--- title: "Gas turbine improvement methods and intercooling" subject: "Power Plant Equipment" chapter: 6 key: "semester_7/power_plant_equipment:gt-methods-intercooling" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/gt-methods-intercooling site: BCT Engineering --- # Gas turbine improvement methods and intercooling Power Plant Equipment · Chapter 6: Gas Turbine Power Plant - Asked in: 2082 Baisakh, 4+4 marks; 2074 Asoj, 8 marks; 2070 Asar, 8 marks (asked 3 times) - Video: [Methods to increase thermal efficiency of gas turbines, by regeneration, intercooling, reheating](https://www.youtube.com/watch?v=wYWzQJE8pcc) — AB CLASSES - 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-methods-intercooling - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-6 ## Question What are the methods of efficiency improvement in gas turbine power plant? Also describe how intercooling improves the efficiency of the plant with the help of a layout and T–S diagram. ## Short answer Methods: regeneration, intercooling, reheating, higher turbine inlet temperature, higher pressure ratio, combined cycles. Intercooling splits compression into stages with intercooler between, reducing compressor work and increasing net output; with regeneration it also raises efficiency. ## Answer The thermal efficiency of a simple open-cycle gas turbine is low (about 20–30 %) because the compressor absorbs a large part of the turbine work (60–70 %) and the exhaust leaves at a high temperature. Its efficiency and output are improved by the following methods. #### Methods of efficiency improvement 1. **Regeneration (heat exchanger):** hot turbine exhaust preheats the compressed air before the combustion chamber, so less fuel is needed for the same turbine inlet temperature. 2. **Intercooling:** compression is done in stages with cooling of air between stages; compressor work falls. 3. **Reheating:** expansion is done in stages; gas is reheated between turbines, raising turbine work. 4. **Higher turbine inlet temperature** using better blade materials and blade cooling. 5. **Higher (optimum) pressure ratio** with high component (isentropic) efficiencies. 6. **Combined cycle / waste-heat recovery:** exhaust heat raises steam for a steam turbine. 7. Water or steam injection, and reducing pressure losses in ducts and combustor. #### Intercooling In intercooling the air is compressed in a low-pressure (LP) compressor, cooled in an **intercooler** at nearly constant pressure back to (ideally) the inlet temperature, and then compressed in a high-pressure (HP) compressor. ```text Air +-----+ +-------+ +-----+ +----+ -1-->| LPC |-2>| Inter |-3>| HPC |-4>| CC | +-----+ | cooler| +-----+ +----+ | +-------+ | |5 +========= shaft =====+====+---+--+ | T |-6-> exhaust Gen <======+------+ ``` ```text T | 5 | /| | / | | 4 / | | |---' | | | 2 6 | | /| / | | / | / | 3 1--' +----------------------- s 1-2 LP comp, 2-3 intercool, 3-4 HP comp, 4-5 heating, 5-6 turbine, 6-1 exhaust ``` **How it improves the plant:** - Work of compression is proportional to the absolute temperature of the air ($W_c = c_p T_{in}[r_p^{(\gamma-1)/\gamma} - 1]$). Cooling the air to $T_3 \approx T_1$ before the HP stage reduces total compressor work. With perfect intercooling and equal stage pressure ratios ($r_{p1} = r_{p2} = \sqrt{r_p}$), compressor work is minimum. - Turbine work is unchanged, so **net work output and work ratio increase** considerably. - However, the air leaves the HP compressor colder ($T_4 < T_2'$ of single-stage), so **more fuel** must be burnt from 4 to 5. Alone, intercooling may give only a small increase (or even a decrease) in efficiency. - When intercooling is used **with a regenerator**, the lower compressor delivery temperature allows more exhaust heat to be recovered, and thermal efficiency increases appreciably. Thus intercooling mainly raises specific output and, combined with regeneration, raises thermal efficiency. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does a regenerator raise efficiency? A. It cools the air before the compressor B. It raises turbine work C. It adds a second turbine D. Exhaust heat preheats compressed air, so less fuel is needed Answer: D. Fuel supplied for the same turbine inlet temperature falls. (Key point: Regeneration) 2. What does intercooling do to compressor work? A. Reduces it, because work is proportional to inlet temperature B. Increases it C. Leaves it unchanged D. Reduces turbine work Answer: A. Cooling between stages lowers the HP stage inlet temperature. (Key point: Intercooling effect) 3. Why may intercooling alone give little efficiency gain? A. Turbine work falls B. Air enters the combustor colder so more fuel is needed C. Compressor work rises D. Pressure ratio drops Answer: B. Net work and work ratio rise but extra fuel offsets the gain. (Key point: Intercooling alone) 4. Minimum compressor work with perfect intercooling needs A. LP stage at the full ratio B. HP ratio double the LP C. Equal stage pressure ratios, each sqrt(rp) D. Ratio of 1 for HP Answer: C. Intermediate pressure = sqrt(p1 p2). (Key point: Optimum stage ratio) ## 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 - [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 - [How a regenerator improves gas turbine output](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-regenerator-explain) — Power Plant Equipment, chapter 6 - [Gas turbine improvement methods and regeneration](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-methods-regeneration) — 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 5:1: power, efficiency, work ratio](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-5-1-work-ratio) — Power Plant Equipment, chapter 6 ## Related in other subjects - [Gas turbine power from isentropic efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Agas-turbine-power-isentropic-efficiency) — Turbo Machines, chapter 1 - [Ideal gas turbine 100 kW: pressure ratio, work, efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aideal-gas-turbine-100kw-pressure-ratio) — Fundamental of Thermodynamics & Heat Transfer, chapter 6 - [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 - [Simple Rankine plant 2 MPa / 20 kPa: efficiency, power](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Arankine-2mpa-20kpa-50kgs) — Fundamental of Thermodynamics & Heat Transfer, chapter 6