Refrigerant

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A refrigerant is a substance or mixture, usually a fluid, used in a heat pump and refrigeration cycle. In most cycles it undergoes phase transitions from a liquid to a gas and back again. Many working fluids have been used for such purposes. Fluorocarbons, especially chlorofluorocarbons, became commonplace in the 20th century, 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.[1]

The ideal refrigerant would have favorable thermodynamic properties, be noncorrosive to mechanical components, and be safe, including free from toxicity and flammability. It would not cause ozone depletion or climate change. Since different fluids have the desired traits in different degree, choice is a matter of trade-off.

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.

Refrigerant environmental issues[edit]

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 (as measured by carbon dioxide equivalent) 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 were developed in the early 21st century that are safe to humans and to the environment, but their application has been held up by regulatory hurdles due to concerns over toxicity and flammability.[2]

History[edit]

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. Occasionally, one may encounter older machines with methyl formate, chloromethane, or dichloromethane (called carrene in the trade).

Chlorofluorocarbons were little used for refrigeration until better synthesis methods, developed in the 1950s, reduced their cost. Their domination of the market was called into question in the 1980s by concerns about depletion of the ozone layer.

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.

Uses[edit]

Refrigerants such as ammonia (R717), carbon dioxide (R744) and non-halogenated hydrocarbons preserve the ozone layer and have no (ammonia) or only a low (carbon dioxide, hydrocarbons) global warming potential.[citation needed] 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). In these settings their toxicity is less a concern than in home equipment.

Emissions from automobile air conditioning are a growing concern because of their impact on climate change.[citation needed] 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).[citation needed] 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. 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.[citation needed] R12 is compatible with mineral oil, while R134a is compatible with synthetic oil that contains esters.[citation needed] GM has announced that it will start using "hydrofluoroolefin", HFO-1234yf, in all of its brands by 2013.[3] Dimethyl ether (DME) is also gaining popularity as a refrigerant,[4] but like propane, it is also dangerously flammable.

Some refrigerants are seeing rising use as recreational drugs, leading to an extremely dangerous phenomenon known as inhalant abuse.[5]

Disposal[edit]

Under Section 608 of the EPA Clean Air Act it is illegal in the United States to knowingly release HFC-134a refrigerants into the atmosphere.[EPA 1]

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.

Various refrigerant reclamation methods are in use to recover refrigerants for reuse.[6]

Refrigerants by class and R-number[edit]

Main article: List of refrigerants

Refrigerants may be divided into three classes according to their manner of absorption or extraction of heat from the substances to be refrigerated:[citation needed]

  • 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:[citation needed]

  • Subtracting 90 from the concatenated numbers of carbon, hydrogen and fluorine atoms, respectively gives the assigned R#.[7]
  • 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.[citation needed]

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.[citation needed]

Notable blends[edit]

R407C pressure-enthalpy diagram, isotherms between the two saturation lines

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.[8]
  • 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.[9]
  • 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.[10]
  • 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.[11]
  • 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.[12]
  • 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.[13]
  • 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.[14] It appears to have gained widespread market acceptance under several trade names.[15]
  • 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).[16][17]
  • 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[edit]

"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 perception that because there is no change of phase, it 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.

An explosion could result from refrigerant-type compressor lubricating oils being compressed together with the air.

Air is so frequently used as a coolant that air cooling is seldom mentioned in this context. Due to the low boiling point of its constituents, air is less often used as a refrigerant.

Water as a refrigerant[edit]

Water - non toxic, low cost, and widely available, is widely used in water cooling, and if evaporated in the process may be called a "refrigerant".

The simplest open-cycle cooling systems, known as swamp coolers in the south-west United States, do not even need power for a compressor, merely a blower fan - humidified air is simply vented to the living space. 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 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.

See also[edit]

References[edit]

  1. ^ Siegfried Haaf, Helmut Henrici "Refrigeration Technology" in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, doi:10.1002/14356007.b03 19
  2. ^ Rosenthal, Elisabeth; Lehren, Andrew (June 20, 2011). "Relief in Every Window, but Global Worry Too". New York Times. Retrieved June 21, 2012. 
  3. ^ GM First to Market Greenhouse Gas-Friendly Air Conditioning Refrigerant in U.S.
  4. ^ http://www.mecanica.pub.ro/frigo-eco/R404A_DME.pdf 101110
  5. ^ 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>.
  6. ^ Guide to Effective Refrigerant Recovery Ritchie Engineering Company
  7. ^ http://www.epa.gov/ozone/geninfo/numbers.html#nonhalons
  8. ^ HCFC - R401A
  9. ^ Cameo Chemicals Refrigerant gas R-404A
  10. ^ Cameo Chemicals Refrigerant gas R-407A
  11. ^ [dead link]
  12. ^ Mixed refrigerants, R-408A page
  13. ^ Mixed refrigerants, R-409A page
  14. ^ http://www.epa.gov/Ozone/snap/refrigerants/lists/homeac.html
  15. ^ Web search
  16. ^ [1]
  17. ^ MO99
  18. ^ a b The Air Cycle Machine compressor technology.
  19. ^ a b c Air as a refrigerant in air conditioning systems in buildings.
  20. ^ Air cycle machine.
  21. ^ Turboexpander.
  22. ^ Current air cooling methods release a trivial amount of oil or other lubricant into the atmosphere.
  1. ^ "Frequently Asked Questions on Section 608". Environment Protection Agency. Retrieved 20 December 2013. 

External links[edit]