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). The use of refrigerant has been increasingly generating concerns, since it was discovered in the 1980s that the most widely used refrigerants were the cause of ozone depletion.
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, 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.
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 relatively high 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 R410a, often marketed under the trade name Puron. Another popular blend of R-32, R-125, and R-134a with a higher critical temperature, and lower GWP than R-410a is R-407c. While the R-22 and other ozone depleting refrigerants are being phased out, they still have value and can be easily sold.
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.
Use of highly purified liquefied propane gas as a refrigerant is gaining favor, especially in systems designed for R-12, R-22 or R-134a. Moreover, propane is nontoxic. An odorant, such as ethyl mercaptan, can be added in trace amounts to alert persons of system leaks.
CFC's or chlorofluorocarbons are used as refrigerants in some commercial air conditioning and refrigeration systems. CFC's are considered to be 100% ozone depleting, meaning that they are the standard for efficiency in the catalytic breakdown of ozone. In most residential air conditioners and many refrigeration systems it is R-22 or Freon which is a hydrochlorofluorocarbon or HCFC. HCFC's are considered to be 5% ozone depleting and are less of a danger to the Earth's vital ozone layer. However, non-ozone layer depleting refrigerants are the most desirable.
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 1410—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, has a global warming potential of 1, but is toxic and potentially lethal in concentrations above 5% by volume. 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.
As of July 1, 1992 it is illegal to release refrigerants into the atmosphere (intentional or accidental) because they can cause severe damage to the ozone layer. When CFCs are removed they should be recycled to clean out any contaminants and return it to a usable condition. Refrigerants should never be mixed together. Some CFCs must be managed as hazardous waste even if recycled, and special precautions are required for their transport, depending on the legislation of the country's government.
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.
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.