Mechanical refrigeration, often referred to simply as refrigeration, is a process by which heat is removed from a location using a man-made heat-exchange system. The system of refrigeration can be cyclic, non-cyclic,thermoelectric or magnetic depending on the application for which refrigeration is needed.
In a data center, computer room air conditioner (CRAC) is the most common use of mechanical refrigeration. It typically involves cyclic heat absorption using a fluid refrigerant and a compressor system. Common air conditioners use an electric motor to operate a compressor and also drive air flow over heat sink coils. The actual cooling principle is based on the phase change of the refrigerant. The compressor causes pressure changes between two segments of a cooling loop maintained at different pressures. When the refrigerant enters the low pressure cooling coil, the refrigerant changes state into a vapor and absorbs heat. The compressor then moves the vapor into a higher pressure condenser where the refrigerant is condensed back into a liquid, liberating its heat into a heat exchange coil before being re-circulated back into the low pressure evaporator coil.
In the early days of mechanical refrigeration, flammable chemicals such as ammonia were used as refrigerant. This proved hazardous and the creation of Freon was thought to be the perfect solution until the discovery of chlorofluorocarbons' (CFCs) harmful effects on the environment. Since then, alternative refrigerants such as Puron have been developed for commercial use.