--- title: "Vapour compression refrigeration cycle" subject: "Industrial Power Distribution and Illumination" chapter: 5 key: "semester_6/industrial_power_distribution_and_illumination:vapour-compression-refrigeration" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/vapour-compression-refrigeration site: BCT Engineering --- # Vapour compression refrigeration cycle Industrial Power Distribution and Illumination · Chapter 5: Electrification of Industrial Building - Asked in: 2068 Magh (old course), 8 marks - Video: [How Vapor Compression Refrigeration Cycle Works | HVAC Refrigeration Cycle](https://www.youtube.com/watch?v=QwSH_a8_COw) — Augmintech - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/vapour-compression-refrigeration - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-5 ## Question With necessary sketch and circuit diagram, explain vapour compression refrigeration cycle and show how cooling effect is produced? ## Short answer A vapour compression cycle uses a compressor, condenser, expansion valve, and evaporator. Refrigerant evaporates at low pressure absorbing heat (cooling effect), then is compressed, condensed rejecting heat, and expanded. The cycle repeats, driven by an electric motor, with COP typically 2–4. ## Answer **Vapour compression refrigeration** is the cycle used in refrigerators, water coolers, cold stores and air conditioners. A refrigerant (R-134a, R-410A, R-600a, ammonia in industry) absorbs heat when it **evaporates** at low pressure inside the cold space and rejects that heat when it **condenses** at high pressure outside. An electric motor-driven compressor supplies the work needed to move heat from cold to hot. #### Sketch of the cycle ```text Heat out (to room/air) ↑ ↑ ↑ +----[ CONDENSER ]----+ | high P, high T | (2)| |(3) liquid | ↓ [COMPRESSOR] [EXPANSION VALVE] ↑ driven by | | motor M | (4) low P, (1)| low P vapour | cold mix +----[ EVAPORATOR ]---+ ↓ ↓ ↓ Heat in (from food / room) ``` #### Working (four processes) 1. **Compression (1→2):** the compressor draws low-pressure, low-temperature vapour from the evaporator and compresses it. Its pressure and temperature rise well above room temperature (superheated vapour). 2. **Condensation (2→3):** hot vapour passes through the condenser coils (fins at the back of a fridge, or outdoor unit of an AC). Heat flows to the surrounding air, and the refrigerant condenses into high-pressure liquid. 3. **Expansion (3→4):** the liquid passes through the expansion valve or capillary tube. Its pressure drops suddenly; part of it flashes to vapour and its temperature falls far below the space temperature. 4. **Evaporation (4→1):** the cold liquid–vapour mixture flows through the evaporator inside the cabinet/room. It takes latent heat from the food or air and boils into vapour. **This heat absorption is the cooling (refrigerating) effect.** The vapour returns to the compressor and the cycle repeats. #### Electrical circuit of a domestic refrigerator ```text L ──[Thermostat]──[OLP]──┬──[Relay coil]── Run winding ──┐ │ │ └─[Relay contact]─ Start winding┤ N ───────────────────────────────────────────────────────┘ Door switch ── Lamp (in parallel across L–N) ``` - **Thermostat** switches the compressor motor on when the cabinet warms and off when the set temperature is reached. - **Starting relay** (current relay or PTC) connects the start winding for a moment at starting; **overload protector (OLP)** trips the motor on overcurrent or overheating. - The compressor motor is a hermetically sealed single-phase induction motor. **Coefficient of performance:** COP = refrigerating effect / work input = (h₁ − h₄)/(h₂ − h₁), typically 2–4, i.e. 1 kW of electric input removes 2–4 kW of heat. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In which part does the cooling (refrigerating) effect occur? A. In the condenser, where the refrigerant condenses B. In the evaporator, where the refrigerant boils absorbing latent heat C. In the compressor, where pressure rises D. In the expansion valve, where heat is rejected Answer: B. The cold liquid-vapour mixture absorbs latent heat from the space in the evaporator and boils; this is the refrigerating effect. (Key point: Heat absorption) 2. What does the compressor do? A. Cools the vapour to a liquid B. Raises pressure and temperature of the vapour from the evaporator, supplying work C. Drops the pressure suddenly D. Stores the refrigerant Answer: B. The compressor takes low-pressure vapour and compresses it to a hot high-pressure superheated vapour using motor work. (Key point: Compressor role) 3. What happens in the expansion valve or capillary tube? A. Pressure rises and the liquid heats up B. Pressure drops suddenly, some liquid flashes and temperature falls below the space temperature C. The refrigerant condenses D. Heat is rejected to the room Answer: B. Throttling lowers pressure; part flashes to vapour and the mixture becomes very cold. (Key point: Expansion valve) 4. How is COP defined for this cycle? A. Work input / refrigerating effect B. Refrigerating effect / work input = (h1 - h4)/(h2 - h1) C. Heat rejected / heat absorbed D. Compressor power / thermostat setting Answer: B. COP = heat extracted in evaporator divided by compressor work, typically 2-4. (Key point: COP) ## Related in Industrial Power Distribution and Illumination - [Battery charger circuit and charging methods](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Abattery-charger-two-methods) — Industrial Power Distribution and Illumination, chapter 9 ## Related in other subjects - [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 - [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