--- title: "Why reversed Carnot cycle is not used" subject: "Applied Thermodynamics" chapter: 3 key: "semester_3/applied_thermodynamics:reversed-carnot-cycle-limitations" source: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/reversed-carnot-cycle-limitations site: BCT Engineering --- # Why reversed Carnot cycle is not used Applied Thermodynamics · Chapter 3: Refrigeration - Asked in: year not given, 5 marks - Video: [Reversed Carnot Cycle](https://www.youtube.com/watch?v=2zagp5NLdyw) — TutorialsPoint - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/reversed-carnot-cycle-limitations - Whole chapter: https://www.bctengineer.com/md/semester_3/applied_thermodynamics/chapter-3 ## Question Why is the reversed Carnot cycle not used as the practical refrigeration cycle? Compare it with the vapour compression cycle. ## 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. ## Answer The reversed Carnot cycle (isothermal heat absorption, isentropic compression, isothermal heat rejection, isentropic expansion) has the maximum COP between two temperatures, $\text{COP} = T_L/(T_H - T_L)$. It is used only as a standard of comparison, for these reasons: 1. **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. 2. **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. 3. **Reversible heat transfer:** truly isothermal heat transfer across a negligible temperature difference needs an infinitely large heat exchanger or very slow heat transfer. 4. **Slow, bulky plant** for a given capacity, and the cycle is difficult to control. 5. 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 | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The reversed Carnot cycle is used A. in almost all refrigerators B. in air-conditioners C. in place of throttling D. only as a standard of comparison, since it has the maximum COP Answer: D. It sets the upper bound COP = TL/(TH - TL). (Key point: Use of Carnot) 2. Why is wet compression a problem? A. Liquid droplets damage valves and cylinder walls, and isentropic compression of a mixture is difficult B. Vapour is too hot C. It lowers pressure D. It needs no work Answer: A. A liquid-vapour mixture is hard to compress and harms the compressor. (Key point: Wet compression) 3. In the practical cycle the expander is replaced by A. a second compressor B. a throttle valve, since expander work is small versus cost and friction C. a turbine producing large work D. a condenser Answer: B. Throttling is isenthalpic and simple. (Key point: Expander) 4. Compared with the reversed Carnot cycle, the vapour compression cycle has A. higher COP B. wet compression C. lower COP but compresses only dry vapour D. isentropic expansion in an expander Answer: C. Throttling and superheat reduce COP but make the plant practical. (Key point: Comparison) ## Related in Applied Thermodynamics - [Simple vapour compression cycle](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Asimple-vcr-cycle-ts-ph-diagrams) — Applied Thermodynamics, chapter 3 - [Ideal versus actual vapour compression cycle](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Aideal-vs-actual-vcr-cycle) — Applied Thermodynamics, chapter 3 - [Ideal vapour compression cycle for 5 TR](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Avcr-ideal-cycle-numerical) — Applied Thermodynamics, chapter 3 - [VCR cycle with subcooling and superheating](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Avcr-subcooling-superheating-numerical) — Applied Thermodynamics, chapter 3 - [Actual VCR cycle with compressor efficiency](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Avcr-actual-compressor-efficiency-numerical) — Applied Thermodynamics, chapter 3 ## Related in other subjects - [Vapour compression refrigeration cycle](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Avapour-compression-refrigeration) — Industrial Power Distribution and Illumination, chapter 5 - [Vapor compression refrigeration cycle p-h and T-s](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Avapor-compression-refrigeration-ph-ts) — Fundamental of Thermodynamics & Heat Transfer, chapter 6 - [Rankine cycle 2 MPa: efficiency vs Carnot](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Arankine-cycle-2mpa-efficiency-vs-carnot) — Fundamental of Thermodynamics & Heat Transfer, chapter 6 - [First law for control mass; cycle; power and refrigeration](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Afirst-law-control-mass-cycle) — Fundamental of Thermodynamics & Heat Transfer, chapter 4