Refrigeration

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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 because of 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 empirical 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 hydrochlorofluorocarbons has arisen, because of the regulatory differences among these groups.