The vapour-compression cycle
Nearly every fridge, chiller, air conditioner and heat pump runs the same four-stage loop. Step through it below: heat is absorbed in the evaporator, the compressor lifts the refrigerant's pressure and temperature, the condenser rejects the heat, and the expansion valve resets the loop. The machine moves about three units of heat per unit of electricity, which is why heat pumps beat every fuel-burning appliance.
A closed loop of refrigerant passes through four components. Two move heat (evaporator in, condenser out), one moves the refrigerant (compressor — the only place you pay for energy), and one resets it (expansion valve). Step through each in turn.
Go deeper
The full vapour-compression cycle lesson adds the energy balance and a worked chiller example, and the rest of the Refrigeration & Heat Pumps course builds from this loop to COP, diagnostics and the savings that follow. For the heating side, start with the heat pump lesson.