In much earlier days, mechanical refrigeration systems were usually used for large applications and the refrigerants most commonly used for these systems were carbon dioxide and ammonia. Soon small units, domestic units and commercial units were also developed and along with these, new refrigerants like methyl chloride and sulfur-dioxide. However, except for ammonia, none of the other refrigerants were found to be safer and almost all were discarded. In the year 1920, halocarbon refrigerants were developed, which were found to be very safe and effective and even today they are the most extensively used refrigerants.
The completely halogenated hydrocarbons are called chlorofluorocarbons (CFCs) in which all the atoms of hydrogen are replaced by chlorine and fluorine atoms. The chlorine atoms present in the CFC refrigerants are found to have been a major cause of the depletion of the ozone layer and in adding to the greenhouse effect; hence the use of these refrigerants has been restricted.
Another group of the halogenated hydrocarbons used as refrigerants is called hydrochlorofluorocarbons (HCFCs). In these compounds one or more atom of hydrogen are retained in the molecule. Since they contain lesser number of chlorine atoms, they cause fewer negative effects to the atmosphere than the CFCs. While the productions of CFCs have been phased out, the production of HCFCs is scheduled to be phased out by 2030.
Another group of halocarbons is called hydrofluorocarbons (HFCs) that have no atom of chlorine. They don’t cause any affect on the ozone layer, hence they are considered to be the viable replacements for CFCs, the most popular being R-134a.
The halogenated hydrocarbons have a number of properties that make them excellent refrigerants. Even today when the use of CFCs is being restricted it has been tough finding the alternatives. In fact some of the most useful alternatives are also the halogenated hydrocarbons that have no bad effect on the atmosphere.
The refrigerants are manufactured by different brand names. The most popular name for these refrigerants has been “Freon" given by E.I.Du Pont de Nemours and Co. (USA). A universal numbering system has been adopted to identify the refrigerants of the same chemical composition. Some of the most commonly used halocarbons as refrigerants are: R-11, R-12, R-22 and R-502. Here R stands for the “refrigerant" and the number after it indicates the number of fluorine atoms in the molecule of the hydrocarbon. For instance in R-11 there is one atom of fluorine while in R-12 there are two. R-11 is CCL3F and R-12 is CCl2F2. R-502 is the combination of R-22 and R-115. The numbering for the refrigerants has been standardized by ASHRAE.
Read more on how CFCs effect the environment and what we can do about it:
Ozone Layer Friendly Refrigerants: Alternatives to CFCs