Vapour-compression cycle

The vapour-compression cycle is the practical refrigeration cycle used by essentially every refrigerator, freezer, air conditioner, and heat pump in service. A working fluid circulates through four components. The compressor — the only place work enters — raises cold low-pressure vapour to high pressure and, in doing so, to a temperature above the surroundings'. The condenser lets that hot vapour give up heat and condense to liquid: this is Q_h. The throttle valve, a narrow capillary, drops the high-pressure liquid to the low-pressure side with no work extracted, a Joule–Thomson expansion that flashes part of it to vapour and plunges its temperature below the cold space's. The evaporator lets the remaining liquid boil, absorbing latent heat from inside the box: this is Q_c.

The phase changes are the point. Latent heat dwarfs sensible heat: boiling a kilogram of isobutane absorbs roughly 300 kJ, while warming the same kilogram by ten degrees absorbs about 17 kJ. By condensing and boiling rather than merely cooling and warming, a given mass of circulating fluid moves an order of magnitude more heat per lap, which is why every practical refrigerator boils something and why the cycle's performance is governed by the refrigerant's saturation curve rather than by its heat capacity.

The cycle is deliberately irreversible in one place. The throttle is an unrestrained expansion that produces no work and generates entropy, and replacing it with a turbine that recovered that work would raise the COP — but the gain is small, the extracted work is minuscule, and the hardware is expensive and unreliable. Accepting the throttling loss is the main reason a real device sits near half of its reverse-Carnot ceiling rather than at it.

Jacob Perkins patented the cycle in London in 1834 using ether as the working fluid. The refrigerant, not the cycle, is what changed over the next two centuries: toxic ammonia, sulphur dioxide, and methyl chloride until the 1920s; Thomas Midgley Jr.'s chlorofluorocarbons from 1928, banned by the Montreal Protocol after Molina and Rowland showed in 1974 that they destroy stratospheric ozone; hydrofluorocarbons until the 2016 Kigali Amendment began phasing down those as potent greenhouse gases; and now a return to natural fluids — isobutane in domestic fridges, carbon dioxide in supermarket and heat-pump systems.