3.1 General function and theory

For a liquid inside a vessel to boil, the pressure over the liquid surface most correspond to the boiling temperature. The gas produced due to the addition of heat must be removed continuously if the same boiling point, i.e. the same pressure, is to be maintained.

In a refrigerant system, the "vessel" referred to above is the evaporator, and the device removing the gas is the compressor. The compressor pumps gas from the evaporator and compresses it, i.e. increases its pressure. The energy required for the compression normally comes from electricity.

The red compression line in Figure 3.1 symbolizes an ideal isentropic compression, i.e. compression at constant entropy, in which there is no heat exchange with the surroundings. In reality, however, there are always some heat losses from the vapor due to mechanical friction in the equipment, i.e. flow frictional work between the fluid and the wall, and potential leakage in the compressor.

Figure 3.1 illustrates a comparison between a realistic case (blue) and the theoretical isentropic compression (red). The difference between isentropic and actual compression can be expressed by the isentropic compressor efficiency, ηIS.

The efficiency of a compressor can be indicated in various ways. Another example is volumetric efficiency, ηVOL, which is the ratio between the actual vapor volume and the theoretical maximum volume that can be contained in the compressor cylinder:

It is always desirable to achieve high compressor efficiency to minimize the compressor work and maintain conditions in the refrigerant system. The compression ratio shows the ratio between the high-pressure and low-pressure sides of the compressor:

A high compression ratio, i.e. a large pressure difference between the low-pressure and the high-pressure sides, requires more compressor work.​​​​​​​​

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