The theoretical possible heat removal per pound of air circulated in a cooling tower
depends on the temperature and moisture content of the air. An indication of the
moisture content of the air is the wet-bulb temperature. Ideally, the wet-bulb temperature
is the lowest temperature to which the water can be cooled. In practical applications, the
cold water temperature approaches but does not equal the air wet-bulb temperature
because it is impossible to contact all the water with fresh air as the water drops through
the fill to the basin. The magnitude of approach to the wet-bulb temperature is dependent
on tower design. Important factors are air-to-water contact time, amount of fill surface
and break up of water into drops [4]. The closer the approach, i.e. the smaller the
difference between wet-bulb temperature and cold water temperature, the more
expensive the cooling tower design will be.