Keeping Things Cool
Any substance, which absorbs heat through vaporization or expansion, is called a refrigerant. In broader sense, the term refrigerant is also applied to such cooling systems as cold water or brine solutions. As commonly interpreted, refrigerants include those working mediums that pass through the cycle of evaporation, recovery, compression and condensation. Thus, circulating cold mediums are not primary refrigerants, nor are cooling systems such as ice and solid carbon dioxide.
Desirable Properties Of Refrigerants
Desirable refrigerants are those which possess chemical, physical, and thermodynamic properties that permit their efficient application and service, in practical designs of refrigerating equipment. In addition, if the volume of charge is large, there should be no danger to health and property in case of its escape. A great variety of substances, such as butane, carbon tetrachloride, ethane, and hexane have been applied to refrigeration systems, but found to have little practical use. These materials are either highly explosive or flammable, or possess other combinations of undesirable properties.
Air is one of the earliest refrigerants and was widely used in World War I whenever a completely nontoxic material was required. Although air is free of cost and completely safe, its low coefficient of performance makes it unable to compete with the modern nontoxic refrigerants.
Ammonia is one of the oldest and widely used of all refrigerants. It is flammable and highly toxic. It is widely used in commercial and large industrial reciprocating compression systems where high toxicity is secondary.
It is a colorless and odorless gas, which is heavier than air. It is nontoxic and nonflammable but has extremely high operating pressures. In former years it was used for marine refrigeration, theatre air conditioning systems, and for hotel refrigeration systems.
These refrigerants use ethane and methane as bases, and are the most important group of refrigerants being used in modern technology. These are used in a variety of applications, such as reciprocating compression refrigeration, and rotary compressors.
Other refrigerants are methyl chloride, sulphur dioxide, hydrocarbon refrigerants, methyl chloride, and azeotropes.
Selection Of Refrigerants
No substance has proved to be the ideal working medium, under all operating conditions. The characteristics of some refrigerants make them desirable for use with reciprocating compressors. In some applications toxicity is of negligible importance, whereas in others, such as comfort cooling, a nontoxic and nonflammable refrigerant is essential. Therefore, in selecting the correct refrigerant, it is necessary to determine those properties which are most suitable, and to choose the most closely approaching ideal for the particular application.