Compound or Multi Stage Vapour Compression Refrigeration System:
In simple vapour compression refrigeration system, the low pressure vapour refrigerant from the evaporator is compressed in single stage (or in a single compressor) and then delivered to a condenser at a high pressure. But sometimes, the vapour refrigerant is required to be delivered at a very high pressure as in the case of low temperature refrigerating systems. In such cases either the vapour refrigerant should be compressed by employing a single stage compressor with a very high pressure ratio between the condenser and evaporator or it should be compressed in two or more compressors placed in series. The compression carried out in two or more compressors is called compound or multi stage vapour compression refrigeration system.
In vapour compression refrigeration systems, the major operating cost is the energy supplied to the system in the form of mechanical work. Thus any method of increasing coefficient of performance is advantageous so long as it does not involve too heavy increase in other operating costs, as well as initial plant cost and consequent maintenance, Since the coefficient of performance of a refrigeration system is the ratio of refrigerating effect to the compression work, the coefficient of performance can be increased either by increasing the refrigerating effect or by decreasing the compression work.
A little consideration will show that in a vapour compression system, the compression work is largely reduced if the refrigerant is compressed very close to the saturated vapour line. This can be achieved by compressing the refrigerant in more stages with intermediate intercooling. But it is economical only where the pressure ratio is considerable as would be the case when very low evaporator temperatures are desired or when high condenser temperature may be required. The compound or multi stage vapour compression refrigeration system is generally economical in large plants.
The refrigerating effect can be increased by maintaining the condition of the refrigerant in more liquid state at the entrance to the evaporator. This can be achieved by expanding the refrigerant very close to the saturated liquid line. It may be noted that by subcooling the refrigerant and by removing the flashed vapour, as they are during multi-stage expansion, the expansion can be brought close to the liquid line.