Cooling towers are used to conserve or recover water. In one design, the hot water from the condenser is pumped to the tower. There, it is sprayed into the tower basin. The temperature of the water decreases, as it gives up heat to the air circulating through the tower. Some of the towers are rather large, since they work with condensers yielding 1600 ton of cooling capacity (see Fig. 8-14).
Most of the cooling that takes place in the tower results from the evaporation of part of the water as it falls through the tower.
The lower the wet bulb temperature of the incoming air, the more efficient the air is in decreasing the temperature of the water being fed into the tower. The following factors influence the efficiency of the cooling tower:
■ Mean difference between vapor pressure of the air and pressure in the tower water
■ Length of exposure time and amount of water surface exposed to air
■ Velocity of air through the tower
■ Direction of air flow relative to the exposed water surface (parallel, transverse, or counter)
Theoretically, the lowest temperature to which the water can be cooled is the temperature of the air (wet bulb) entering the tower. However, in practical terms, it is impossible to reach the temperature of the air In most instances, the temperature of the water leaving the tower will be no lower than 7 to 10°F (−14 to −12°C) above the air temperature.
The range of the tower is the temperature of the water going into the tower and the temperature of the water coming out of the tower. This range should be matched to the operation of the condenser for maximum efficiency.