Radical changes in the selection and use of refrigerants in response to environmental issues have taken place during the last 25 years, a story which can be traced with the aid of a ‘Refrigerant Time Line’ (Figure 3.1).
The earliest mechanical refrigeration used air as a working fluid. The introduction of the vapour compression cycle enabled more compact and effective systems. At first the only practical fluids were carbon dioxide and ammonia. One of the major requirements was preservation of meat on the long sea voy — 30 ages from New Zealand and Australia to Europe, and for this ammonia was
Unsuitable owing to its toxic nature. Carbon dioxide, although requiring much higher pressures, was used. Methyl chloride, although toxic and very unpleasant, was used in some smaller systems.
A revolution came about with the invention of the chlorofluorocarbon (CFC) R12 by Midgley in the early 1930s. This refrigerant and other members of the CFC family seemed to possess all the desirable properties. In particular they were non-toxic, non-flammable and with good thermodynamic properties and oil miscibility characteristics. The CFCs R12, R11, R114 and R502 together with hydrochlorofluorocarbon (HCFC) R22 became the definitive refrigerants. They enabled the expansion of refrigeration into the commercial, domestic and air-conditioning sectors. Ammonia with its excellent thermodynamic properties and low cost continued in many industrial applications. Environmental concerns have now driven the development of replacements for the chlorine containing compounds.
A summary table (Table 3.1) gives the key properties of the main refrigerants in use today together with their typical application ranges; low (-25 to -40°C), medium (-5 to -25°C) and high (+10 to -5°C) temperature.
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