--- title: "Rankine cycle working principle" subject: "Fundamental of Thermodynamics & Heat Transfer" chapter: 6 key: "semester_2/fundamental_of_thermodynamics_and_heat_transfer:rankine-cycle-working-principle" source: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/rankine-cycle-working-principle site: BCT Engineering --- # Rankine cycle working principle Fundamental of Thermodynamics & Heat Transfer · Chapter 6: Thermodynamic Cycles - Asked in: 2073 Bhadra, 6 marks - Video: [RANKINE CYCLE (Simple and Basic)](https://www.youtube.com/watch?v=QFZN71MY71o) — EarthPen - 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/rankine-cycle-working-principle - Whole chapter: https://www.bctengineer.com/md/semester_2/fundamental_of_thermodynamics_and_heat_transfer/chapter-6 ## Question Explain the working principle of an Rankine cycle with P-V and T-S diagram. ## Short answer The Rankine cycle is the vapor power cycle: pump raises pressure of saturated liquid isentropically, boiler adds heat at constant pressure to produce steam, turbine expands steam isentropically, and condenser rejects heat at constant pressure returning to saturated liquid. ## Answer The Rankine cycle is the ideal vapour power cycle used in steam power plants. It removes the practical difficulties of the Carnot vapour cycle: the steam is fully condensed to liquid, so a small pump (not a compressor) can raise the pressure. ```text Boiler (Q_in) 2 +----------+ 3 | | Pump Turbine --> W_out | | 1 +----------+ 4 Condenser (Q_out) ``` #### Processes 1. **1-2: Isentropic compression in the pump.** Saturated liquid at the condenser pressure is pumped to the boiler pressure. The pump work, w_p = v_f (P₂ − P₁), is very small. 2. **2-3: Constant-pressure heat addition in the boiler.** The compressed liquid is heated, evaporated and (usually) superheated to state 3. 3. **3-4: Isentropic expansion in the turbine.** The steam expands to the condenser pressure and produces work. The exit is a wet mixture (or slightly superheated). 4. **4-1: Constant-pressure heat rejection in the condenser.** The wet steam condenses to saturated liquid. #### Diagrams ```text T-s diagram P-v diagram T P | 3 | 2 ____________________ 3 | .-'''-. .'| | | \ | .' '.' | | | '. | 2' /____________3' | | | '-. | / \ | | 1 _________________________ 4 | / \| +----------------------------------> v | 2 / |\ | 1/___________________4 \ | / \ +-----------------------------> s (2 lies just above 1; 2'-3' boiling, 3'-3 superheating) ``` #### Energy relations (per kg) - w_p = h₂ − h₁ - q_in = h₃ − h₂ - w_t = h₃ − h₄ - q_out = h₄ − h₁ **η = [(h₃ − h₄) − (h₂ − h₁)]/(h₃ − h₂)** Efficiency improves with higher boiler pressure and temperature (superheating) and lower condenser pressure. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does the Rankine cycle use a pump rather than a compressor? A. Pumps are cheaper B. To reduce boiler pressure C. Steam cannot be compressed D. Steam is fully condensed to liquid, so little work is needed Answer: D. Complete condensation avoids compressing vapour, removing the Carnot vapour cycle difficulty. (Key point: Why Rankine) 2. Process 4-1 takes place in: A. Pump isentropically B. Condenser at constant pressure C. Turbine isentropically D. Boiler at constant pressure Answer: B. Wet steam condenses to saturated liquid, rejecting heat. (Key point: Process 4-1) 3. In the turbine: A. Steam is heated B. Liquid is pumped C. Steam is throttled D. Steam expands isentropically producing work Answer: D. 3-4 is isentropic expansion to condenser pressure. (Key point: Process 3-4) 4. Efficiency improves with: A. Lower boiler pressure B. Removing superheater C. Higher boiler pressure and temperature, lower condenser pressure D. Higher condenser pressure Answer: C. These raise mean heat-addition temperature or lower rejection temperature. 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