A refrigerant is a substance or mixture. Usually, a fluid, used in a heat pump and refrigeration cycle that can extract heat from another body or substance. Ice, cold water, cold air, etc. can be treated as refrigerants.
Desirable Properties of a Refrigerant:
1. Vapor density:
To enable the use of smaller compressors and other equipment the refrigerant should have smaller vapor density.
2. Enthalpy of vaporization:
To ensure the maximum heat absorption during refrigeration, a refrigerant should have a high enthalpy of vaporization.
3. Thermal Conductivity:
Thermal conductivity of the refrigerant should be high for faster heat transfer during condensation and evaporation.
4. Dielectric strength:
In hermetic arrangements, the motor windings are cooled by refrigerants vapor on its way to the suction valve of the compressor. Therefore, the dielectric strength of refrigerant is an important property in hermetically sealed compressor units.
5. Critical temperature:
In order to have a broad range of isothermal energy transfer, the refrigerant should have a critical temperature above the condensing temperature.
6. Specific heat:
To have the minimum change in entropy during the throttling process, the specific heat should be minimum. For this, the liquid saturation line should be almost vertical.
7. Leak tendency:
The refrigerant may leak out of the system. The problems with a leakage are wearing out of joint or the material used for the fabrication of the system. A denser refrigerant will have fewer tendencies to leak as compared to higher density refrigerant. The detection of leaks should be easy to loss of refrigerant. Leakage can be identified quickly if the refrigerant has a distinct color or odor.
The refrigerant used in air conditioning, food preservation, etc. should not be toxic as they will come into contact with human beings. Refrigerants will affect human health if they are poisonous.
9. Cost of refrigerants:
The quantity of refrigerant used in industries is very less. The price of the refrigerants is generally high when compared to other chemicals in the industry. Shallow industry professional will not take necessary action to control the leaks.
Refrigerants should be available near the usage point. It must be sourced and procured within a short period to enable the user in case of leaks, maintenance schedules, etc.
Properties of Commonly Used Refrigerants:
1. Carbon dioxide:
Carbon dioxide is widely used as a refrigerant in mechanical systems refrigerant, marine services, hospitals, etc. due to its excellent safety properties. It is odorless, non-toxic, non-flammable, non-explosive and non-corrosive.
2. Sulfur dioxide:
Sulfur dioxide was widely used as refrigerant during the early 20th century. However, its use has been restricted nowadays because of its many inherent disadvantages. It is highly toxic, non-flammable, non-explosive, non-corrosive and works at low pressures
Ammonia is one of the earliest types of refrigerants which is still widely used in many applications due to its inheritance excellent thermal properties, It is toxic in nature, flammable explosive under certain conditions, it has low specific volume¸ high refrigerating effect, low piston displacement in case of reciprocating compressors make it an ideal refrigerant for cold storage’s, ice plants, packing plants, skating rinks breweries, etc.
Freon-11 (Trichlorofluoromethane) is used under low operating pressures; it is non-toxic, non-corrosive and non-flammable. Due to low operating pressure and high displacement, it is used in systems employing centrifugal compressors. It is used for air-conditioning applications.
Freon-12 (Dichloro difluoromethane) is non-flammable, non-toxic and non-explosive. It is highly chemically stable. If it is brought into contact with open flame or heater elements, it decomposes into highly toxic constituents. It has not only excellent safe properties but also condenses at moderate pressure under normal atmospheric conditions.
6. Cryogenic refrigerants:
Cryogenic refrigerants are those refrigerants which produce minus temperature in between range -157°C to -273°C in the refrigerated space. The cryogenic refrigerants have a shallow boiling point at atmospheric pressure. Some of the widely used cryogenic refrigerants are Helium, Nitrogen, Oxygen, Hydrogen.
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