- Practice · 5 marks
Why reversed Carnot cycle is not used
Short answer
The reversed Carnot cycle involves wet compression and expansion which damage compressor valves and require an expander with small net work. It needs infinitely large heat exchangers for isothermal heat transfer and is impractical. The vapour compression cycle replaces isentropic expansion with throttling, uses dry compression, and achieves practical heat transfer, resulting in lower but usable COP.
Why is the reversed Carnot cycle not used as the practical refrigeration cycle? Compare it with the vapour compression cycle.
The reversed Carnot cycle (isothermal heat absorption, isentropic compression, isothermal heat rejection, isentropic expansion) has the maximum COP between two temperatures, . It is used only as a standard of comparison, for these reasons:
- Wet compression: the compression starts in the two-phase region. Liquid droplets damage the valves and cylinder walls, and it is hard to compress a liquid-vapour mixture isentropically.
- Expansion engine: the isentropic expansion would need an expander handling a wet mixture. Its work output is very small compared with its cost and friction losses, so it is replaced by a throttle valve.
- Reversible heat transfer: truly isothermal heat transfer across a negligible temperature difference needs an infinitely large heat exchanger or very slow heat transfer.
- Slow, bulky plant for a given capacity, and the cycle is difficult to control.
- Operating close to the critical point is difficult.
Comparison with the vapour compression cycle
| Point | Reversed Carnot | Vapour compression |
|---|---|---|
| Compression | Wet, isentropic | Dry (into the superheated region), isentropic |
| Expansion | Isentropic, in an expander | Throttling, isenthalpic |
| Practical use | None, standard only | Almost all refrigerators and air conditioners |
| COP | Highest possible | Lower (losses in throttling and superheat) |
| Compressor | Must handle liquid | Handles only vapour |
✓ Answer checked by Claude Opus 5.5 (high effort)
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