Air conditioning refrigerants may be divided into three classes according to their manner of absorption or extraction of heat from the substances to be refrigerated:
- Class 1: This class includes refrigerants that cool by phase change (typically boiling), using the refrigerant’s latent heat.
- Class 2: These refrigerants cool by temperature change or ‘sensible heat’, the quantity of heat being the specific heat capacity x the temperature change. They are air, calcium chloride brine, sodium chloride brine, alcohol, and similar nonfreezing solutions. The purpose of Class 2 refrigerants is to receive a reduction of temperature from Class 1 refrigerants and convey this lower temperature to the area to be air-conditioned.
- Class 3: This group consists of solutions that contain absorbed vapors of liquefiable agents or refrigerating media. These solutions function by nature of their ability to carry liquefiable vapors, which produce a cooling effect by the absorption of their heat of solution. They can also be classified into many categories.
The R-# numbering system was developed by DuPont corporation (which owns the Freon trademark) and systematically identifies the molecular structure of refrigerants made with a single halogenated hydrocarbon. The meaning of the codes is as follows:
- Adding 90 to the number gives three digits which stands for the number of carbon, hydrogen and fluorine atoms, respectively.
- Remaining bonds not accounted for are occupied by chlorine atoms.
- A suffix of a lower-case letter a, b, or c indicates increasingly unsymmetrical 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 2 carbon atoms, 2 hydrogen atoms, and 4 fluorine atoms, 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 air conditioning 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.
- R-401A is a HCFC zeotropic blend of R-32, R-152a, and R-124. It is designed as a replacement for R-12.
- R-404A is a HFC “nearly azeotropic” blend of 52 wt.% R-143a, 44 wt.% R-125, and 4 wt.% R-134a. It is designed as a replacement of R-22 and R-502 CFC. Its boiling point at normal pressure is -46.5 °C, its liquid density is 0.485 g/cm3.
- R-406A is a zeotropic blend of 55 wt.% R-22, 4 wt.% R-600a, and 41 wt.% R-142b.
- R-407A is a HFC zeotropic blend of 20 wt.% R-32, 40 wt.% R-125, and 40 wt.% R-134a.
- R-407C is a zeotropic hydrofluorocarbon blend of R-32, R-125, and R-134a. The R-32 serves to provide the heat capacity, R-125 decreases flammability, R-134a reduces pressure.
- R-408A is a zeotropic HCFC blend of R-22, R-125, and R-143a. It is a substitute for R-502. Its boiling point is -44.4 °C.
- R-409A is a zeotropic HCFC blend of R-22, R-124, and R-142b. Its boiling point is -35.3 °C. Its critical temperatiure is 109.4 °C.
- R-410A is a near-azeotropic blend of R-32 and R-125. The US Environmental Protection Agency recognizes it as an acceptable substitute for R-22 in household and light commercial air conditioning systems. It appears to have gained widespread market acceptance under several trade names.
- R-500 is an azeotropic blend of 73.8 wt.% R-12 and 26.2 wt.% of R-152a.
- R-502 is an azeotropic blend of R-22 and R-115.