--- title: "Combined plant introduction and popular designs" subject: "Power Plant Equipment" chapter: 8 key: "semester_7/power_plant_equipment:combined-intro-designs" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-intro-designs site: BCT Engineering --- # Combined plant introduction and popular designs Power Plant Equipment · Chapter 8: Combined Power Plant - Asked in: 2081 Bhadra, 2+6 marks - 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-intro-designs - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8 ## Question Give short introduction about a combined power plant. Also draw the layout with necessary labels for popular designs of combined power plant. ## Short answer A combined cycle plant integrates a gas turbine topping cycle with a steam turbine bottoming cycle, reaching 50–60% efficiency. Popular designs: unfired/supplementary fired HRSG; exhaust as combustion air for a fired boiler; exhaust used to heat feed water. ## Answer #### Introduction 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. Main parts: compressor, combustion chamber, gas turbine, heat recovery boiler or fired boiler, steam turbine, condenser, feed pump and two generators. The arrangement depends on how the exhaust heat is used; the three popular designs are shown below. #### 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the combined plant's basic idea? A. Heat rejected by the topping cycle is used as heat input for the bottoming cycle B. Both cycles reject heat to the same stack C. A steam plant is run twice D. Gas is compressed in the steam turbine Answer: A. Two cycles are in series, so rejected heat from the first is reused. (Key point: Definition) 2. In Design 2, how is the gas turbine exhaust used? A. As hot oxygen-rich combustion air for a fired boiler, replacing the air preheater B. To heat only the condenser C. To compress air D. As fuel Answer: A. Exhaust with about 15 % O2 supports combustion in the fired boiler. (Key point: Design 2 feature) 3. Which design is the most common modern one, reaching 50-60 % efficiency? A. Unfired or supplementary fired HRSG B. Exhaust heating feed water only C. Pressurised boiler only D. Diesel-steam combination Answer: A. The HRSG design raises all steam from exhaust heat and is the most common. (Key point: Design 1 feature) ## Related in Power Plant Equipment - [Three popular combined cycle designs](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-three-designs) — Power Plant Equipment, chapter 8 - [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 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 plant layouts, performance and economics](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-layout-economics) — Power Plant Equipment, chapter 8 - [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× ## Related in other subjects - [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 - [Development cycle; storage plant layout](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Adev-cycle-storage-layout) — Hydropower Engineering, chapter 2 - [Flow patterns in plant layout](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_engineering_and_management%3Aflow-patterns-in-production-layout) — Industrial Engineering and Management, chapter 2 - [Components of a hydroelectric power plant](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Ahydroelectric-plant-components) — Fluid Machines, chapter 2