What is the lowest temperature a cooling tower can operate at?

• A. Average dew point
• B. Wet bulb temperature
• C. Dry bulb temperature
• D. Ambient air temperature

Answer: (B)

Evaporative cooling in a cooling tower can only drop the water temperature to about the wet-bulb temperature of the surrounding air. As water evaporates, it carries away latent heat, cooling the remaining water. But evaporation can continue only until the air becomes fully saturated with moisture; at that point the wet-bulb temperature is the lowest achievable temperature. So the water leaving the tower tends toward the air’s wet-bulb temperature, often with a small approach above it due to inefficiencies. The dry-bulb temperature is just the air’s actual temperature without humidity, the dew point is simply where air becomes saturated, and ambient air temperature is not the limiting factor.