In a cooling tower, cooling of the main mass of water is obtained by the evaporation of a small proportion into the airstream. Cooled water leaving the tower will be 3-8 K warmer than the incoming air wet bulb temperature. (See also Chapters 21 and 22.) The quantity of water evaporated will take up its latent heat equal to the condenser duty, at the rate of about 2430 kJ/kg evaporated, and will be approximately
= 0.41 X 10-3 kg/(skW)
For a condenser load of 400 kW, evaporation would be at a rate of 0.16 kg/s.
Cooled water from the drain tank is taken by the pump and passed through the condenser. The warmed water then passes back to sprays or distribution
troughs at the top of the tower and falls in the upgoing airstream, passing over packings which present a large surface to the air. Evaporation takes place, the vapour obtaining its latent heat from the body of the water, which is therefore cooled (see Figure 6.6).
Condenser water pump
Figure 6.6 Cooling tower circuit
The power consumption of the tower can be reduced by fan cycling or fan speed control under light load conditions. An induced draught tower in which the fan is in the outlet air stream can achieve 10-15% performance with the fan idle. This is not the case with a forced draught tower where the fan is located in the inlet air stream.
Posted in Refrigeration and Air Conditioning