Airconditioning can be considered as the addition of refrigeration to the heating and ventilation of a building in order to provide cooling and dehumidification, that is to extract heat and moisture from the conditioned space; the extraction of heat occurs at the air coil, and the refrigerant, or the chilled water, as the case may be, conveys this heat away from the conditioned area. Ultimately this heat is discharged elsewhere either usefully, in the form of reheat, or for heating at another point as in a heat pump, or is discharged into the atmosphere as a waste product. It is for this last purpose, that the cooling tower may be used so that the heat is dissipated under low-grade conditions.
At the refrigeration plant, the heat is dissipated in the refrigerant condenser, which is usually air or water-cooled; each is a relatively cheap medium. The physical characteristics are such that heat transfer between two gases through the walls of a separating tube or vessle is more difficult, and requires a greater heat transfer surface than between a liquid and a gas. The cost of heat transfer surface is high thus an air cooled condenser may cost two or three times that of water cooled units.
Furthermore, in an air cooled condenser, it is normally only economic to cool the refrigerant to within 10 deg F of the dry bulb temperature, whereas the combination of shell and tube condenser and evaporative cooling tower can operate at least 5 deg F below these conditions. This temerature reduction can save upto 10% in refrigeration compressor designrunning horsepower.