In cooling tower operation, which temperature reading is used to specify the lower limit?

• A. Globe temperature
• B. Wet bulb temperature
• C. Dry bulb temperature
• D. Ambient temperature

Answer: (B)

The key idea is evaporative cooling. In a cooling tower, heat is removed from water primarily by evaporation, which depends on how much moisture the surrounding air can hold. The wet-bulb temperature combines air temperature and humidity and represents the lowest temperature to which air can be cooled by evaporation under those conditions. Therefore, the leaving water temperature of a cooling tower cannot be lower than the ambient wet-bulb temperature; this sets the practical lower limit.

Dry-bulb temperature is just the ambient air temperature without accounting for humidity, so it doesn’t reflect how far evaporative cooling can actually drive the water temperature. Globe temperature includes radiant heat (like sun exposure) and isn’t the controlling factor for the evaporative limit. Ambient temperature is another way of referring to surrounding air temperature, which again doesn’t capture the evaporative cooling capability.