Endothermic and exothermic phenomena
General principles - the theory of refrigerating machines

To transfer heat from a low-temperature medium to a higher-temperature medium, you need to :

• an endothermic phenomenon, taking place at the temperature T_F of the cold source to extract heat from the cold external environment;

• a thermodynamic process, to raise the thermal level of this heat from T_F to T_C;

• an exothermic phenomenon, taking place at the temperature T_C of the hot well to reject heat to the hot external environment.

These phenomena may occur simultaneously in the various parts of the system, which thus produces cold or, more generally, continuously displaces heat. If they take place sequentially, the various phenomena mentioned above occur one after the other. The production of cold, or more generally the displacement of heat, then takes place discontinuously or intermittently.