In the very early days (1920’s) variants of ether were probably used.
In common use a little later and generally in order
Sulphur dioxide
Methyl Chloride
Ammonia
R12 Dichlorodifluoromethane
R134aTetrafluoroethane
R290 Propane
R600 Iso-Butane
Sulphur Dioxide is a noxious refrigerant and has long since fallen out of favour due to it’s toxicity.
Methyl chloride was not a particularly good refrigerant but adequate except that it is not compatible with aluminium and most refrigerator evaporators and many compressor components are made from aluminium. In the last five or ten years at least four refrigeration mechanics have died from explosions caused by this incompatibility when Methyl Chloride was used as a cheaper refrigerant.
Ammonia is another noxious gas which is somewhat flammable and also not compatible with copper containing or aluminium componentry. It is common in absorption refrigerators but they too are a limited market and falling a little out of favour since sealed unit compressors are now so small and quiet and do not require a flame or flammable gas or liquid to operate. There are many that use an electric element to generate the heat required but because of the necessity for steel tubing and components there is a weight and manufacturing penalty during manufacture and transport.
R12 is not very environmentally friendly but otherwise is quite a good refrigerant for this market.
R134a is a refrigerant which was introduced as an interim replacement for R12 until a suitable long term refrigerant is developed. Not too bad, but there are some environmental concerns.
R290 and R600 are both flammables and therefore explosive in the right conditions. They require some special consideration when being used but the market seems to be adopting them as the new standard. They require special leak detection, charging and evacuation equipment.
I am making an enormous assumption here that you are talking about domestic (not Dometic because that is one manufacturer) refrigerators and use the abbreviation “fridge”. This places you as an American so you would probably mean “household”rather than domestic.
In the commercial and industrial market there are at least another 70 gases commonly used.
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