--- title: "Working of combined gas-steam plant" subject: "Power Plant Equipment" chapter: 8 key: "semester_7/power_plant_equipment:combined-gas-steam-working" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-gas-steam-working site: BCT Engineering --- # Working of combined gas-steam plant Power Plant Equipment · Chapter 8: Combined Power Plant - Asked in: 2076 Asoj, 10 marks - Video: [Combined Gas Turbine - Vapor Power Plant (Theory & Problem Solving)](https://www.youtube.com/watch?v=ouOlKZURoO4) — Ahsan - 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-gas-steam-working - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8 ## Question Explain the working of combined gas-steam power plant with neat figure and also show the process on T-S diagram. ## Short answer Air is compressed, burnt with fuel, and expanded in the gas turbine driving G1; exhaust at 500-600 C enters the HRSG, where it heats feed water, evaporates and superheats steam. Steam expands in the steam turbine driving G2, is condensed, and pumped back; the T-s diagram shows Brayton cycle 1-2-3-4 on top and Rankine cycle c-d-a-b below. ## Answer A **combined power plant** (combined cycle plant) uses two thermodynamic cycles in series, so that the heat rejected by the first (**topping**) cycle becomes the heat input of the second (**bottoming**) cycle. The most common is the **gas turbine - steam turbine combined cycle**: a gas turbine (Brayton cycle, heat added at 1100-1500 C) exhausts at 450-600 C, and this exhaust raises steam for a steam turbine (Rankine cycle, heat rejected at 30-45 C). Overall efficiency of 50-60% is achieved, compared with 30-40% for each plant alone. #### Layout ```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 |<---+ +-----------+ ``` #### Components - **Gas turbine unit:** axial compressor, combustion chamber and gas turbine on one shaft with generator G1. - **Heat recovery steam generator (HRSG):** banks of economiser, evaporator (with drum) and superheater placed in the exhaust duct; may have a duct burner for supplementary firing. - **Steam unit:** steam turbine with generator G2, condenser, condensate and feed pumps, deaerator. #### 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. #### T-s diagram ```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) ``` Process summary: | Process | Gas cycle | Steam cycle | |---|---|---| | Compression / pumping | 1-2 compressor | c-d feed pump | | Heat addition | 2-3 combustion chamber | d-a in HRSG (from gas 4-5) | | Expansion | 3-4 gas turbine | a-b steam turbine | | Heat rejection | 4-5 to HRSG, 5 to stack | b-c condenser | #### Efficiency $$ \eta_{cc} = \frac{W_{GT} + W_{ST}}{Q_{fuel}} = \eta_{GT} + \eta_{ST} - \eta_{GT}\eta_{ST} $$ With $\eta_{GT} = 0.35$ and $\eta_{ST} = 0.30$: $\eta_{cc} = 0.35 + 0.30 - 0.105 = 0.545$, about 54.5%. Typical modern plants reach 55-60%. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In a gas-steam combined plant, how are the two cycles related? A. Heat rejected by the gas (topping) cycle becomes the heat input of the steam (bottoming) cycle B. Steam exhaust is fed into the gas turbine compressor C. Both cycles burn fuel separately and share only the generator D. Heat rejected by the steam cycle is the heat input of the gas cycle Answer: A. The Brayton (topping) cycle exhausts at 450-600 C and this heat raises steam for the Rankine (bottoming) cycle. (Key point: Topping and bottoming cycles) 2. What does the HRSG do in the combined cycle? A. Compresses air before the combustion chamber B. Uses the gas turbine exhaust to heat, evaporate and superheat feed water C. Condenses the steam leaving the steam turbine D. Burns fuel to drive the steam turbine directly Answer: B. The HRSG contains economiser, evaporator and superheater in the exhaust duct, converting exhaust heat into steam (d-a). (Key point: Role of the HRSG) 3. Which sequence correctly describes the steam cycle processes c-d, d-a, a-b, b-c? A. Condensation, pumping, turbine expansion, heating B. Heating in HRSG, pumping, condensation, expansion C. Pumping, heating in HRSG, expansion in steam turbine, condensation D. Expansion, heating, pumping, condensation Answer: C. Feed pump (c-d), HRSG heat addition (d-a), steam turbine expansion (a-b), then condenser (b-c). (Key point: Process sequence) 4. For eta_GT = 0.35 and eta_ST = 0.30, the combined efficiency eta_GT + eta_ST - eta_GT*eta_ST equals A. 0.65 B. 0.105 C. 0.545 D. 0.455 Answer: C. 0.35 + 0.30 - 0.105 = 0.545, about 54.5%. (Key point: Combined efficiency) ## Related in Power Plant Equipment - [Working and performance of combined plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-working-performance) — Power Plant Equipment, chapter 8 - [Combined plant components and T-s diagram](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-components-ts) — Power Plant Equipment, chapter 8, asked 4× - [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× - [Advantages of combined power plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-advantages) — Power Plant Equipment, chapter 8 - [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 - [Combined cycle advantages and working principle](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-advantages-working) — Power Plant Equipment, chapter 8, asked 2× ## 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 - [Operational characteristics of combined cycle plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Accgt-operational-characteristics) — Power Plant Design, chapter 1 - [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× - [Three-process gas cycle: net work and heat](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Athree-process-cycle-pv-const-800kpa) — Fundamental of Thermodynamics & Heat Transfer, chapter 4