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.
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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.
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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.
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. Dimethyl ether (DME) is also gaining popularity as a refrigerant, 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.
Disposal
Under Section 608 of the United States' Clean Air Act it is illegal to knowingly release refrigerants into the atmosphere.
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.
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:
- 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:
- For saturated hydrocarbons, subtracting 90 from the concatenated numbers of carbon, hydrogen and fluorine atoms, respectively gives the assigned R#.
- If bromine is present, the number is followed by a capital B and then the number of bromine atoms.
- 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.
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 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 standards organization.
- The R-700 series is made up of non-organic refrigerants, also designated by ASHRAE.
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
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 temperature 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 molecular 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 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, 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 because there is no change of phase, and is therefore too inefficient to be practical in most applications. 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, and almost completely 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
- Brine (refrigerant)
- Eurammon - initiative for natural refrigerants
- Industrial gas
- List of refrigerants
- Refrigerant reclamation
References
External links
- US Environmental Protection Agency page on the GWPs of various substances
- Green Cooling Initiative on alternative natural refrigerants cooling technologies
- International Institute of Refrigeration
Source of the article : Wikipedia
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