Carnot cycle
The Carnot cycle is a reversible cycle so the four processes that comprise it, two isothermal and two isentropic, can also be reversed. When a Carnot cycle runs reversely, it is called a "reversed Carnot cycle". A refrigerator or heat pump that acts on the reversed Carnot cycle is called a Carnot refrigerator or Carnot heat pump respectively. In the first stage of this cycle, the refrigerant absorbs heat isothermally from a low-temperature source, , in the amount . Next, the refrigerant is isentropically compressed and its temperature rises to that of the high-temperature source, . Then at this high temperature, the refrigerant rejects heat isothermally in the amount . Also during this stage, the refrigerant changes from a saturated vapor to a saturated liquid in the condenser. Lastly, the refrigerant expands isentropically until its temperature falls to that of the low-temperature source, .