Today in my Thermodynamics class, we learned about the general process of how a commercial refrigerator works. This process is referred to the Refrigeration Cycle. This most typical cycle consists of four components: compressor, condenser, evaporator, and expansion valve.
In this cycle, a refrigerant enters the compressor as a vapor, The vapor is compressed at constant entropy and exits the compressor superheated. The superheated vapor travels through the condenser which first cools and removes the superheat and then condenses the vapor into a liquid by removing additional heat at constant pressure and temperature. The liquid refrigerant goes through the expansion valve where its pressure decreases drastically. That results in a mixture of liquid and vapor at a lower temperature and pressure. The cold liquid-vapor mixture then travels through the evaporator coil or tubes and is completely vaporized by cooling the warm air being blown by a fan across the evaporator coil or tubes. The resulting refrigerant vapor returns to the compressor inlet to complete the thermodynamic cycle.
The Coefficient of Performance aka COP is found by taking the heat removed divided by the net work. In addition to the refrigeration cycle we were also briefed on the Rankine Cycle and the Brayton Cycle, all which were examples of how a thermodynamic cycle of a heat engine is able to convert heat into mechanical work. However, keep in mind a very important rule of the 2nd Principle of Thermodynamics: There is no process that the only change that will be produced is the transformation of heat to work!!!
Sources: Fundamentals of Engineering Thermodynamics, by Howell and Buckius, McGraw-Hill, New York.