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Explain with neat sketch a complete multistage vapour compression system.

Subject : Refrigeration and Air Conditioning

Topic : Vapour Compression Refrigeration System

Difficulty : Medium

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  • Applications like cold storages, liquefaction of gases, chemical industries etc. demand very low evaporator temperatures and pressures, reaching even up to -150°C in some cases.
  • Hence difference between condenser and evaporator pressure increases. So when pressure ratio required is more than 8 to 10, a single stage vapour compression cycle is not efficient. Hence multistage VCRS with intercooling is adopted.
  • This reduces the work of compression while keeping refrigerating effect the same. Thus COP of cycle increases, which leads to improved performance of cycle and decreased operational cost.

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  • Process 1-2 represents isentropic compression in L.P. compressor from evaporator pressure to intermediate pressure. The vapour is then passed through intercooler where it is cooled at constant pressure (2-3) and then compressed to condenser pressure (3-4) by the H.P. compressor.
  • The vapour cooling can be achieved by water or flash intercooler depending upon the refrigerant and application.

Disadvantages:

1) The complexity of compressor body increases

2) Manufacturing and operating costs increase

3) Resistance losses increase in piping, valves and other equipments

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