Swamp coolers, or evaporative coolers, are a type of refrigeration system that uses evaporation to cool air, differing from traditional air conditioning which uses a phase change heat pump with a compressor to cool air. Swamp coolers are designed especially for areas that are low-humidity but where the air is hot. Operating a swamp cooler in these areas can be more cost effective than traditional air conditioning.

To cool the air, the fan in the swamp cooler brings air from outside through vents in the sides through damp pads. The hot air evaporates the air in the pads which then humidifies the air. The pads are constantly dampened to continue the cooling process. Once cooled, the moistened air is blown into the building via a fan through a vent in the roof or wall. Air will reach saturation, meaning that it cannot hold any more water, so the vent must be on and functioning well to pump air into the building bring in more outside air in order to produce maximum efficiency.

Direct Evaporative (Open Circuit) Cooling is used to lower the temperature of air by using latent heat of evaporation. This is the most generic swamp cooler and is what is usually found on residential homes. In this process, as the water is changed to vapor, warm dry air is changed to cool moist air. The energy in the air does not change. The heat in the air is used to evaporate the water.

Indirect Evaporative (Closed Circuit) Cooling is basically the same thing as direct evaporative cooling, but the air is used to cool a heat exchanger which then cools the environment. The cooled moist air never comes in direct contact with the conditioned environment.

Two-stage evaporative coolers do not produce humidity levels as high as that produced by traditional swamp coolers. Swamp coolers use a lot less energy of traditional air conditioners. However, they are not ideal for humid climates because the increased air humidity may cause the occupants to be uncomfortable. In the first stage, air passes through a heat exchanger that is cooled from the outside by humidity. This way, the air does not yet pick up any humidity. In the second stage, the air passes through water-soaked pads where it picks up humidity. Because the air is already cooled, it picks up less humidity than warmer air.

Cooler pads are traditionally made of excelsior (aspen wood wool fiber) which is put in a containment net. Lately, more modern materials, such as plastics and melamine paper are being used in cooler pads. Wood absorbs water, so it is more efficient at cooling passing air than some synthetic materials. The thickness of the pads plays a big part in the cooling efficiency. Thicker cooling pads allow for longer air contact.