Water-cooled fans use the principle from physics that evaporating liquids absorb heat. The fan pulls air across wet pads to increase evaporation and circulate the chilled air. Unlike more common air conditioning systems that operate with a closed loop to recycle refrigerant, evaporative coolers are open systems, and the water increases the humidity in the air as the unit converts it from a liquid to a vapor.
The ancient Egyptians used evaporation to cool large pots of water. They wrapped wet towels around the clay pots and placed them on the rooftop to allow the sun to increase evaporation rates. Benjamin Franklin noted that changing from sweaty clothes to dry ones made him feel warmer. He experimented with evaporation by placing wet fabric around the bulb of a thermometer. By using spirits that evaporate faster than water, Franklin was able to lower the temperature to below freezing.
The effectiveness of water-cooled fans depends on the relative humidity in the area. While some users refer to evaporative coolers as swamp coolers, they do not work well in swamps or other areas where humidity is high. However, in desert regions, where low humidity maximizes the evaporation, the swamp cooler can lower the air temperature by as much as 15 degrees Fahrenheit.