--- title: "Three popular combined cycle designs" subject: "Power Plant Equipment" chapter: 8 key: "semester_7/power_plant_equipment:combined-three-designs" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-three-designs site: BCT Engineering --- # Three popular combined cycle designs Power Plant Equipment · Chapter 8: Combined Power Plant - Asked in: 2080 Baisakh, 4+4 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-three-designs - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8 ## Question Draw the three popular designs of the combined steam and gas turbine cycle. Explain one of them with T-s diagram. ## Short answer The three designs are: unfired or supplementary fired heat recovery boiler (HRSG), exhaust used as combustion air in a fired boiler, and exhaust used for feed water heating. In the HRSG design, gas turbine exhaust at 500-600 C raises all steam; overall efficiency is 50-60%. ## Answer The three popular ways of combining a gas turbine with a steam cycle are: (1) unfired/supplementary fired heat recovery boiler, (2) exhaust used as combustion air in a fired boiler, and (3) exhaust used for feed water heating. #### Design 1: Unfired (or supplementary fired) heat recovery boiler (HRSG) ```text air 1 fuel | | v v +------+ 2 +-----------+ 3 +------+ | Comp |---------->| Combustion|------>| GT |===> G1 +------+ | chamber | +------+ || +-----------+ || | ++=============== shaft =============++ | 4 hot exhaust v (~500-600 C) stack <-- 5 --+-------------------------------+ | HRSG (economiser, | | evaporator, superheater) | +-------------------------------+ ^ d | a steam | v +------+ +------+ | Pump | | ST |===> G2 +------+ +------+ ^ c | b | +-----------+ | +---| Condenser |<---+ +-----------+ ``` The GT exhaust raises all the steam in a heat recovery boiler. A duct burner may add supplementary fuel. This is the most common modern design (efficiency 50-60%). #### Design 2: Exhaust used as combustion air in a fired boiler ```text air fuel |1 | v v [C]--2-->[CC]--3-->[GT]==> G1 | 4 hot exhaust (~15% O2) v used as combustion air fuel --> +-----------------+ a +----+ | Fired steam |---->| ST |==> G2 stack <----| boiler | +----+ +-----------------+ | b ^ d v | +------+ c +-----------+ +--| Pump |<----| Condenser | +------+ +-----------+ ``` The hot, oxygen-rich exhaust replaces the air preheater and forced draft air of a conventional fired boiler. Steam turbine output is large compared to gas turbine output. #### Design 3: Exhaust heats the 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 | +-----------------+ ``` The exhaust heats the condensate/feed water of a conventional steam plant, replacing bled-steam feed heaters; the boiler supplies the rest of the heat. #### Explanation of design 1 (heat recovery boiler) with T-s diagram Working: 1. Air is compressed in the compressor (1-2), burnt with fuel in the combustion chamber (2-3) and expanded in the gas turbine (3-4), which drives the compressor and generator G1. 2. Exhaust gas at about 500-600 C enters the **HRSG**. Here it heats the feed water in the economiser, evaporates it in the evaporator and superheats it in the superheater (d-a). The cooled gas goes to the stack at about 100-150 C (4-5). 3. Steam expands in the **steam turbine** (a-b) and drives generator G2. 4. Exhaust steam is condensed (b-c) and pumped back to the HRSG (c-d). 5. Optionally, a duct burner before the HRSG burns extra fuel (supplementary firing) to raise steam output. Heat added at high temperature in the gas cycle and rejected at low temperature in the steam condenser gives a high overall efficiency. ```text T ^ 3 | /| | / | Gas cycle 1-2-3-4 | / | (Brayton, top) | / | | 2 | | | 4 | a | / 4-5: exhaust heats | /| | 5 steam (d-a) in HRSG | / | | / 5-1: gas to stack | d | |/ | | | 1 | c--b Steam cycle c-d-a-b | (Rankine, bottom) +--------------------------> s (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. Which list gives the three popular combined cycle designs? A. HRSG, exhaust as combustion air in fired boiler, exhaust heating feed water B. Reheat, regeneration, supercritical C. Pelton, Francis, Kaplan D. Diesel, petrol, gas Answer: A. These are the three arrangements of gas turbine exhaust use described. (Key point: Three designs) 2. What is the order of steam cycle processes in design 1? A. c-d pump, d-a heating in HRSG, a-b turbine, b-c condenser B. a-b pump, b-c boiler, c-d turbine C. c-d condenser, d-a turbine D. d-a pump only Answer: A. Water is pumped, heated and evaporated/superheated in the HRSG, expanded and condensed. (Key point: Steam cycle in HRSG) 3. At what approximate temperature does the gas leave the HRSG to the stack? A. 100-150 C B. 500-600 C C. 900 C D. 10 C Answer: A. After giving heat to the steam cycle the gas is cooled to about 100-150 C. (Key point: Stack temperature) 4. On the T-s diagram of design 1, what happens along 4-1? A. Hot exhaust gives heat to the HRSG B. The compressor raises pressure C. Steam is condensed D. Fuel is burnt Answer: A. 4-1 is the heat rejection of the gas cycle, supplied to the HRSG. (Key point: T-s meaning) ## Related in Power Plant Equipment - [Combined cycle advantages and three designs](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-advantages-designs) — Power Plant Equipment, chapter 8, asked 2× - [Combined cycle merits, demerits and designs](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-merits-designs) — Power Plant Equipment, chapter 8, asked 2× - [Combined plant introduction and popular designs](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-intro-designs) — 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 - [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 ## 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 - [Closed gas turbine cycle components; thermal efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aclosed-gas-turbine-cycle-efficiency) — Fundamental of Thermodynamics & Heat Transfer, chapter 6 - [Three-process gas cycle: net work, heat, type](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Athree-process-cycle-pv-sketch) — Fundamental of Thermodynamics & Heat Transfer, chapter 4, asked 2× - [Modifications of the basic gas turbine cycle](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Avariations-basic-gas-turbine-cycle) — Turbo Machines, chapter 6