“Refrigerants are operating fluids used in refrigeration, in air-conditioning and heat pump systems. They absorb heat from an area, such as a conditioned space, and reject it in another, such as the exterior, usually through, respectively, the evaporation and condensation [of the fluid itself]. – “ASHRAE Handbook-Fundamentals”.
A refrigerant shall meet a certain number of requirements related to safety, chemical stability, environmental properties, thermodynamic properties and compatibility with the materials in its surroundings. There is no perfect solution across all the above-mentioned characteristics (in particular for the thermodynamic properties), so usually it is a matter of finding the right compromise.
The most important requirement is the chemical stability.
Commonly, a refrigeration system is expected to operate for many years, which means that all the fluid’s properties shall remain the same: it would be impossible for the system to work properly if the refrigerant might decompose or react to form something different.
In theory, any coolant should have low toxicity and be inflammable. The ASHRAE 34 classifies the refrigerants according to their toxicity and their inflammability. An “A” stands for the refrigerants with the “lowest degree of toxicity”, which corresponds to a permissible exposure limit of 400 ppm or above, while refrigerants of type “B” have a “high grade of toxicity “.
Flammability is measured with a scale from “1” (low flammable fluids) to “3” for the most flammable liquids, such as hydrocarbons. The flammability class “2” has a further subclass (“2L”) for refrigerants with low flammability, as defined by a burning rate of less than 10 cm/s.