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A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase transition 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 propane. Many refrigerants are important ozone depleting and global warming inducing compounds that are the focus of worldwide regulatory scrutiny.
The ideal refrigerant has favorable thermodynamic properties, is noncorrosive to mechanical components, and is safe (including nontoxic, nonflammable, and environmentally benign). 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 pressures. The inert nature of many Halons, Chlorofluorocarbons (CFC) and Hydrochlorofluorocarbons (HCFC), with the benefits of them being nonflammable and nontoxic, made them good choices as refrigerants, but their stability in the atmosphere and their corresponding global warming potential and ozone depletion potential raised concerns about their usage. In order from the highest to the lowest potential of ozone depletion are Bromochlorofluorocarbon, CFC then HCFC.
Though HFC and PFC are non-ozone depleting, many have global warming potentials that are thousands of times greater than CO2. Other refrigerants such as propane and ammonia are not inert, and are flammable or toxic if released. New refrigerants have been developed that are safe to humans and to the environment, but their application has been held up by regulatory hurdles.
Early mechanical refrigeration systems employed sulfur dioxide, methyl chloride and ammonia. Being toxic, sulfur dioxide and methyl chloride rapidly disappeared from the market with the introduction of CFCs.
Following legislative regulations on ozone depleting chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), substances used as substitute refrigerants such as perfluorocarbons (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 refrigerants.
Ammonia (R717) has been used in industrial refrigeration plants for more than 130 years and is thought to be environment-friendly, economical, and energy-efficient. Carbon dioxide (R744) has a similarly long tradition in refrigeration technology.
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. 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 non-halogenated 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 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 that contains esters. GM has announced that it will start using Hydrofluoroolefin, HFO-1234yf, in all of its brands by 2013. Dimethyl ether (DME) is also gaining popularity as a refrigerant, but like propane, it is also dangerously flammable.
As of July 1, 1992 it is illegal in the United States to release refrigerants into the atmosphere (intentional or accidental). When refrigerants are removed they should be recycled to clean out any contaminants and return them to a usable condition. Refrigerants should never be mixed together outside of facilities licensed to do so for the purpose of producing blends. Some refrigerants 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 by class and by R-number
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 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.
Below are some notable blended HFC mixtures. There exist many more (see list of refrigerants). All R-400 (R-4xx) and R-500 (R-5xx) hydroflurocarbons are blends, as noted above.
- R-401A is a HCFC zeotropic blend of R-22, 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-438A another HFC blended replacement for R-22, with five components: R-32, R-125/R-134a, R-600, and R-601a, blended in respective ratios 8.5+.5,-1.5%; 45±1.5%; 44.2±1.5%; 1.7+.1,-.2%; 0.6+.1,-.2%. The mean ‘’mo’’lecular weight of this mix is 99, resulting in the tradename ISCEON MO99 from manufacturer DuPont (a line of blended HFC products developed initially by Rhodia, and sold to DuPont).
- 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.
Air as a refrigerant
Air has been used for residential, automobile, and turbine-powered aircraft air-conditioning and/or cooling. The reason why air is not more widely used as a general-purpose refrigerant is the perception that the use of air is too inefficient to be practical.
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, and almost completely harmless to plants and animals. An explosion could result from refrigerant type compressor lubricating oils being compressed together with the air.
Water or air as a refrigerant
The simplest refrigerant is water. Non toxic, low cost, and widely available. The simplest cooling systems, known as swamp coolers in the south-west United States, do not even need power for a compressor, merely a blower fan - evaporated water is simply vented to the living space, where it serves to increase humidity also. However, drawbacks are multiple and severe as well. The total cooling power of the unit is limited by the fact that neither coolant nor air is recirculated. A swamp cooled home will have a constant supply of fresh, not too-dry air, but if the air outside is already humid, cooling power is severely limited. This is why such units are not found in areas of frequent and high humidity, such as the south-east United States. Furthermore, if the temperature outside is severely too hot, such as over 110 degrees F or 43 °C, the unit will not cool the air sufficiently for comfort even if the dew point outside is very low.
- Siegfried Haaf, Helmut Henrici "Refrigeration Technology" in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, doi:10.1002/14356007.b03 19
- Rosenthal, Elisabeth; Lehren, Andrew (June 20, 2011). "Relief in Every Window, but Global Worry Too". New York Times. Retrieved June 21, 2012.
- GM First to Market Greenhouse Gas-Friendly Air Conditioning Refrigerant in U.S.
- http://www.mecanica.pub.ro/frigo-eco/R404A_DME.pdf 101110
- Harris, Catharine. "Anti-inhalant Abuse Campaign Targets Building Codes: ‘Huffing’ of Air Conditioning Refrigerant a Dangerous Risk." The Nation's Health. American Public Health Association, 2010. Web. 05 Dec. 2010. <http://thenationshealth.aphapublications.org/content/39/4/20.extract>.
- HCFC - R401A
- http://cameochemicals.noaa.gov/chemical/26023 Refrigerant gas R-404A
- http://cameochemicals.noaa.gov/chemical/26024 Refrigerant gas R-407A
- Mixed refrigerants, R-408A page
- Mixed refrigerants, R-409A page
- Web search
- The Air Cycle Machine compressor technology.
- Air as a refrigerant in air conditioning systems in buildings.
- Air cycle machine.
- Current air cooling methods release a trivial amount of oil or other lubricant into the atmosphere.