--- title: "Rankine efficiency and impulse versus reaction" subject: "Power Plant Equipment" chapter: 7 key: "semester_7/power_plant_equipment:rankine-efficiency-impulse-reaction" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/rankine-efficiency-impulse-reaction site: BCT Engineering --- # Rankine efficiency and impulse versus reaction Power Plant Equipment · Chapter 7: Thermal (Steam) Power Plant - Asked in: 2080 Baisakh, 4+4 marks - Video: [Difference Between Impulse And Reaction Turbine](https://www.youtube.com/watch?v=tntddHeuCvw) — BEST MECHANICAL ENGINEERING - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/rankine-efficiency-impulse-reaction - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-7 ## Question What are the ways by which efficiency of the Rankine cycle can be increased? Differentiate between impulse and reaction steam turbine. ## Short answer Efficiency rises with higher boiler pressure, superheating, lower condenser pressure, reheating, and regenerative feed heating. In impulse turbines, pressure drops only in nozzles with constant moving-blade pressure; reaction turbines drop pressure in both fixed and moving blades, yielding higher efficiency but more stages. ## Answer #### Ways to increase the efficiency of the Rankine cycle Rankine efficiency $\eta = \dfrac{W_T - W_P}{Q_s}$ increases when heat is added at a higher mean temperature or rejected at a lower temperature: 1. **Higher boiler pressure** - higher saturation temperature during boiling. 2. **Superheating** to a higher turbine inlet temperature. 3. **Lower condenser pressure** - lower heat rejection temperature (limited by cooling water temperature). 4. **Reheating** the partly expanded steam. 5. **Regenerative feed heating** with bled steam. 6. Recovering flue gas heat with **economiser and air preheater**; using **supercritical pressure** or **combined/binary cycles**. #### Impulse vs reaction steam turbine | Point | Impulse turbine | Reaction turbine | |---|---|---| | Pressure drop | Whole drop occurs in the nozzles only | Drop occurs in both fixed and moving blades | | Pressure on moving blades | Constant over the moving blades | Falls continuously across moving blades | | Force on blades | Impulse of high-velocity jet only | Impulse + reaction of steam accelerating in the blade passage | | Blade profile | Symmetrical, constant passage area | Aerofoil shape, converging passage | | Fixed element | Nozzles | Fixed (guide) blades acting as nozzles | | Steam admission | Partial or full (nozzle arcs) | Full admission all round | | Blade velocity / speed | Very high speed per stage (needs compounding) | Lower speed, many stages | | Number of stages | Fewer | More (for same pressure drop) | | Efficiency | Lower | Higher | | Size / cost | Smaller, cheaper for given power | Larger, costlier (more stages) | | Tip leakage | Small (no pressure difference across blades) | Larger, needs close clearances and dummy piston | | Example | De Laval, Curtis, Rateau | Parsons turbine | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Rankine cycle efficiency is given by: A. (W_T + W_P)/Q_s B. (W_T - W_P)/Q_s C. W_P/W_T D. Q_s/(W_T - W_P) Answer: B. Net work (turbine minus pump) divided by heat supplied. (Key point: Rankine efficiency expression) 2. What limits how low the condenser pressure can be made? A. Cooling water temperature B. Boiler pressure C. Superheater metallurgy D. Number of heaters Answer: A. Heat rejection temperature cannot fall below the available cooling water temperature. (Key point: Condenser pressure limit) 3. Where does the pressure drop occur in an impulse turbine? A. Only in the nozzles; pressure is constant over moving blades B. Equally in fixed and moving blades C. Only in the moving blades D. Only in the condenser Answer: A. In an impulse turbine the whole pressure drop occurs in the nozzles, and moving blades see constant pressure. (Key point: Pressure drop in impulse vs reaction) 4. What force acts on the blades of a reaction turbine? A. Impulse of the jet only B. Impulse plus reaction from steam accelerating in the blade passage C. Gravity only D. Centrifugal force only Answer: B. Steam expands in moving blades too, giving a reaction force in addition to the impulse. (Key point: Reaction turbine force) 5. Which statement is correct? A. Reaction turbines have fewer stages and partial admission B. Impulse turbines have symmetrical blades and may use partial admission; reaction turbines have aerofoil blades, full admission and more stages C. Both use identical blade profiles D. Impulse turbines have higher efficiency and more stages Answer: B. Impulse: symmetrical blades, nozzle arcs, fewer stages. Reaction: aerofoil blades, full admission, more stages, higher efficiency. (Key point: Stages and admission) ## Related in Power Plant Equipment - [Reheat Rankine cycle: 20 MPa, 500°C](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Arankine-reheat-20mpa) — Power Plant Equipment, chapter 7 - [Impulse versus reaction turbines](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aimpulse-vs-reaction) — Power Plant Equipment, chapter 7, asked 2× - [Throttled reheat cycle with 50 bar steam](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Athrottled-reheat-50-bar) — Power Plant Equipment, chapter 7 - [Open cycle gas turbine with intercooling](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-intercooling-sketch) — Power Plant Equipment, chapter 6 - [Open cycle gas turbine on heavy oil](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-heavy-oil-capacity) — Power Plant Equipment, chapter 6 - [Reheat cycle at 14 bar: ideal efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Areheat-14-bar-efficiency) — Power Plant Equipment, chapter 7 ## Related in other subjects - [Impulse and reaction steam turbines compared](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Aimpulse-reaction-steam-turbine-types) — Fluid Machines, chapter 5 - [Rankine cycle plots and efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Arankine-cycle-efficiency) — 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 - [Single-stage impulse turbine blade work and efficiency](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Ade-laval-turbine-velocity-diagram-numerical) — Fluid Machines, chapter 5