--- title: "Combined plant components and T-s diagram" subject: "Power Plant Equipment" chapter: 8 key: "semester_7/power_plant_equipment:combined-components-ts" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-components-ts site: BCT Engineering --- # Combined plant components and T-s diagram Power Plant Equipment · Chapter 8: Combined Power Plant - Asked in: 2069 Chaitra, 4 marks; 2076 Chaitra, 4 marks; 2074 Asoj, 4 marks; 2070 Chaitra, 4 marks (asked 4 times) - Video: [Combined Steam And Gas Turbine Power Plant](https://www.youtube.com/watch?v=uD0kH-0HME8) — 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/combined-components-ts - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8 ## Question Sketch the basic components of a combined power plant. Also sketch the corresponding processes on T-s diagram. ## Short answer A combined cycle couples a gas turbine topping cycle with a steam turbine bottoming cycle using a heat recovery steam generator. The T-s diagram shows the Brayton and Rankine cycles, with exhaust heat transferred to steam, achieving 50–60% overall efficiency. ## Answer A **combined cycle power plant** joins a gas turbine (Brayton) cycle as the **topping cycle** and a steam turbine (Rankine) cycle as the **bottoming cycle**. The hot gas turbine exhaust (about 500-600 C) raises steam in a **heat recovery steam generator (HRSG)**. #### Basic components ```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 |<---+ +-----------+ ``` - **Gas turbine unit:** compressor (Comp), combustion chamber, gas turbine (GT), generator G1. - **HRSG / waste heat boiler:** economiser, evaporator and superheater heated by GT exhaust (sometimes with supplementary firing). - **Steam unit:** steam turbine (ST), generator G2, condenser, feed pump. #### 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) ``` The heat rejected by the gas cycle in the HRSG (4-5) is used as heat input to the steam cycle, so the overall efficiency (about 50-60%) is higher than either cycle alone. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In a gas turbine - steam turbine combined plant, which cycle is the topping cycle? A. Rankine steam cycle B. Brayton gas turbine cycle C. Diesel cycle D. Regenerative steam cycle Answer: B. The gas turbine (Brayton) works at the highest temperature, so it is the topping cycle; the steam (Rankine) cycle is the bottoming cycle. (Key point: Topping and bottoming cycles) 2. What is the function of the heat recovery steam generator (HRSG)? A. It compresses the air for the gas turbine B. It uses hot gas turbine exhaust to raise steam for the steam turbine C. It condenses the exhaust steam D. It burns fuel to drive the generator directly Answer: B. The HRSG (economiser, evaporator, superheater) takes heat from the 500-600 C exhaust to generate steam. (Key point: Role of HRSG) 3. On the T-s diagram, what does process 4-1 of the gas cycle represent in a combined plant? A. Heat rejected from the gas cycle, used as heat input to the steam cycle B. Compression in the compressor C. Combustion at constant pressure D. Expansion in the steam turbine Answer: A. The gas cycle's rejected heat (4-1) is given to the HRSG and becomes heat input for the steam cycle. (Key point: T-s link between cycles) ## Related in Power Plant Equipment - [Advantages of combined power plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-advantages) — Power Plant Equipment, chapter 8 - [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× - [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 - [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 introduction and popular designs](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-intro-designs) — Power Plant Equipment, chapter 8 ## Related in other subjects - [Components of a hydroelectric power plant](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Ahydroelectric-plant-components) — Fluid Machines, chapter 2 - [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 - [Basic psychrometric processes](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Abasic-psychrometric-processes) — Applied Thermodynamics, chapter 4 - [Components of a run-of-river plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aror-components-functions) — Power Plant Design, chapter 3, asked 2×