In evaporative cooling devices, the approach is defined as the temperature difference between which elements?

• A. The temperature difference between entering air and leaving air
• B. The temperature difference between the average temperature of the circulating water leaving the device and the average wet-bulb temperature of the entering air
• C. The temperature difference between the entering fluids
• D. The rate of heat transfer between fluids

Answer: (B)

In evaporative cooling devices, the approach is the temperature difference between the averaged temperature of the circulating water leaving the device and the average wet-bulb temperature of the entering air. The wet-bulb temperature represents the lowest temperature the air can reach through evaporation, so the leaving water can’t get cooler than that limit. The closer the leaving water temperature is to the entering air’s wet-bulb, the more efficient the cooling is. A larger gap means less effective heat transfer. The other ideas—how much the air cools, the temperatures of entering fluids, or the rate of heat transfer—don’t describe this specific measure of how close the leaving water gets to the air’s evaporation-limited temperature.