The use of refrigerating machinery to condition air in summer is usually expensive i~ terms of both capital and operating costs. It has become essential, therefore, to investigate cooling devices that consume less energy than conventional air-conditioning devices and methods found to improve their operating characteristics. A simple and. economical substitute for refrigeration that can achieve air-conditioning in hot. dry climates is the evaporative cooling of air. The evaporative cooler is considerably cheaper than refrigeration equipment to install and operate and is suitable for cooling rooms, residences, factories and small commercial establishments. Moreover, it affords the possibility of using solar energy as the input, because of the small amount of power required for its operation.

Evaporative cooling has been known for a long time and has found numerous practical applications. When water evaporates into the air being cooled, the process is termed direct evaporative cooling. When the evaporative principle is used to cool air or water, which in turn cools the supply air without a change in humidity, the process is termed indirect evaporative cooling.