What is most commonly used to transfer heat in mechanical refrigeration systems?

In mechanical refrigeration systems, refrigerants are the primary medium used to transfer heat. Refrigerants have unique thermodynamic properties that allow them to absorb heat from the environment (such as inside a refrigerator or air conditioner) and release it to another medium (usually outside air or water) when they undergo phase changes. This property is critical in refrigeration cycles, where the refrigerant evaporates at low pressure to absorb heat and then condenses at high pressure to release that heat.

The effectiveness of refrigerants in transferring heat is due to their ability to exist in different phases—liquid and gas—within the refrigeration cycle, allowing them to absorb and release significantly different amounts of heat energy. This makes them highly efficient for the purpose of cooling.

Other substances, such as water, air, and thermal oils, typically have their own applications in heating and cooling processes but do not possess the same properties or efficiency as refrigerants in mechanical refrigeration systems. While water might be used in some cooling applications, it does not cycle through as integral a role in these systems as refrigerants do.