Refrigerant

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]

Desirable properties

The ideal refrigerant would have favorable thermodynamic properties, be noncorrosive to mechanical components, and be safe, including freedom 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-offs.

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 appropriate choice of operating pressures.

Environmental issues

The inert nature of many halons, chlorofluorocarbons (CFC), and hydrochlorofluorocarbons (HCFC), particularly CFC-11 and CFC-12, made them preferred choices among refrigerants for many years because of their nonflammability and nontoxicity. However, their stability in the atmosphere and their corresponding global warming potential and ozone depletion potential raised concerns about their usage. This led to their replacement with HFCs and PFCs, especially HFC-134a, which are not-ozone depleting, and have lesser global warming potentials. However, these refrigerants still have global warming potentials thousands of times greater than CO2. Therefore, they are now being replaced in markets where leaks are likely, by using a third generation of refrigerants, most prominently HFO-1234yf, which have global warming potentials much closer to that of CO2.

Some other refrigerants such as propane and ammonia are not inert, and are flammable or toxic if released.

In order from the highest to the lowest potential of ozone depletion are: Bromochlorofluorocarbon, CFC then HCFC.

New refrigerants were developed in the early 21st century that are safer for the environment, but their application has been held up due to concerns over toxicity and flammability.[2]

History

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

Refrigerants such as ammonia (R717), carbon dioxide and non-halogenated hydrocarbons do not deplete 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, marine shipping, 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. 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 (also known as R-134a in North America) —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. R12 is compatible with mineral oil, while R134a is compatible with synthetic oil that contains esters. GM announced that it would 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

Under Section 608 of the United States' Clean Air Act it is illegal to knowingly release 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

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 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:

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 abbreviations HFC- for hydrofluorocarbons, CFC- for chlorofluorocarbons, and HCFC- for hydrochlorofluorocarbons has arisen, because of the regulatory differences among these groups.

Notable blends

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.

Air as a refrigerant

Air is so frequently used as a coolant that air cooling is seldom mentioned in a refrigeration context. Due to the low boiling point of its constituents and low heat-carrying capacity, air is infrequently used as a refrigerant.

Air has been used for residential,[18] automobile, and turbine-powered aircraft[19][20] air-conditioning and/or cooling. The reason why air is not more widely used as a general-purpose refrigerant is because there is no change of phase, and is therefore too inefficient to be practical in most applications.[18] It has been suggested that 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,[18] and almost completely[21] harmless to plants and animals.

However, an explosion could result from vapors or atomized refrigerant-type compressor lubricating oils being compressed together with air, in a process similar to a diesel engine.

Water as a refrigerant

Water—natural, non toxic, low cost, environmentally friendly, and widely available—is widely used in water cooling, and if evaporated in the process may be called a "refrigerant". Water also commonly serves as a heat transfer and storage material ,and in large systems it may actually fill all of these roles.

The simplest and lowest cost 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, so humidified cooled air is simply vented into the living space. Portable free-standing units can be obtained at discount stores for less than $200. However, if these systems are improperly implemented the drawbacks are multiple and severe.

The total cooling power of the unit is limited by the fact that neither coolant nor air can be recirculated. If the cooling unit does not have a supply of fresh dry air and the waste air is not effectively vented, stagnant humid air will make the space more uncomfortable than if it were merely ventilated.

An additional limitation of such systems would be that 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.

If the temperature outside is severely hot, above 110 °F (43 °C), the simple unit will not cool the air sufficiently for comfort even if the dew point outside is very low. In these instances more-complex systems such as two stage, indirect-direct or hybrid will be needed.

While all the drawbacks can be addressed in various ways, the complexity and initial cost of these systems increases to the point that the installation cost comes into competition with common refrigerant based direct cooling systems. At this price point, direct cooling systems are often chosen even though the long term energy cost of evaporative systems may be lower.

See also

References

  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. ↑ 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. ↑ "Numbering Scheme for Ozone-Depleting Substances and their Substitutes". Retrieved 25 December 2015.
  8. ↑ HCFC - R401A
  9. ↑ Cameo Chemicals Refrigerant gas R-404A
  10. ↑ Cameo Chemicals Refrigerant gas R-407A
  11. ↑ "Comparison of R407C and R417A heat transfer coefficients and pressure drops during flow boiling in a horizontal smooth tube". ResearchGate. Retrieved 2015-09-09.
  12. ↑ Mixed refrigerants, R-408A page
  13. ↑ Mixed refrigerants, R-409A page
  14. ↑ "Significant New Alternatives Policy (SNAP) Program". Archived from the original on 1 June 2002. Retrieved 25 December 2015.
  15. ↑ Web search
  16. ↑
  17. ↑ "This page has moved.". Retrieved 25 December 2015.
  18. 1 2 3 Air as a refrigerant in air conditioning systems in buildings.
  19. ↑ Air cycle machine.
  20. ↑ Turboexpander.
  21. ↑ 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.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.