--- title: "Effects of subcooling and superheating" subject: "Applied Thermodynamics" chapter: 3 key: "semester_3/applied_thermodynamics:subcooling-superheating-effects" source: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/subcooling-superheating-effects site: BCT Engineering --- # Effects of subcooling and superheating Applied Thermodynamics · Chapter 3: Refrigeration - Asked in: year not given, 5 marks - Video: [Comparison of Superheating and Subcooling for VCRS](https://www.youtube.com/watch?v=FxKjxVrnYEY) — Sunil Mohod - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/subcooling-superheating-effects - Whole chapter: https://www.bctengineer.com/md/semester_3/applied_thermodynamics/chapter-3 ## Question 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. ## 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. ## Answer #### (a) Subcooling Cooling the liquid below the saturation temperature at condenser pressure (3 to 3'). - Enthalpy $h_3$ falls, so the refrigerating effect $h_1 - h_{3'}$ 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 $h_{1'} - h_1$; 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. ```text p-h diagram (not to scale) p | 3'--3-----------2 | | | / | | | / | 4' 4-----1--1' +---------------------- h ``` Points: 3 to 3' = subcooling, 1 to 1' = superheating. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does subcooling do to COP? A. Decreases it because work rises B. Increases it, since refrigerating effect rises with unchanged compressor work C. No effect D. Makes compressor work vanish Answer: B. Lower h3 raises h1 - h3 while the work stays the same. (Key point: Subcooling effect) 2. Subcooling the liquid causes flash vapour in the expansion valve to A. increase B. remain unchanged C. decrease D. become all vapour Answer: C. A colder liquid flashes less. (Key point: Flash vapour) 3. Useless superheat (heat from the surroundings) causes A. a higher refrigerating effect B. a higher COP C. lower compressor work D. no refrigerating effect gain but more work, so COP falls Answer: D. The cold space gains no cooling, but specific volume and work increase. (Key point: Useless superheat) 4. Why is a small superheat always provided? A. To prevent liquid slugging in the compressor B. To reduce the condenser size C. To increase delivery pressure D. To lower the evaporator temperature Answer: A. Superheat ensures only vapour enters the compressor. (Key point: Why superheat) ## Related in Applied Thermodynamics - [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 - [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 - [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 - [Work of single-stage compressor without clearance](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Asingle-stage-compressor-work-derivation) — Applied Thermodynamics, chapter 2 ## 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 - [Compressor and turbine performance characteristics](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Acompressor-turbine-performance-characteristics) — Turbo Machines, chapter 3 - [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