A refrigerant is a compound used in a heat cycle that undergoes a phase change from a gas to a liquid and back.

The ideal refrigerant has good thermodynamic properties, is noncorrosive, 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, and a relatively high density in gasous form. 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.

Corrosion properties are a matter of materials compatibility with the components used for the compressor, piping, evaporator, and condenser. Saftey considerations include toxicity and flammability.

Until concerns about depletion of the ozone layer arose in the 1980s, the most widely used refrigerants were the halomethanes R-12 and R-22, with R-12 being more common in automotive air conditioning and small refrigerators, and R-22 being used for residential and light commercial air conditioning, refrigerators, and freezers. Some very early systems used R-11 because its low boiling point allows low-pressure systems to be constructed, reducing the mechanical strength required for components. R-134a and certain blends are now replacing chlorinated compounds.

Numbering

The R-# numbering system was developed by DuPont and systematically identifies the molecular structure of refrigerants made with a single halogenated hydrocarbon. The meaning of the codes is as follows:

• Rightmost digit - Number of fluorine atoms per molecule.
• Tens digit - One plus the number of hydrogen atoms per molecule.
• Hundreds digit - The number of carbon atoms minus one. Omitted for methyl halides, which have only one carbon atom.
• Thousands digit - Number of double bonds in the molecule. This is omitted when zero, and in practice is rarely used, since most candidate compounds are unstable.
• A suffix with a capital B and a number indicates the number of bromine atoms, when present. This is rarely used.
• Remaining bonds not accounted for are occupied by chlorine atoms.
• A suffix of a lower-case letter a, b, or c indicates increasingly unbalanced isomers.
• As a special case, the R-400 series is made up of zeotropic blends (those where the boiling point of constituent compounds differs enough to lead to changes in relative concentration due to fractional distillation) and the R-500 series is made up of so-called azeotropic blends. The rightmost digit is assigned arbitrarily by ASHRAE, an industry organization.

For example, R-134a has 4 fluorine atoms, 2 hydrogen atoms, 2 carbon atoms, with an emperical formula of tetrafluoroethane. The "a" suffix indicates that the isomer is unbalanced by one atom, giving 1,1,1,2-tetrafluoroethane. R-134 without the "a" suffix would have a molecular structure of 1,1,2,2-tetrafluoroethane -- a compound not especially effective as a refrigerant.

The same numbers are used with an R- prefix for generic refrigerants, with a "Propellant" prefix (e.g. "Propellant 12") for the same chemical used as a propellant for an aerosol spray, and with trade names for the compounds, such as "Freon 12." Recently, a practice of using HFC- for hydrofluorocarbons , , CFC- for chlorofluorocarbons, and HCFC- for hydrocholorfluorocarbins has arisen, due to the regulatory differences among these groups.