. It needs less space.

Disadvantages of Induced Draught Cooling Tower :

1. Fan power is high compared to forced fan power of forced draught cooling tower since it needs to handle hot air of higher specific volume.
2. Maintenance cost is high.
3. Fan bearings needs to be cooled.