Theory and operation - Open circuit cooling towers
Exploiting a simple, natural physical principle, thanks to which the forced evaporation of a minimal quantity of water produces a lowering of the temperature of the main mass of water, evaporative cooling towers still today represent the most widely employed system of cooling in civil and industrial applications.
The minimum outlet temperature theoretically obtainable from an evaporative cooling tower is that of the wet bulb temperature of the atmospheric air measured in the area in which the unit is installed: this value is always lower than the dry bulb temperature (unless, of course, the air is already saturated).
In practice, due to the effects of performance factors related to air saturation, an adequately sized cooling tower manages to cool the circulating water down to a temperature 2-3 °C above the wet bulb temperature.
It is on this basis that many designers and equipment manufacturers size their cooling circuits and heat exchangers, already planning, from the first design stage, the use of cooling tower water ensuring optimum plant efficiency.