- Practice · 6 marks
Ideal versus actual vapour compression cycle
Short answer
Actual vapour compression cycle differs due to superheating at compressor inlet, subcooling at condenser exit, pressure drops in evaporator and condenser, and irreversibilities in compression. The actual cycle has lower COP than the ideal because of increased compressor work and non-isentropic processes.
Explain why the actual vapour compression refrigeration cycle differs from the ideal cycle. Differentiate between the ideal and actual cycles and show the actual cycle on a T-s diagram.
The ideal cycle assumes isentropic compression, no pressure drops and exact saturation at the compressor inlet and condenser outlet. A real plant departs from this because of friction, heat transfer across a temperature difference and practical safety requirements.
| Point | Ideal cycle | Actual cycle |
|---|---|---|
| Compressor inlet | Dry saturated vapour | Slightly superheated (to protect against liquid carry-over) |
| Compression | Isentropic, reversible | Polytropic with friction; entropy increases; needs more work |
| Pressure in evaporator and condenser | Constant | Pressure drops due to friction and bends |
| Condenser exit | Saturated liquid | Subcooled liquid |
| Suction and delivery lines | Insulated, no loss | Heat gain in suction line; heat loss from delivery line |
| Valves | No loss | Pressure drop through suction and delivery valves |
| COP | Higher | Lower |
Causes of deviation in more detail
- Superheating in the evaporator exit and suction line increases specific volume and compressor work slightly, but ensures dry vapour.
- Subcooling in the condenser (and liquid line) gives extra refrigerating effect.
- Pressure drops in the evaporator, condenser and lines require the compressor to start at a lower suction pressure and deliver at a higher pressure.
- Compressor irreversibility: isentropic efficiency of 70 to 85 %; part of the work appears as extra heat in the vapour.
T
| 2 actual
| 2s ideal
| 3'--3-----2' | |
| | | | |
| 4---------1--1'
+------------------------- s
1-1' superheating; 1'-2s ideal compression; 1'-2 actual compression (entropy rises); 2-2'-3-3' desuperheating, condensation and subcooling; 3'-4 throttling.
✓ Answer checked by Claude Opus 5.5 (high effort)
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