A refrigerant is a working fluid used in the refrigeration cycle of air conditioning systems and heat pumps where in most cases they undergo a repeated phase transition from a liquid to a gas and back again.
Refrigerants are used in a Direct Expansion (DX) system to transfer energy from one environment to another, typically from inside a building to outside (or vice versa) commonly known as an “air conditioner” or “heat pump”. Refrigerants can carry per kg 10 times more energy than water and 50 times more than air.
Refrigerants are controlled substances due to 1) High Pressures (100-145 psi), 2) Extreme temperatures (-50°C to 145°C), 3) Flammability A1 class non-flammable, A2/A2L class flammable & A3 class extremely flammable/explosive and 4) Toxicity B1-low, B2-medium & B3-high, as classified by ISO 817 & ASHRAE 34.
Refrigerants must only be handled by qualified/certified engineers to the relevant classes of refrigerant eg in the UK C&G 2079 if A1 class PLUS C&G 6187-2 if A2/A2L & A3 class refrigerants.