Cold store, domestic refrigerator or air conditioner, it makes no difference: all mechanical refrigeration rests on the same four-step cycle. The refrigerant evaporates at low pressure in the evaporator and draws heat from the space; the compressor compresses this vapour, raising its pressure and temperature; the condenser turns the hot vapour back into liquid by giving its heat to the outside air; and the expansion valve drops the pressure of the liquid and sends it back to the evaporator. The loop then repeats.
Heat does not flow from cold to hot by itself; what drives the cycle is the electricity consumed by the compressor. That is why the energy a refrigeration system uses depends not on how much heat it removes but on the temperatures it moves that heat between: rejecting heat from a −25 °C blast freezer to 40 °C outside air takes far more electricity than rejecting it from a 0 °C room.
In practice the cycle is built around four main parts: the evaporator inside the room, the condenser and compressor outside (the condensing unit), and the expansion valve at the evaporator inlet. Between them sit auxiliary components such as the liquid receiver, filter drier, sight glass, solenoid valve and pressure switch; each has its own entry in this glossary.
Whether the cycle is running properly can be told from two measurements: the superheat at the evaporator outlet and the subcooling at the condenser outlet. If both values are in the right range, the refrigerant charge, valve setting and heat exchange are in order.