--- title: "Open cycle gas turbine with intercooling" subject: "Power Plant Equipment" chapter: 6 key: "semester_7/power_plant_equipment:gt-intercooling-sketch" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/gt-intercooling-sketch site: BCT Engineering --- # Open cycle gas turbine with intercooling Power Plant Equipment · Chapter 6: Gas Turbine Power Plant - Asked in: 2080 Baisakh, 8 marks - Video: [Open Cycle Gas Turbine with Intercooling - Gas Turbines - Power Engineering](https://www.youtube.com/watch?v=j8QOxVz_2fE) — Ekeeda - 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-intercooling-sketch - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-6 ## Question Draw a sketch of a simple open cycle constant pressure gas turbine power plant with intercooling and describe the method to improve its thermal efficiency considering intercooling and T-s diagram. ## Short answer Intercooling splits compression into LP and HP stages with cooling between them, reducing compressor work because work is proportional to inlet temperature. Optimum intermediate pressure is sqrt(p1 p2). It increases net work and work ratio; combined with regeneration it noticeably raises thermal efficiency. ## Answer A **simple open-cycle constant-pressure gas turbine** draws atmospheric air, compresses it, burns fuel in it at constant pressure, expands the hot gas in a turbine and exhausts it to the atmosphere (Brayton/Joule cycle). With **intercooling**, compression is split into LP and HP stages with an air cooler between them. #### Sketch of the plant ```text water in/out || Air->[LPC]-2->[ IC ]-3->[HPC]-4->[CC]-5->[ T ]-6-> 1 || || ^ || exh ++======shaft======++======|=======++==>Gen fuel ``` LPC = low-pressure compressor, IC = intercooler, HPC = high-pressure compressor, CC = combustion chamber, T = turbine. #### T–s diagram ```text T | 5 | /| | / | | 4 / | | |---' | | | 2 6 | | /| / | | / | / | 3 1--' +----------------------- s 1-2 LPC, 2-3 intercooling at p_i, 3-4 HPC, 4-5 combustion at p_2, 5-6 turbine, 6-1 exhaust (p_1) ``` #### How intercooling improves the plant - Compressor work per kg is $W = c_p T_{in}\left[r^{(\gamma-1)/\gamma} - 1\right]$, i.e. proportional to inlet temperature. Cooling the air from $T_2$ back to $T_3 \approx T_1$ at the intermediate pressure makes the HP stage work much smaller. - Total compressor work is **minimum** when intercooling is perfect ($T_3 = T_1$) and the stage pressure ratios are equal: $p_i = \sqrt{p_1 p_2}$. - Turbine work is unchanged, so **net work and work ratio increase**; a smaller plant gives the same output. - Because air now leaves the HPC colder ($T_4$ lower), more fuel is needed in 4–5, so intercooling alone gives only a small change in efficiency. When combined with a **regenerator**, the larger gap between exhaust temperature $T_6$ and $T_4$ allows more heat recovery, and **thermal efficiency rises noticeably**. - Other ways to raise efficiency: regeneration, reheating, higher turbine inlet temperature, optimum pressure ratio and better component efficiencies. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Where is the intercooler placed? A. After the turbine B. Before the LP compressor C. In the combustion chamber D. Between the LP and HP compressors Answer: D. Air is cooled between compression stages. (Key point: Plant layout) 2. Total compressor work is minimum when A. Cooling is perfect (T3=T1) and stage pressure ratios are equal B. HP ratio is largest C. No cooling is done D. Stage ratios differ widely Answer: A. p_i = sqrt(p1 p2). (Key point: Minimum work) 3. Why does intercooling alone change efficiency little? A. Turbine work drops B. Colder HPC exit needs more fuel in 4-5 C. Compressor work rises D. Mass flow falls Answer: B. Net work rises but heat added rises too. (Key point: Effect on fuel) 4. Why does intercooling help with a regenerator? A. It lowers turbine temperature B. It removes the need for fuel C. Larger gap between exhaust T6 and T4 allows more heat recovery D. It raises T4 Answer: C. Thermal efficiency then rises noticeably. (Key point: With regenerator) ## Related in Power Plant Equipment - [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 - [Methods to improve gas turbine efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-efficiency-methods) — Power Plant Equipment, chapter 6, asked 2× - [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 - [Gas turbine improvement methods and intercooling](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-methods-intercooling) — Power Plant Equipment, chapter 6, asked 3× ## Related in other subjects - [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 - [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 - [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