Any substance that absorbs heat through expansion or vaporization may be termed a refrigerant. In broad sense, refrigerant is also applied to secondary cooling medium as brine solution or cold water. As commonly interpreted, however, refrigerant includes only those working medium which passes through cycle of evaporation, recovery, compression and liquefaction. Thus circulating cold mediums, ice and solid carbon dioxide are not primary refrigerants. The primary refrigerants directly take part in refrigeration system whereas secondary refrigerants are first cooled with the help of primary refrigerant and are further used for cooling purposes.