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Effects of subcooling and superheating

Short answer

Subcooling of liquid refrigerant increases refrigerating effect without changing compressor work, thus improving COP. Superheating vapour at compressor inlet prevents liquid slugging; if heat comes from the refrigerated space (useful superheat), refrigerant effect may increase, but compressor work rises, so COP change depends on refrigerant. Useless superheat reduces COP.

Discuss the effect of (a) subcooling of the liquid refrigerant and (b) superheating of the vapour entering the compressor on the performance of a vapour compression refrigeration cycle. Show both on a p-h diagram.

(a) Subcooling

Cooling the liquid below the saturation temperature at condenser pressure (3 to 3').

  • Enthalpy h3h_3 falls, so the refrigerating effect h1−h3′h_1 - h_{3'} increases.
  • Compressor work is unchanged, so COP increases.
  • Flash vapour formed in the expansion valve decreases.
  • It is obtained with a larger condenser, a liquid-to-suction heat exchanger or a separate subcooler.

(b) Superheating

Heating the vapour above the saturation temperature at evaporator pressure (1 to 1').

  • If the heat comes from the cold space (useful superheat), the refrigerating effect rises by h1′−h1h_{1'} - h_1; however, the specific volume of the vapour increases and compressor work also increases. The COP may rise or fall, depending on the refrigerant.
  • If the heat comes from the surroundings (useless superheat), the refrigerating effect does not increase but the work does, so the COP falls.
  • It prevents liquid slugging (wet compression) in the compressor, so a small superheat is always provided.
 p-h diagram (not to scale)
 p
 |   3'--3-----------2
 |   |   |         /
 |   |   |       /
 |   4'  4-----1--1'
 +---------------------- h

Points: 3 to 3' = subcooling, 1 to 1' = superheating.

✓ Answer checked by Claude Opus 5.5 (high effort)

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