--- title: "Waste heat methods and feed water heating layout" subject: "Power Plant Equipment" chapter: 8 key: "semester_7/power_plant_equipment:combined-waste-heat-feedwater" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-waste-heat-feedwater site: BCT Engineering --- # Waste heat methods and feed water heating layout Power Plant Equipment · Chapter 8: Combined Power Plant - Asked in: 2082 Baisakh, 2+3+3 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-waste-heat-feedwater - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8 ## Question Briefly discuss the common methods employed to utilize waste heat in combined power plants. Sketch the layout of a combined power plant in which waste heat of gas turbine outlet is used to heat feed water. Explain its working with processes on T-s diagram. ## Short answer GT exhaust passes through a gas-to-water feed water heater, heating condensate to near boiling and replacing bled-steam heaters. The hot feed water then enters a fired steam boiler, increasing overall efficiency by about 3–5%. ## Answer #### Methods of utilising waste heat Common methods of utilising the gas turbine exhaust (waste) heat: 1. **Heat recovery steam generator (HRSG), unfired:** exhaust passes through an economiser, evaporator and superheater and raises steam for a steam turbine. Simplest and most common; steam turbine gives about one-third of the plant output. 2. **Supplementary fired HRSG:** extra fuel is burnt in a duct burner in the exhaust (it still contains about 15-16% O2) to raise gas temperature and steam output/temperature; increases output and flexibility. 3. **Exhaust-fired (fully fired) boiler:** GT exhaust is used as hot **combustion air** for a conventional fired steam boiler, replacing the air preheater. 4. **Feed water heating:** exhaust heats the boiler feed water in a gas-to-water heat exchanger (economiser), replacing some or all bled-steam feed heaters of a steam plant. 5. **Pressurised (supercharged) boiler:** combustion takes place under pressure in the boiler furnace; after raising steam, the gases expand in a gas turbine that drives the compressor. 6. **Cogeneration / process heat:** exhaust or exhaust-raised steam is used for industrial process heating, district heating or absorption cooling. 7. **Regenerator / recuperator:** exhaust preheats the compressed air of the gas turbine itself (regenerative gas turbine). #### Layout: gas turbine exhaust used to heat feed water ```text air fuel | 1 | v v [C]--2-->[ CC ]--3-->[GT]==> G1 || | 4 exhaust gas ++=====shaft=========+ v +--------------------------+ stack <-5| Feed water heater (gas | | to water heat exchanger) | +--------------------------+ ^ d | e hot feed water | v +------+ +----------+ a +----+ | Pump | | Steam |---->| ST |==> G2 +------+ | boiler | +----+ ^ c +----------+ | b | ^ fuel v | +-----------------+ +-----------| Condenser | +-----------------+ ``` Working: 1. Air is compressed (1-2), burnt with fuel in the combustion chamber (2-3) and expands in the gas turbine (3-4), driving the compressor and generator G1. 2. The hot exhaust (about 450-550 C) passes through the **feed water heater** (an economiser-type gas-to-water heat exchanger) and leaves to the stack at 5. 3. Condensate from the steam plant is pumped (c-d) and heated by the exhaust gas (d-e) nearly up to the boiling temperature. This replaces the bled-steam feed heaters, so steam that would have been extracted now expands fully in the turbine and produces extra power. 4. The hot feed water enters the conventional **fired steam boiler**, where fuel supplies the latent heat and superheat (e-a). 5. Steam expands in the steam turbine (a-b) driving G2, is condensed (b-c) and the cycle repeats. The steam boiler is still fully fired, so this scheme gives a modest gain (about 3-5% points) and is often used to **repower** an existing steam plant by adding a gas turbine. #### T-s diagram ```text T ^ 3 | /| | / | gas cycle 1-2-3-4, | 2 | exhaust 4-5 heats | | 4 feed water d-e | | / | |5 | 1 a | f---------g /| | e | | / | | d | | c---------------b +---------------------------> s c-d pump, d-e heating by GT exhaust, e-f-g-a heating in fired boiler, a-b steam turbine, b-c condenser (gas and steam drawn per kg of their own fluid, schematic only) ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the scheme where gas turbine exhaust heats feed water, what does the exhaust replace? A. The steam turbine B. The bled-steam feed heaters C. The condenser D. The compressor Answer: B. The gas-to-water heat exchanger replaces some or all bled-steam feed heaters. (Key point: Feed water heating method) 2. Why does this scheme give extra steam turbine power? A. Steam that would have been bled now expands fully in the turbine B. The pump is removed C. The boiler is no longer fired D. The exhaust drives the steam turbine directly Answer: A. Without bleeding, all the steam expands to condenser pressure and produces more work. (Key point: Extra power) 3. After the exhaust heats feed water (d-e), what does the fired boiler supply? A. Latent heat and superheat (e-a) B. Only pump work C. Compressor air D. Condensation Answer: A. Feed water is brought near boiling by the exhaust; the fuel-fired boiler provides evaporation and superheating. (Key point: Boiler role) 4. What gain does this scheme give and what is it often used for? A. About 3-5 percentage points; repowering an existing steam plant B. About 30 points; new nuclear plants C. No gain; peak shaving only D. About 50 points; hydro plants Answer: A. Because the steam boiler is still fully fired, the gain is modest, and the scheme is used to repower existing steam plants. (Key point: Gain and use) ## Close variants set in other exams - [Combined plant heating feed water with exhaust](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-feedwater-layout) — Power Plant Equipment, chapter 8, asked 2× ## Related in Power Plant Equipment - [Methods to utilize waste heat in combined plants](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-waste-heat-methods) — Power Plant Equipment, chapter 8, asked 2× - [Feed water heating combined circuit and reasons](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-feedwater-reasons) — Power Plant Equipment, chapter 8 - [Working of combined gas-steam plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-gas-steam-working) — Power Plant Equipment, chapter 8 - [Combined cycle components and waste heat use](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-waste-heat-utilization) — Power Plant Equipment, chapter 8 - [Regenerative scheme with open feed water heater](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aregenerative-open-heater-layout) — Power Plant Equipment, chapter 7 - [Reheat gas turbine power plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-reheat-plant) — Power Plant Equipment, chapter 6 ## Related in other subjects - [Combined cycle gas turbine versus hydro](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Accgt-vs-hydro) — Power Plant Design, chapter 1 - [Gas turbine 60 MW: heat loss and areas](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Agas-turbine-60mw-heat-loss-areas) — Fundamental of Thermodynamics & Heat Transfer, chapter 4