Air Conditioning is a system used for controlling the internal climate of a building. This may be necessary for some industrial processes (as in cotton mills), for laboratory work, or to maintain comfortable conditions in very hot or cold climates. An extreme instance is afforded by the pressurised cabin of the modern airliner.
Essentially an air-conditioning system is a forced ventilation system in which, instead of air being drawn in from outside the building, the inside air is recirculated, filtered, washed, heated or cooled, and its humidity adjusted. In cold weather this system results in a considerable economy in heat, especially if the shell of the building is designed to give a high level of insulation; double glazing may be installed, for example, as windows will not need to be opened to admit fresh air.
Air conditioning is widely adopted in many countries. Cooling systems, incorporating refrigeration plant, are much more costly, but this is proving to be a very worthwhile investment as factory and office personnel are more productive when working in comfortable conditions.