Refrigeration - Basic Cycle Concepts General Heat energy always flows naturally from a higher to a lower temperature level. That is, hot areas naturally cool off and cold areas naturally warm up. Therefore, moving heat from a lower to a higher temperature requires the input of work (or heat), usually to create a pressure differential in the cycle refrigerant. The refrigerant (acting as a heat transfer fluid) is used to transfer heat energy from a lower temperature to a higher temperature. The refrigerant is evaporated at a temperature lower than the desired temperature in the freezer or cooler. The condensing temperature of the refrigerant is increased by compression so that it can either be rejected to the environment or recovered as useful heat. The basic refrigeration cycle, with all steps combined, is shown: Step One, Evaporation: Liquid refrigerant at a sufficiently low pressure is brought into contact with the heat source (the medium to be cooled). The refrigerant absorbs heat and boils, producing a low-pressure vapor. The heat exchanger used for this process is called the evaporator. Step Two, Compression: The compressor raises the pressure of the refrigerant vapor, normally using an electric motor drive. This increases the temperature at which the vapors will condense to a temperature above the temperature of the heat sink. Most common compressors are reciprocating (piston and cylinder) or screw (looking much like an old meat grinder) compressor designs. Step Three, Condensing: The high-pressure refrigerant gas now carrying the heat energy absorbed at the evaporator plus the work energy from the compressor, enters the condenser. Since the refrigerant's condensing temperature is higher than that of the heat sink, heat transfer will take place, condensing the refrigerant from a high-pressure vapor to a high-pressure liquid. Step Four, Expansion: The condensed liquid's pressure is reduced (called "throttled") to the lower pressure evaporator using a valve, orifice plate or capillary tube device. In actual practice, the condenser cools the refrigerant a bit more, subcooling it below the condensing temperature. This is an important efficiency improving attribute to the cycle, since it reduces the amount of refrigerant liquid that has to evaporate (it is called flashing at this stage in the cycle) to a gas in the expansion valve to reduce the pressure and temperature of the liquid entering the evaporator. This reduction in flash gas is important to improve system performance.