A refrigerant is a compound used in a heat cycle that reversibly undergoes a phase change from a gas to a liquid. Traditionally, fluorocarbons, especially chlorofluorocarbons, were used as refrigerants but they are being phased out because of their ozone depletion effects.
Other refrigerants include ammonia, sulfur dioxide, carbon dioxide and non-halogenated hydrocarbons such as methane. The ideal refrigerant has good thermodynamic properties, is unreactive chemically, and safe. The desired thermodynamic properties are a boiling point somewhat below the target temperature, a high heat of vaporization, a moderate density in liquid form, a relatively high density in gaseous form, and a high critical temperature.
Since boiling point and gas density are affected by pressure, refrigerants may be made more suitable for a particular application by choice of operating pressure. These properties are ideally met by the chlorofluorocarbons. Until concerns about depletion of the ozone layer arose in the 1980s, the most widely used refrigerants were the halomethanes R-11, R-12 and R-22.