A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a liquid to a gas. Traditionally, fluorocarbons, especially chlorofluorocarbons, were used as refrigerants, but they are being phased out because of their ozone depletion effects. Other common refrigerants used in various applications are ammonia, sulfur dioxide, and non-halogenated hydrocarbons such as methane.[1]
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The ideal refrigerant has favorable thermodynamic properties, is unreactive chemically, 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. These properties are ideally met by the chlorofluorocarbons.
Corrosion properties are a matter of materials compatibility with the mechanical components: compressor, piping, evaporator, and condenser. Safety 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 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 by 2020. 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 R-410A, 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 (Global Warming Potential) 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.
Following the ban on chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), substances used as substitute refrigerants such as fluorocarbons (FCs) and hydrofluorocarbons (HFCs) have also come under criticism. They are currently subject to prohibition discussions on account of their harmful effect on the climate. In 1997, FCs and HFCs were included in the Kyoto Protocol to the Framework Convention on Climate Change. In 2006, the EU adopted a Regulation on fluorinated greenhouse gases, which makes stipulations regarding the use of FCs and HFCs with the intention of reducing their emissions. The provisions do not affect climate-neutral natural refrigerants.
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 CFCs. Ammonia (R717) has been used in industrial refrigeration plants for more than 130 years and is deemed to be environment-friendly, economical, and energy-efficient. The natural refrigerant carbon dioxide (R744) has a similarly long tradition in refrigeration technology.[2]
Occasionally, one may encounter older machines which used other transitional refrigerants such as methyl formate, chloromethane, or dichloromethane (called carrene in the trade). Perhaps the most common of these to still retain a charge are the methyl formate Monitor Top refrigerators produced by General Electric.
Use of highly purified propane as a refrigerant is gaining favor, especially in systems designed for R-22. Although propane is non-toxic its mixture with air in certain proportions is explosive. An odorant, such as ethyl mercaptan, can be added in trace amounts to alert persons of system leaks.
Natural refrigerants such as ammonia, carbon dioxide and non-halogenated hydrocarbons preserve the ozone layer and have no (ammonia) or only a low (carbon dioxide, hydrocarbons) global warming potential.[3] They are used in air-conditioning systems for buildings, in sport and leisure facilities, in the chemical/pharmaceutical industry, in the automotive industry and above all in the food industry (production, storage, retailing). New applications are opening up for natural refrigerants for example in vehicle air-conditioning.
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. R12 is compatible with mineral oil, while R134a is compatible with synthetic oil.[4] GM has announced that it will start using Hydrofluoro olefin, HFO-1234yf, in all of its brands by 2013.[5] This new refrigerant has a GWP rating of 4 and is not a blend.[6] Dimethyl ether (DME) is also gaining popularity as a refrigerant.[7]
Some refrigerants, such as tetrafluoroethane, are seeing rising use as recreational drugs, leading to an extremely dangerous phenomenon known as inhalant abuse.[8]
As of July 1, 1992 it is illegal in the United States 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:
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:
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 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.
"Air cycle is not a new technology. At the turn of the century air cycle or 'cold air machines' were available from companies such as J & E Hall... These were used on board ships and by food producers and retailers to provide cooling for their food stores."[18]
Air has been used for residential,[19] automobile,[18] and turbine-powered aircraft[20][21] air-conditioning and/or cooling. The reason why air is not more widely used as a general-purpose refrigerant is the misperception that the use of air is too inefficient to be practical.[19]
Yet, with suitable compression and expansion technology, air can be a practical (albeit not the most efficient) refrigerant, free of the possibility of environmental contamination or damage,[19] and almost completely[22] harmless to plants and animals.