Cooling towers are essentially water conservation or recovery devices. Cooling towers are the largest mass and heat transfer devices used extensively wherever water is used as a cooling medium as in the case of water-cooled condensers.
Principle of cooling tower
A cooling tower is heat-rejection equipment where the warm water from the condenser is pumped over the top of the cooling tower from where it falls or is sprayed down to the tower basin. The temperature of the water is reduced as it gives up heat air circulating through the tower.
Most of the heat transfer from the water to the air is accomplished by the evaporation of condensing water and the same sensible heat transfer between them.
CLASSIFICATION OF COOLING TOWERS
Based on the direction of air flow
- Parallel flow cooling tower
- air flow and water are flow are in the same direction.
- Counter flow cooling tower
- air flow and water flow are in the opposite direction.
- Cross flow cooling tower
- air flows across the flow of water.
Based on the draught
- Natural draught cooling tower or atmospheric tower
- Mechanical draught cooling tower
(a) Induced draught cooling tower
(b) Forced draught cooling tower
NATURAL DRAUGHT COOLING TOWER
The difference of pressure is known as draught. The pressure difference causes the air or gas to flow.
Natural draught is produced due to the difference in weight (density) the cold air (outside the tower) and hot air (inside the tower).
The outside air of the cooling tower is heavier than the hot air inside the cooling tower. Therefore, the pressure inside the chimney is less; hence outside air will flow through the tower, which makes the natural draught.
- The schematic sketch of a natural draught cooling tower.
- The hot water from the condenser outlet is sprayed down from the header installed at the top of the cooling tower through a series of nozzles fixed in the header.
- Due to the natural draught, the air flows across the cooling tower when, the water gives its heat to the air due to evaporation.
- The cool water gets collected in the sump, from where, it is taken to the condenser through a pump.
- Packing materials inside the tower, are used.
- to increase thre wetted surface
- to break up the water into droplets and
- to slow down the water descending to the bottom of the tower which causes an effective cooling
- Large natural draught cooling towers are popularly used in power plant for cooling the water supply to the condenser.
NATURAL DRAUGHT COOLING TOWER
When the air circulation through the tower is with the help of a blower, the tower is called the mechanical cooling tower.
If the draught is produced by a fan or blower which draws the air through the tower called induced draught.
If the draught is produced by a fan or blower which forces the air through the tower called forced draught.
- A schematic representation of forced draught and induced cooling tower.
- The hot wter from the condenser outlet is sprayed down from the header installed at the top of the cooling tower through a series of nozzles fixed in the header.
- Eliminators are provided at the top of the tower to prevent the air carrying water droplets with it.
- Small mechanical draught cooling towers are preferred for oil refineries and other process industries as well as for central air conditioning plants.