--- title: "Reheat gas turbine power plant" subject: "Power Plant Equipment" chapter: 6 key: "semester_7/power_plant_equipment:gt-reheat-plant" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/gt-reheat-plant site: BCT Engineering --- # Reheat gas turbine power plant Power Plant Equipment · Chapter 6: Gas Turbine Power Plant - Asked in: 2073 Chaitra, 6+2 marks - Video: [Reheating in Gas turbine](https://www.youtube.com/watch?v=rdY4qjlw7Nk) — Gear Institute Mechanical Engineering Videos - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/gt-reheat-plant - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-6 ## Question Explain the Reheat gas turbine power plant with neat sketch. What are the applications of reheat cycle over the open cycle gas turbine power plant? ## Short answer Reheat cycle expands gas in HP and LP turbines with reheating between them, raising net work and work ratio. Used in peak-load plants, aircraft afterburners, combined cycles, and where high specific output from a compact unit is needed. ## Answer In a **reheat gas turbine plant**, expansion is done in two turbines (HP and LP). After partial expansion in the HP turbine, the gas is heated again in a **reheat combustion chamber** to nearly the maximum temperature, and then expanded in the LP turbine. Because gas turbine exhaust contains a lot of excess air (A/F ratio 60–100), extra fuel can be burnt directly in the reheater. #### Sketch ```text Fuel Fuel | | 1 2 v 3 4 v 5 6 -->[C]-->[CC1]-->[HPT]-->[CC2]-->[LPT]--> exh || || || ++=====shaft===++======shaft===++==> Gen ``` #### T–s diagram ```text T | 3 5 | * * | /| /| | / | / | | / *----' * 6 | 2 * 4 | (6' without | | / reheat is | *--------' lower) | 1 +---------------------- s ``` 1–2 compression, 2–3 combustion, 3–4 HP expansion, 4–5 reheating at intermediate pressure, 5–6 LP expansion, 6–1 exhaust. #### Working and effect - Turbine work per kg is proportional to inlet temperature. Raising the gas from $T_4$ back to $T_5 \approx T_3$ makes the LP turbine produce more work than a single turbine would from 4 onward. Total turbine work $W_t = c_p(T_3 - T_4) + c_p(T_5 - T_6)$ increases. - Compressor work is unchanged, so **net work output and work ratio increase**. Maximum work occurs when the pressure ratios of the two turbines are equal: $p_4 = \sqrt{p_3 p_6}$. - Heat supplied also increases and exhaust temperature $T_6$ is higher, so reheat alone gives little or no gain in efficiency; with a **regenerator**, the hot exhaust is used to preheat the air and efficiency also rises. #### Applications / advantages of reheat over the simple open cycle - Where **high specific output** is needed from a compact, light plant: **peak-load and standby power stations**, mobile/emergency sets. - **Aircraft jet engines** (afterburning is a form of reheat to increase thrust for take-off and combat) and marine propulsion. - **Combined-cycle plants**: the hotter exhaust from a reheat turbine gives more heat to the HRSG, increasing steam-cycle output (e.g. sequential-combustion turbines). - Plants combined with regeneration (and intercooling) for high efficiency in base-load service. - Reduces the size of plant (air flow) for a given output, so lower capital cost per kW. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the reheat gas turbine arrangement? A. Air is heated before the compressor B. Expansion in HP and LP turbines with a second combustion chamber between them C. Compression in two stages with cooling between D. Exhaust gas preheats compressed air Answer: B. Gas partially expands in the HP turbine, is reheated by burning fuel in the excess air, then expands in the LP turbine. (Key point: Reheat arrangement) 2. Why does reheating increase net work output? A. Compressor work is reduced B. Turbine work rises because the LP turbine inlet is heated back to about T3, while compressor work is unchanged C. Mass flow increases D. Pressure ratio of compressor rises Answer: B. Turbine work is proportional to inlet temperature, so reheat adds work with the same compressor work. (Key point: Why net work increases) 3. For maximum turbine work, the intermediate pressure should be: A. Equal to the exhaust pressure B. Equal to the maximum pressure C. The square root of (p3 x p6), equal pressure ratios across both turbines D. Half of the maximum pressure by arithmetic mean Answer: C. Maximum work occurs when both turbines have equal pressure ratios: p4 = sqrt(p3 p6). (Key point: Optimum split) 4. What is the effect of reheat alone on efficiency, and where is it useful? A. It greatly increases efficiency in every plant B. It gives little or no efficiency gain, but is useful for high output in compact plants or with regeneration C. It always halves fuel use D. It decreases net work Answer: B. Extra fuel and hotter exhaust cancel the gain; with regeneration efficiency also improves, and the main benefit is high specific output. (Key point: Effect on efficiency and use) ## Related in Power Plant Equipment - [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 - [Gas turbine with intercooling, reheat and regeneration](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-intercool-reheat-regen) — Power Plant Equipment, chapter 6 - [Gas turbine with reheat and 90% regenerator](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-reheat-regenerator-1400k) — Power Plant Equipment, chapter 6 - [Gas turbine 6:1: power output and efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-6-1-power-efficiency) — Power Plant Equipment, chapter 6, asked 2× - [Gas turbine 6:1: generator power output](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-6-1-power-output) — Power Plant Equipment, chapter 6, 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 - [Actual gas turbine cycle with component efficiencies](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aactual-brayton-cycle-numerical) — Turbo Machines, chapter 3 - [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 - [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