--- title: "Steam plant improvement methods and regeneration" subject: "Power Plant Equipment" chapter: 7 key: "semester_7/power_plant_equipment:steam-methods-regeneration" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/steam-methods-regeneration site: BCT Engineering --- # Steam plant improvement methods and regeneration Power Plant Equipment · Chapter 7: Thermal (Steam) Power Plant - Asked in: 2069 Chaitra, 8 marks; 2073 Chaitra, 8 marks (asked 2 times) - Video: [Improvements of Steam Power Plant](https://www.youtube.com/watch?v=eVUjBTZgAxg) — Engr. Daniyal - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/steam-methods-regeneration - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-7 ## Question List the common methods used for the performance improvement of the steam turbine power plants. Explain how regeneration increases efficiency of the plant. ## Short answer Common methods: raising steam pressure/temperature, lowering condenser pressure, reheating, regeneration, using economizer/air preheater. Regeneration extracts steam from turbine stages to preheat feedwater, raising the mean temperature of heat addition, reducing heat rejection, and net work reduction is less than heat saved, improving efficiency by 10–15%. ## Answer #### Methods of improving performance of a steam power plant The efficiency of a Rankine (steam) cycle is $\eta = 1 - \dfrac{T_{\text{low}}}{T_{m,\text{in}}}$ roughly, so it improves by **raising the mean temperature of heat addition** $T_{m}$ or **lowering the temperature of heat rejection**. Common methods: 1. **Increasing boiler (steam) pressure** - raises the saturation temperature at which heat is added. 2. **Superheating the steam** (higher turbine inlet temperature) - raises mean temperature of heat addition and dryness at exhaust. 3. **Reducing condenser pressure** (better vacuum) - lowers heat rejection temperature. 4. **Reheating** - steam partly expanded in HP turbine is reheated in the boiler and expanded in LP turbine. 5. **Regenerative feed water heating** - steam bled from the turbine heats the feed water in feed water heaters. 6. **Using economiser and air preheater** - recover heat from flue gases. 7. **Supercritical / binary vapour and combined cycles** - use very high pressure, or a topping cycle (gas turbine) whose waste heat runs the steam cycle. 8. Reducing losses: better turbine blading, insulation, reduced throttling. #### How regeneration increases efficiency ```text 1 +--------+------>+---------------+ +-----+ | Boiler | | Turbine |===| Gen | | + S/H | +---------------+ +-----+ +--------+ | m kg (2) | (1-m) kg (3) ^ 7 v v | +-----------+ +-----------+ +--------+ 6 | Open FWH | | Condenser | | Pump 2 |<---| (mixing) | +-----------+ +--------+ +-----------+ | 4 ^ 5 | +--------+ | | Pump 1 |<--------+ +--------+ ``` **Regeneration** means heating the feed water, before it enters the boiler, with steam **bled (extracted)** from the turbine at one or more intermediate stages. In the simple Rankine cycle the cold feed water (about 40 °C) enters the boiler and is heated by the hot flue gases over a large temperature range. This low-temperature part of the heat addition lowers the mean temperature of heat addition and so lowers efficiency. In the regenerative cycle a fraction $m$ kg of steam per kg is bled from the turbine at state 2 and used to heat the feed water in a **feed water heater (FWH)**. The remaining $(1-m)$ kg expands to the condenser. Why efficiency increases: - The feed water enters the boiler hotter (state 7 instead of 5), so the heat supplied in the boiler falls from $(h_1 - h_5)$ to $(h_1 - h_7)$, and all of it is added at a higher mean temperature. In the limit (infinite heaters) the cycle approaches the Carnot efficiency. - The **latent heat of the bled steam is not rejected in the condenser**; it is returned to the feed water. Heat rejected per kg of boiler steam is only $(1-m)(h_3 - h_4)$. - Work output per kg falls slightly (bled steam does not expand fully), but heat supplied falls more, so the ratio $W/Q$ rises (typically by 10-15% with several heaters). - Other benefits: less thermal stress in the boiler, smaller condenser and LP blade height (less steam flow at the exhaust), deaeration in an open heater. Efficiency (one open heater): $$ \eta = \frac{(h_1 - h_2) + (1-m)(h_2 - h_3) - W_p}{h_1 - h_7}, \qquad m = \frac{h_6 - h_5}{h_2 - h_5} $$ ```text T ^ 1 | /| | f----------g | | / | | 7 | | 6 . . . . . . . 2 bleed (m kg) | / | | 5 | | 4-----------------3 (1-m) kg +-------------------------> s 4-5 pump 1, 5 and 2 mix in FWH -> 6, 6-7 pump 2, 7-f-g-1 boiler, 1-2-3 turbine, 3-4 condenser ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Efficiency of a Rankine cycle is improved by which of the following? A. Lowering the mean temperature of heat addition B. Raising the mean temperature of heat addition or lowering heat rejection temperature C. Raising the condenser pressure D. Reducing the boiler pressure Answer: B. Efficiency is roughly 1 - T_low/T_m,in, so a higher mean heat-addition temperature or a lower rejection temperature raises it. (Key point: Principle of improving Rankine efficiency) 2. Why does reducing the condenser pressure (better vacuum) improve plant performance? A. It raises the boiler temperature B. It lowers the heat rejection temperature C. It increases the moisture in the feed water D. It removes the need for a feed pump Answer: B. A lower condenser pressure means a lower saturation temperature at which heat is rejected, so efficiency rises. (Key point: Condenser pressure) 3. What is regeneration in a steam power plant? A. Reheating HP turbine exhaust in the boiler B. Heating feed water with steam bled from the turbine C. Recovering heat from flue gases in the economiser D. Raising steam to supercritical pressure Answer: B. Regeneration heats the feed water before the boiler using steam extracted at intermediate turbine stages. (Key point: What regeneration is) 4. Which is the main reason regeneration increases cycle efficiency? A. Work output per kg of steam rises sharply B. The latent heat of bled steam is not rejected in the condenser and feed water enters the boiler hotter C. The condenser pressure is lowered D. Boiler pressure is increased Answer: B. Bled steam's latent heat goes to the feed water, cutting heat rejection and the low-temperature part of heat addition, so mean heat-addition temperature rises. (Key point: Why regeneration raises efficiency) 5. With bleeding, work per kg of boiler steam falls slightly, yet efficiency rises. Why? A. Heat supplied falls by a larger proportion than work B. Pump work becomes zero C. Heat supplied increases more than work D. Turbine isentropic efficiency increases Answer: A. Heat supplied falls from (h1-h5) to (h1-h7), a larger relative drop than the work lost, so W/Q increases. (Key point: Work versus heat in regeneration) ## Related in Power Plant Equipment - [Gas turbine improvement methods and regeneration](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-methods-regeneration) — Power Plant Equipment, chapter 6 - [Steam plant improvement methods and reheating](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asteam-methods-reheating) — Power Plant Equipment, chapter 7, asked 4× - [Steam plant components and performance improvement](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asteam-components-performance) — Power Plant Equipment, chapter 7 - [Regeneration in steam plant with P-V and T-S](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asteam-regeneration-pv-ts) — Power Plant Equipment, chapter 7 - [Steam improvement methods, reheating, layout and T-S](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asteam-methods-reheat-layout) — Power Plant Equipment, chapter 7, asked 4× - [Gas turbine improvement methods and reheating](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-methods-reheat) — Power Plant Equipment, chapter 6 ## Related in other subjects - [Adiabatic steam turbine 50 MW: mass flow, isentropic efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Asteam-turbine-5mpa-500c-50mw) — Fundamental of Thermodynamics & Heat Transfer, chapter 5 - [Adiabatic steam turbine 70% isentropic efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aadiabatic-steam-turbine-70-percent-moisture) — Fundamental of Thermodynamics & Heat Transfer, chapter 5 - [Steam turbine 10 MPa, 550°C: isentropic efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Asteam-turbine-10mpa-550c-isentropic-eff) — Fundamental of Thermodynamics & Heat Transfer, chapter 5 - [Simple Rankine plant 2 MPa / 20 kPa: efficiency, power](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Arankine-2mpa-20kpa-50kgs) — Fundamental of Thermodynamics & Heat Transfer, chapter 6