The rate of heat transfer in any cooling tower system depends on the:

• A. The difference between the inlet air relative humidity and the inlet air absolute humidity.
• B. The difference between the temperature of the water in and the temperature of the air out.
• C. The amount and temperature of the make-up water
• D. The difference between the inlet water temperature and the inlet air wet bulb temperature

Answer: (D)

The rate of heat transfer in a cooling tower is governed by the driving force of the heat transfer, which is the temperature difference between the hot water and the air’s cooling potential. The air’s wet-bulb temperature represents how cold the air can become when it’s cooled by evaporating water, so the most meaningful measure is the difference between the incoming water temperature and the incoming air’s wet-bulb temperature. A larger gap means more heat can be transferred from the water to the air, through both sensible cooling and evaporation.

Other factors mentioned, like differences in humidity, the water entering versus leaving temperatures, or the makeup water amount, influence how efficiently the tower operates or how much water is evaporated, but they do not set the fundamental driving force for heat transfer the way the water–air wet-bulb temperature difference does.