The vapor-compression refrigeration cycle moves heat from a cold region to a warm environment using a circulating refrigerant, the working fluid that evaporates and condenses inside the system.

The idealized cycle contains:

1. ideally isentropic compression of low-pressure vapor;
2. constant-pressure heat rejection and condensation;
3. throttling to a lower pressure;
4. constant-pressure heat absorption and evaporation.

Per unit mass,

$$w_c=h_2-h_1,$$

$$q_H=h_2-h_3,$$

$$q_L=h_1-h_4,$$

and the throttling valve satisfies

$$h_3=h_4.$$

The refrigerator coefficient of performance is

$$COP_R=\frac{q_L}{w_c}=\frac{h_1-h_4}{h_2-h_1}.$$

The same machine used for space heating is a heat pump, for which the useful effect is $q_H$.

This cycle explains why refrigeration relies on phase change: evaporation absorbs substantial heat at low temperature, while condensation rejects it at higher pressure and temperature.