01-01-2013, 04:28 PM
A Seminar Report On Different refrigerants used in refrigeration
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ABSTRACT
The life blood of refrigerating system is the refrigerant circulating inside the system .It
carries heat from the refrigerated space or body to the outside surrounding .Refrigerant is
nothing but the substance that absorbs heat either by expansion or vaporisation and
rejects it through condensation in the condenser .This topic includes history of
refrigerants, classification of refrigerants, thermodynamic and chemical properties of
refrigerants .
In 1869, changes were made in the system to enable use of ammonia as refrigerant.
General Motors formed a new company Du Pont for this task, which introduced the first
popular refrigerant Freon-12.
We are aware that there is layer of ozone (O3) gas in stratosphere upto 50 km from
earth’s surface called ozonosphere all around the earth, but due to use of CFC
(chloroflurocarbon) as refrigerants it affects the ozone layer which called Ozone
depletion and it causes Ozone hole. The another phenomenon which is causing concern
is global warming.
This topic also includes what are the alternative refrigerants of CFC’s and what are its
effect on human being or environment. What will be the future of refrigerants. Natural
refrigerants are very useful in future. In 1974, it was noticed that chlorofluorocarbons
(CFCs) like R-11, R-12 used have ozone depletion potential (ODP). After few
preliminary discussion, in 1987, an agreement was signed by representatives of many
countries in Montreal, Canada. This Montreal Protocol has set timetable for phasing out
of CFC gases. The amended Montreal Protocol signed by India along with 165 other
countries plans phasing out of both CFCs and HCFCs. India does not have option of
HCFC as short term alternative to CFCs.
Introduction
At the end of the last millennium, there were many “Top Ten” lists made, including
one listing the greatest invention of the twentieth century. Along with space flight and
computers, refrigeration made the top ten because without refrigeration, food
preservation would not be possible.
Webster’s dictionary defines refrigerants as “a substance used in a refrigerating cycle
or directly such as ice for cooling”. The refrigerant is heat carrying medium which during
their cycle (i.e. compression, condensation, expansion and evaporation) in the
refrigeration system absorb heat from a low temperature system and discard the heat so
absorbed to a higher temperature system. One of the most important applications of
refrigerants used in refrigeration has been the preservation of perishable food products by
storing them at low temperature. In olden days refrigeration was achieved by natural
means such as the use of ice or evaporative cooling (like Carbon Dioxide and Ammonia).
Refrigeration as it is known these days is produced by artificial means. Though it is very
difficult to make a clear demarcation between natural and artificial refrigeration, it is
generally agreed that history of artificial refrigeration began in the year 1755, when the
Scottish professor William Cullen made the first refrigerating machine, which could
produce a small quantity of ice in the laboratory. Flammable substances such as propane
and isobutane are also used as refrigerants. To this group, CFCs, HCFCs and HFCs can
be added. ASHRAE(1) Standard 34, Designation and Safety Classification of
Refrigerants, lists over 100 refrigerants, although many of these are not used on a regular
commercial HVAC basis.
Refrigerant Properties
Physical properties
Stability and Inertness
An ideal refrigerant should not form higher boiling point liquids. Some refrigerants
disintegrate forming non-condensable gases which causes high condensing pressure and
vapour lock. It occurs due to reaction with metals. In order avoid this, refrigerant should
be inert. Refrigerants must be stable otherwise it decomposes and forms poisonous and
corrosive products.
Viscosity
The refrigerant in the liquid and vapour states should have low viscosity because
pressure drops in passing through liquid and suction lines are small. The heat transfer
through condenser and evaporator is improved at low viscosity.
Thermal conductivity.
The refrigerant in the liquid and vapour states should have high thermal conductivity
which required in finding heat transfer coefficients in evaporators and condensers.
Corrosive property
The corrosive property of a refrigerant must be taken into consideration while selecting
refrigerant. The Freon group of refrigerants are non corrosive with practically all metals.
Ammonia is only used with iron or steel. Sulphur dioxide is non-corrrosive to all metals
in the absence of water because sulphur dioxide reacts with water and forms sulphuric
acid.
Chemical property
Toxicity
Toxicity of a substance is a relative matter. Almost anything can be toxic in some
dosage. Some non-toxic refrigerants (all fluorocarbon refrigerants) when mixed with
certain percentage of air become toxic. From the consideration of comparative hazard to
life from gases and vapours, compounds have been divided into six groups by
Underwriters Laboratories (4). Group six contains compounds with a very low degree of
toxicity. It includes CFCs. Group one, at the other end of the scale, includes the most
toxic substances such as sulphur dioxide, R-717 (Ammonia). These refrigerants are also
strong irritants. Therefore these refrigerants are not used in domestic refrigeration and
comfort air conditioning. The use of toxic refrigerants is only limited to cold storages.
Flammability
Hydrocarbons, such as methane, ethane, propane, and butane are highly explosive and
flammable. Ammonia is also explosive in mixture with air in concentration of 16 to 25
percent by volume of ammonia. The halo-carbon refrigerants are neither flammable nor
explosive. Some of HFCs which are being considered as alternative to CFCs because of
the ozone layer depletion problem.
Solubility of water
Water is only slightly soluble in R-12. At -18○C, it will hold six parts per million by
weight. The solution formed is very slightly corrosive to any of the common metals. The
solubility of water with R-22 is more than R-12 by a ratio 3 to 1. If more water is present
than can be dissolved by the refrigerant, the ice will be formed which chokes the
expansion valve or capillary tube used in throttling system. This may be avoided by
proper dehydration of refrigerating unit before charging and by use silica gel drier of
liquid line. Ammonia is highly soluble in water. Due to this reason, a wetted cloth is put
at the point of leak to avoid harm to the persons working in ammonia refrigerating plants.
Ozone Loss Effect
Ozone exists in two major layers of the atmosphere (5), see Figure 3. The layer closest
to the earth's surface is the troposphere. The natural background levels of ozone in the
troposphere play a beneficial role to the ecosystem. Ozone aids in the oxidation of many
pollutants, such as carbon monoxide (CO) and methane (CH4) and plays a key role in
ecosystem cleansing. The downside of tropospheric ozone, often called "bad ozone", is
the damage it can do when it comes into direct contact with living organisms. It is
harmful to breathe, damages vegetation, and is a main ingredient in the creation of urban
smog. The troposphere generally extends from the earth's surface to a height of about 9.6
km, where it meets the second layer, the stratosphere. The stratosphere or "good ozone"
layer extends from about 9.6 km up to 48 km and protects life on earth from the sun's
harmful ultraviolet (UV) rays. Stratospheric ozone has been diminished by CFCs and
other refrigerants-propellants released into the atmosphere. These compounds are broken
down by the sun's UV rays and release chlorine and bromine molecules that destroy the
"good ozone". Scientists estimate that one chlorine atom can destroy 100,000 ozone
molecules over its life in the stratosphere. With less ozone in the stratosphere, more UV
rays hit earth, warming it up and increasing the risk of skin cancer. The protective ozone
layer extends from 8 km (upper troposphere) up throughout the whole stratosphere.
Global warming
It has been noticed that certain gases have a tendency to go stratosphere and forms a
layer which acts like a glass in a green house. Therefore, these gases are referred as green
house gases (GHGs). In green house effect, the solar heat of short wavelength enters
through the glass house but does not get radiated back through the glass and converts into
heat having long wave radiations. This long wavelength radiation is reflected by the glass
and the heat is trapped inside the glass house to keep the plants green. Similarly, the solar
heat enters the layer formed by the glasses emitted from earth’s surface but does not get
radiated back to space due to different wavelength. This tends to increase the temperature
of earth’s surface. The global warming can have frightening effect in long run.
UV-A light is a low energy light with wavelengths between 320-400 nm. Only about
5% of the UV-A light is absorbed by ozone and most reaches the surface of the Earth.
UV-B light is of moderate energy and has wavelengths between 290-320 nm. Ozone
absorbs most of the UV-B light before it reaches the surface of the Earth. UV-C light is a
high energy UV light with wavelengths in the range of 200-290 nm.