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.
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
The inert nature of many halons, chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC), with the
benefits of their 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. Some 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 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.[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,
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 in 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.
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#.
- 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.[citation
needed]
- 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.
- The R-700 series is made up of non-organic refrigerants, also desgnated 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 HFC- for hydrofluorocarbons,
CFC- for chlorofluorocarbons, and HCFC- for hydrochlorofluorocarbons has arisen,
because of the regulatory differences among these groups.[citation
needed]
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 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
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."
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 because there is no change of
phase, it 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.
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
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". 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 - humidified air
is simply vented to the living space. Portable free standing units can be
obtained at discount stores for less than $200US. However, drawbacks of these
systems, if improperly implemented 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, stagnent humid air will make the space more uncomfortable than if it
were merely ventilated.
Further information: Thermal
comfort
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, such
as over 110 °F or 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 freon 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.
source :
http://en.wikipedia.org/wiki/Refrigerant
source :
http://en.wikipedia.org/wiki/Refrigerant
http://en.wikipedia.org/wiki/Refrigerant
http://en.wikipedia.org/wiki/Refrigerant
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