Refrigerant

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A refrigerant is a compound used in a heat cycle that undergoes a phase change from a gas to a liquid and back. The two main uses of refrigerants are refrigerators/freezers and air conditioners. Cf. coolant.

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. New production of R-12 ceased in the United States in 1995, and R-22 is to be phased out in 2010. R-134a and certain blends are now replacing chlorinated compounds. One popular 50/50 blend of R-32 and R-125 now being increasingly substituted for R-22 is marketed under the trade name Puron® and another one as R410a. While the R-22, R-12 and other ozone depleting refrigerants are being phased out, they still have value and can be easily sold.

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 gaseous 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. Safety considerations include toxicity and flammability.

Early mechanical refrigeration systems employed sulfur dioxide gas or anhydrous ammonia, with small home refrigerators primarily using the former. Being toxic, sulfur dioxide rapidly disappeared from the market with the introduction of Freon. Ammonia is still used in some large commercial plants, well away from residential areas, where a leak will not cause widespread injuries.

Use of highly purified liquified propane gas as a refrigerant is gaining favor, especially in systems designed for R-12, R-22 or R-134a. As such, it is designated as R-290 and is marketed under the trade name Duracool®. Although propane is flammable, in home and automotive systems it is present in quantities small enough to not pose an undue fire hazard if a system should develop a leak. Moreover, propane is nontoxic. An odorant, such as ethyl mercaptan, can be added in trace amounts to alert persons of system leaks.

Emissions from automotive air-conditioning are a growing concern because of their impact on climate change. From 2011 on, the European Union will phase out refrigerants with a global warming potential (GWP) of more than 150 in automotive air conditioning (GWP = 100 year warming potential of one kilogram of a gas relative to one kilogram of CO₂). This will ban potent greenhouse gases such as the refrigerant HFC-134a – which has a GWP of 1300 – to promote safe and energy-efficient refrigerants. One of the most promising alternatives is the natural refrigerant CO₂ (R-744). Carbon dioxide is non-flammable, non-ozone depleting, non-toxic and has a global warming potential of 1. R-744 can be used as a working fluid in climate control systems for cars, residential air conditioning, hot water pumps, commercial refrigeration, and vending machines.

[edit] 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 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, due to the regulatory differences among these groups.