Assembly of equipment for air treatment consisting of a means for ventilation, air circulation, air cleaning, and heat transfer (either heating or cooling). The unit usually consists of an evaporator or cooling coil, and an electrically driven compressor and condenser combination.

    Control of temperature, humidity, purity, and motion of air in an enclosed space, independent of outside conditions. In a self-contained air-conditioning unit, air is heated in a boiler unit or cooled by being blown across a refrigerant-filled coil and then distributed to a controlled indoor environment.

    Central air-conditioning in a large building generally consists of a main plant located on the roof or mechanical floor and intermittently spaced air-handling units, or fans that deliver air (supply air) through ducts to zones within the building.

   The air then returns (return air) to the central air-conditioning machinery through spaces called plenums to be re cooled (or reheated) and re circulated. Alternate systems of cooling use chilled water, with water cooled by a refrigerant at a central location and circulated by pumps to units with fans that circulate air locally.

    There are three principal types of central air-conditioning systems: all-air, all-water, and air-processed in a central air-handling apparatus. In one type of all-air system, called dual-duct, warm air and chilled air are supplied to a blending or mixing unit in each space. In a single-duct all-air system, air is supplied at a temperature for the space requiring the coldest air, and then reheated by steam or electric or hot-water coils in each space.