Air-conditioning, in simple terms, is the control of temperature, humidity, purity and motion of air in an enclosed space independent of outside conditions. The mechanism combines either the heating of air in a boiler unit or the cooling of air by being blown down across a refrigerant-filled coil located in a self-contained air-conditioning unit. The next step follows with the distribution of the air to a controlled indoor environment. The central air-conditioning in a large building or setup comprises of a main plant that is usually located on the roof of the building or setup. On the other hand, spaced air-conditioning units do the job of delivering air through ducts to zones within the building. Plenums are the spaces that return the air to the central air conditioning machine for cooling (heating) purpose as well as for recirculation purpose. There are alternative system of cooling that uses chilled water in a central location and an eventual distribution or circulation to different units with the help of motors and pumps.
