Evaporative cooling (EC), also known as swamp cooling, is a strategy for cooling air that takes advantage of the drop in temperature that occurs when water that's exposed to moving air begins to vaporize and change to gas. You've probably experienced the effects of evaporative cooling if you've ever changed out of wet clothes because you felt chilled.
An evaporative cooler is basically a large fan that draws warm air through water-moistened pads. As the water in the pads evaporates, the air is chilled and pushed out to the room. The temperature can be controlled by adjusting the airflow of the cooler. Evaporative coolers are rated by the volume of warm/cool air that can be exchanged in one minute (CFM) and by the amount of energy they require to run. EC works best in dry climates; the lower the relative humidity, the easier it is for moisture to evaporate from the pads.
Evaporative cooling is relatively new to the data center, but it's been used for years in homes as an environmentally-friendly alternative to traditional air conditioning. Unlike an air conditioner (AC), an evaporative cooler doesn't use refrigerants, which can be hazardous to the environment. Installation costs are significantly lower, electricity usage is significantly lower and the units themselves are simpler to maintain and operate.
Here is a simple example of how an evaporative cooler works: Distribution lines supply water to the top of the pads. The pads are often made of wood shavings, but they can be made of any material that absorbs and holds moisture and while resisting mildew. When the pads are super-saturated, excess water drips down to the bottom of the cooler and a water pump sends the collected water back to the top of the pads so the process can begin again. Since water is continually lost through evaporation, a float valve, like the one that controls water in a toilet tank, opens the distribution line and adds water to the cooler when the level gets low.
Evaporative coolers got the nickname "swamp coolers" after the wet sheets people in the Southern part of the United States used to make sleeping porches more comfortable on hot summer nights.