Water and air are brought in close contact, for which a fan sucks in air in counter-flow to the water, as a consequence of which part of the water evaporates.

The necessary heat for this, approx.597 Kcal. For every litre of water, is taken from the circuit itself thus producing its cooling. In addition to this, the temperature drop between the hot water and the external air temperature is also used for the cooling. The efficiency of a cooling tower depends mainly on the heat exchange surface that has been assembled, on the good distribution of the water, on the amount of intake air and on the external air conditions.

The theoretical limit of cooling is the temperature in the wet bulb. This temperature can be determined on the Molier i-x diagram if the air temperature and relative humidity are known. The difference between the required cold water temperature and the wet bulb temperature (called cooling limit distance), is significant to determine the size of the tower. The bigger the cooling distance is, the smaller the tower becomes and therefore the more economical. The limit distance must be of at least 3-4º C.