An evaporative air cooler or washed air cooler is a device that cools air by evaporating water. Evaporative cooling is different from other air conditioning systems that use vapor compression or absorption refrigeration cycles.

The working principle of evaporative cooling is based on using the high enthalpy of vaporization of water. The temperature of dry air can be significantly reduced by the phase transition from liquid water to water vapor, which requires much lower energy consumption than the energy consumption of units that operate by refrigeration. In extremely dry climates, it also has the additional benefit of conditioning the air by adding humidity for greater comfort of the home's occupants. Unlike refrigeration, devices that work based on evaporation require a source of water and consume water continuously for their operation.[1]​.

Washed air chillers and wet cooling towers use the same evaporative cooling operating principles, but are optimized for purposes other than cooling air within a building. For example, an evaporative cooler can be designed to cool coils and pipes of a large air conditioning system to increase its efficiency.

Evaporative cooling is especially convenient for climates in which the air is hot and humidity is low. In the United States, the western and mountain states are good locations, and so evaporative coolers are used in cities such as Denver, Salt Lake City, Albuquerque, El Paso (Texas), Tucson, and Fresno (California), where there is adequate water. Evaporative air conditioning is also popular and appropriate in the southern part of Australia. In dry, arid climates, the installation and operation costs of an evaporative cooler can be much less than the costs of a refrigeration cycle air conditioner, often on the order of 80% less. However, evaporative cooling and vapor compression air conditioning systems are sometimes used in combination to obtain optimal results. Some evaporative coolers can also be used as humidifiers in the cold season.

In addition to its use in areas with dry climates, in locations with moderate levels of humidity there are many economically advantageous uses for evaporative cooling. For example, in industrial plants, commercial kitchens, laundry rooms, dry-cleaning laundry rooms, greenhouses, specific site cooling (loading areas, warehouses, factories, construction sites, athletic events, workshops, garages), and indoor agricultural practice areas (poultry, pig, and dairy farms), systems based on evaporative cooling are often used. Finally, in climates with high humidity, the benefits of using cooling by evaporation are very limited. evaporation.[2]​.