Refrigerant is the working fluid in air-conditioning and refrigeration equipment. It circulates in the air-conditioning and refrigeration equipment system and continuously exchanges energy with the outside world through the cyclic change of its own thermal state to achieve the purpose of refrigeration. It is customarily called refrigerant as a refrigeration working medium. or working substance for short.

　　Only substances that can vaporize and condense in the operating temperature range can be used as refrigerants. How did refrigerants develop?

　　How did refrigerants develop?

　　In 1834, ether was the earliest used refrigerant. The disadvantage is that it is flammable and explosive, and the evaporation pressure is lower than atmospheric pressure when extracting low temperature.

　　In 1866 Widhausen proposed the use of carbon dioxide as a refrigerant.

　　Karl Linde used ammonia as a refrigerant in 1870.

　　In 1874, Raoult used sulfur dioxide as a refrigerant.

　　Sulfur dioxide and carbon dioxide used to be relatively important refrigerants in history, but sulfur dioxide is highly toxic and has been used as an important refrigerant for 60 years. But it is non-toxic and safe to use. It has been used as a refrigerant in marine refrigeration units for 50 years before being replaced by Freon in 1955. In recent years, carbon dioxide has received new attention from people because of its good thermal performance and environmental protection properties.

　　The use of azeotropic refrigerants began in the 1950s

　　The use of non-azeotropic refrigerants in the 1960s

　　In the 1980s, scientists confirmed that CFCs (chlorofluorocarbons) are hazardous substances that cause ozone depletion and the greenhouse effect, and began to limit and replace CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons). At present, the most promising alternative refrigerants are HFCs (hydrofluorocarbons) in Freon.

　　In general, the development of refrigerants has gone through the following three stages:

　　The first stage (1830-1930):

　　Ammonia, carbon dioxide, sulfur dioxide, water, etc. are mainly used as refrigerants. Some of them are toxic, some are flammable, and some are inefficient. It took about 100 years.

　　The second stage (1930-1990):

　　Mainly use CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons) refrigerants, which have been used for about 60 years.

　　Three stages (1990-present):

　　Entered a period dominated by HFCs (hydrofluorocarbons).