A thermodynamic cycle used in refrigeration systems. It uses a volatile liquid refrigerant, which is pumped through the cooling coils of an evaporator where the latent heat used in its evaporation cools the materials requiring refrigeration. The vapour is then compressed to a high pressure by a compressor and then condensed in a condenser back to a liquid, liberating heat. The liquid refrigerant is contained in a storage vessel before finally passing through an expansion valve to reduce its pressure and being pumped back to the evaporator and the cycle repeated. In a domestic refrigeration system, the cycle is repeated until the temperature reaches the desired level of around 4^ºC in the cool box and –18^ºC in the freezer compartment. A thermostat is used to maintain a steady state by turning the compressor on and off. See refrigeration.