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.