Which statement best describes the primary cooling mechanism in a cooling tower?

• A. Evaporation
• B. Conduction
• C. Radiation
• D. Convection

Answer: (A)

Evaporation. In a cooling tower, hot water is spread over packing or fill to maximize contact with air. A portion of that water then evaporates, and the energy required to change liquid water to vapor (the latent heat of vaporization) is taken from the remaining water, producing a drop in temperature. This evaporative cooling is the dominant mechanism, while heat transfer by conduction through tower materials and by convection or radiation from water to air plays a much smaller role. The cooling effect can approach the wet-bulb temperature of the air, and the rate of evaporation (and thus the cooling) increases with drier air and higher airflow.