What are Refrigerants?
In the process of refrigeration the heat is carried from the low temperature reservoir to the high temperature (against the second law of thermodynamics). The refrigerant is the fluid that is used in the vapor compression refrigeration system for carrying the heat from the substance or the fluid to be cooled and throw it to the atmosphere. The substance to be cooled is at low temperature and the atmosphere is at higher temperature. During the process of transfer of heat the refrigerant repeatedly gets converted into the vapor state when it absorbs the heat and into the liquid state when it gives up the heat to the atmosphere.
For the chemical fluid to be used as the refrigerant it should possess certain physical, chemical and thermodynamic properties that make them safe and economical to be used as the refrigerant. In refrigeration and air conditioning applications there are wide range of conditions in which the refrigerants have to work, hence different refrigerants possessing suitable properties are used for different applications. There is no single refrigerant that can be called as the ideal refrigerant possessing all the desirable properties.
History of the Refrigerants
In the beginning when the vapor compression system was discovered the refrigeration systems were mostly used for large cooling applications, ammonia and carbon dioxide were used as the refrigerants. These refrigerants were not found to be suitable for the small cooling applications. With the development of the small refrigeration applications for domestic and commercial purposes new refrigerants were discovered like sulfur dioxide and methyl chloride. In the refrigeration applications where centrifugal compressor was used, methylene chloride was used as the refrigerant. Soon methylene chloride and carbon dioxide came to be used extensively in the large air conditioning applications.
In the later years, the use of almost all the above mentioned refrigerants was stopped because of the invention of the newer group of refrigerants named halocarbons. However, ammonia is still being used as the refrigerant due to its highly suitable thermal properties. It is still being used in the large cold storages, ice plants and skating rinks. At present the halocarbons are used extensively as the refrigerants due to a number of desirable properties that they possess.
What are Halocarbon Refrigerants?
The halocarbons are the compounds formed from methane (CH4) and ethane (C2H6) molecules (both are pure hydrocarbons) by replacing one or more hydrogen atoms with halogens like chlorine, fluorine or bromine. The chlorofluorocarbons (CFCs) are formed by replacing some hydrogen atoms in the hydrocarbons with chlorine and fluorine atoms. Thus chlorofluorocarbons are the hydrocarbons with chlorine and fluorine atoms. The CFCs are now used extensively in the refrigeration and the air conditioning applications.
Some of the common CFCs are: CCl3F (R-11), CCL2F2 (R-12), ChClF2(R-22) etc.
The refrigerants are commonly named as Freon. Thus we have Freon 11, 12, 22 etc. Refrigerants are also named by the initial name R, thus we have the refrigerants named as R-11, R-12, R-22 etc.
1) Book: Principles of Refrigeration by Roy J Dossat, fourth edition, Prentice Hall.