Air conditioning is the process of removing heat and controlling the humidity of air in an enclosed space to achieve a more comfortable interior environment by use of powered “air conditioners” or a variety of other methods, including passive cooling and ventilative cooling. Air conditioning operates based on the principles of phase conversion, which is the transformation of a material from one state of matter to another, such as when a material changes from a liquid to a gas. When a liquid to gas change occurs, the material absorbs heat.
Refrigerant is used in an air conditioning system to transfer heat energy. A refrigerant absorbs heat from a room and rejects it outside the house. Cooling in traditional AC systems is accomplished using the vapor-compression cycle, which uses the forced circulation and phase change of a refrigerant between gas and liquid to transfer heat.
The vapor-compression cycle can occur within a unitary, or packaged piece of equipment or within a chiller that is connected to terminal cooling equipment such as a fan coil unit in an air handler on its evaporator side and heat rejection equipment such as a cooling tower on its condenser side.
An air source heat pump shares many components with an air conditioning system, but includes a reversing valve which allows the unit to be used to heat as well as cool a space.Air conditioning equipment will reduce the absolute humidity of the air processed by the system if the surface of the evaporator coil is significantly cooler than the dew point of the surrounding air./wikipedia