Explain the effect of suction pressure with the help of p-h diagram in vapour compression system

Marks: 5M, 6M

Year: Dec2014, Dec2015, Dec2013, May2014

Difficulty: Medium

1 Answer

The suction or evaporator pressure decreases because of frictional resistance of flow of the refrigerant. Let suction pressure decrease from ps to ps’ as shown in diagram and corresponding cycle be 1’-2’-3’-4’.

The following observations can be made:

  1. Decrease in refrigerating effect from (h1-h4) to (h1’-h4’)

  2. Increase in compression work from (h2-h1) to (h2’-h1’)

Therefore with decrease in suction pressure the net effect decreases the COP of the system for the same amount of refrigerant flow. The refrigerating capacity of system decreases and refrigeration cost increases.

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