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Refrigeration Cycle

A refrigeration process basically removes heat from a space were cooling is desired, to a secondary media, e.g. the surroundings. One way of making this happen is by use of a vapor compression cycle. The vapor compression cycle is carried out by a refrigerant system. This system basically consist of four components namely a compressor, a condenser, an evaporator and an expansion valve (see figure) and properly the most important of all namely a refrigerant.

In this cycle two major properties of the refrigerant comes to use, namely that the refrigerant takes up heat when the phase changes from liquid to gas (evaporation) and that it dissipate heat when going from the gas phase to the liquid phase (condensation). The temperature at which a given refrigerant evaporate / condensate is entirely determined by the pressure; at low pressure the refrigerant evaporate / condensate at low temperatures and vice versa. By going through the refrigeration cycle it will become clear how these properties comes to use. Stating at the inlet of the compressor the refrigerant is in a gas phase at low pressure and temperature (follow the arrows in the figure), by compressing the refrigerant in the compressor the temperature as well as the pressure rises. At the outlet of the compressor the gaseous refrigerant now have higher temperature than the surroundings which enables a heat transfer to the surroundings. Form the compressor outlet the refrigerant flows into the condenser; the condenser is basically a heat exchanger that enables the heat transfer from the warm gaseous refrigerant to the surroundings. Because of the high pressure the refrigerant starts to condensate changing it's phase from gas into liquid. In it's way trough the condenser the refrigerant dissipate so much heat that at the outlet of the condenser it is completely in a liquid phase. At the outlet of the condenser the refrigerant is in a liquid phase at high pressure and from here it passes on to the expansion valve. Across the expansion there is a major pressure drop from the high pressure side to the low pressure side. This causes an expansion of the liquified refrigerant from a high pressure small volume to a low pressure large volume. This abrupt change in pressure causes some of the refrigerant to evaporate, this phase change causes the temperature of the refrigerant to drop (the valve is insulated and can not take up heat from the surroundings). At the outlet of the expansion valve the refrigerant is now partly liquid partly gas (in the two phase region) and has a low temperature and a low pressure. From the outlet of the expansion valve the refrigerant flows to the evaporator; the evaporator is basically a heat exchanger that enables heat transfer with the surroundings e.g. a cold storage room (se figure). Because of the low pressure the evaporation temperature is low. If the pressure is low enough the refrigerant evaporates at for instance 0C. By passing the refrigerant through an evaporator it enables the refrigerant to take up heat from a surrounding that has a higher temperature maybe 5C, such as a cold storage room. At the outlet of the evaporator all the refrigerant has evaporated and flow on to the compressor. At the inlet of the compressor the refrigerant is in a gas phase at low pressure and temperature by this the cyclic refrigeration process is closed, we are back were we started!