--- title: "Vapor compression refrigeration cycle p-h and T-s" subject: "Fundamental of Thermodynamics & Heat Transfer" chapter: 6 key: "semester_2/fundamental_of_thermodynamics_and_heat_transfer:vapor-compression-refrigeration-ph-ts" source: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/vapor-compression-refrigeration-ph-ts site: BCT Engineering --- # Vapor compression refrigeration cycle p-h and T-s Fundamental of Thermodynamics & Heat Transfer · Chapter 6: Thermodynamic Cycles - Asked in: 2070 Magh, 6 marks - Video: [T-s and p-h diagram of Vapour Compression Refrigeration System - Nithy lecture](https://www.youtube.com/watch?v=-cQil8eYUA0) — Nithyanandam T - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/vapor-compression-refrigeration-ph-ts - Whole chapter: https://www.bctengineer.com/md/semester_2/fundamental_of_thermodynamics_and_heat_transfer/chapter-6 ## Question Explain the working of simple vapor compression refrigeration cycle with corresponding processes in p-h and T-s diagrams. ## Answer The simple vapour compression cycle is the most common refrigeration cycle (domestic refrigerators, air conditioners). A refrigerant (R-134a, ammonia) circulates through the system: it absorbs heat in the evaporator and rejects it in the condenser, while a compressor supplies work. ```text Condenser (Q_H out) 2 +-------------------+ 3 | | Compressor Expansion valve | | 1 +-------------------+ 4 Evaporator (Q_L in) ``` #### Processes 1. **1-2: Isentropic compression.** Saturated (or slightly superheated) vapour is compressed in the compressor to high pressure and temperature. Work W is supplied. 2. **2-3: Constant-pressure heat rejection.** The hot vapour first de-superheats, then condenses to saturated liquid in the condenser, rejecting Q_H to the surroundings. 3. **3-4: Throttling (isenthalpic expansion).** The liquid expands through the expansion valve or capillary tube to the low pressure, with h₃ = h₄. Its temperature falls and part of it flashes to vapour. 4. **4-1: Constant-pressure heat absorption.** The cold wet mixture evaporates in the evaporator, absorbing Q_L from the refrigerated space, and leaves as saturated vapour. #### Diagrams ```text p-h diagram p | .-'''-. | .' '. | 3 /___________\_______ 2 | /| \ / | / | \ / | / | \ / | / | \/ | / 4 _______________1 | / \ +---------------------------------> h T-s diagram T | 2 | .'| | .-'''-. .' | | .' '.' | | 3 /__________2' | | /: \ | | / : \ | | / : \ | | / 4 ___________\1 |/ \ +-------------------------> s 1-2 isentropic compression 2-2'-3 condensation (2' = saturated vapour) 3-4 throttling 4-1 evaporation (3-4 is vertical on p-h, and dotted on T-s because it is irreversible) ``` #### Performance - Refrigeration effect: q_L = h₁ − h₄ - Compressor work: w = h₂ − h₁ - Heat rejected: q_H = h₂ − h₃ **COP = q_L/w = (h₁ − h₄)/(h₂ − h₁)** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Process 3-4 is: A. Isothermal compression B. Constant-pressure heating C. Isentropic expansion D. Isenthalpic throttling, h3 = h4 Answer: D. The valve causes a pressure drop at constant enthalpy; vertical on p-h. (Key point: Throttling) 2. Heat is absorbed in: A. Condenser B. Compressor C. Expansion valve D. Evaporator, 4-1 at constant pressure Answer: D. The wet mixture evaporates absorbing Q_L. (Key point: Evaporator) 3. Compression 1-2 is: A. Isothermal B. Isenthalpic C. Isentropic, raising pressure and temperature D. Constant pressure Answer: C. Work W is supplied; h rises. (Key point: Compressor) 4. COP is: A. (h2 - h1)/(h1 - h4) B. (h1 - h4)/(h2 - h1) C. (h2 - h3)/(h2 - h1) D. (h1 - h4)/(h2 - h3) Answer: B. Refrigeration effect over compressor work. 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