--- title: "Open and closed Brayton cycles" subject: "Power Plant Equipment" chapter: 6 key: "semester_7/power_plant_equipment:brayton-open-closed" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/brayton-open-closed site: BCT Engineering --- # Open and closed Brayton cycles Power Plant Equipment · Chapter 6: Gas Turbine Power Plant - Asked in: 2076 Asoj, 6 marks - Video: [Comparison Between The Open Cycle And Closed Cycle Gas Turbine](https://www.youtube.com/watch?v=5yGP8-sneKI) — 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/brayton-open-closed - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-6 ## Question Explain the closed and open Brayton cycle. ## Short answer Open cycle takes in fresh air, compresses, burns fuel internally, exhausts to atmosphere, simple and light. Closed cycle recirculates the same fluid, heats externally, uses a cooler, allows any fuel and no blade fouling but requires large heat exchangers. ## Answer The **Brayton (Joule) cycle** is the ideal cycle for gas turbine plants. It has four processes: isentropic compression (1–2), constant-pressure heat addition (2–3), isentropic expansion (3–4) and constant-pressure heat rejection (4–1). Its ideal efficiency is $\eta = 1 - \dfrac{1}{r_p^{(\gamma-1)/\gamma}}$. #### Open Brayton cycle ```text Air in Fuel 1 --> [C] -2-> [CC] -3-> [T] -4-> Exhaust \==================/ ==> Gen ``` - Atmospheric air is drawn in, compressed, mixed with fuel and **burnt directly** in the combustion chamber; products expand in the turbine and are **exhausted to the atmosphere**. - Heat rejection 4–1 takes place in the atmosphere, so a fresh charge is used every cycle. - Advantages: simple, light, compact, low cost, quick start; no cooler needed. Used in aircraft engines and most power-station gas turbines. - Disadvantages: working fluid is combustion gas, so turbine blades suffer deposits, erosion and corrosion; only clean fuels (gas, light oil) can be used; output depends on atmospheric conditions; filters needed. #### Closed Brayton cycle ```text Heater (external heat) +--> [Heat exch.] -3-> [T] -4-+ |2 | [C] <-1- [Cooler] <------------+ \=======shaft=======/ ==> Gen ``` - The **same working fluid** (air, helium, CO₂, nitrogen) circulates continuously. Heat is supplied **externally** in a heat exchanger (by burning any fuel, or from a nuclear reactor) and rejected in a **pre-cooler** before the compressor. - Advantages: clean working fluid, so no blade fouling or erosion; any fuel (coal, nuclear) can be used; higher pressure level gives smaller components and **better part-load efficiency** (output controlled by changing pressure level); gases with better properties (helium) can be used. - Disadvantages: needs large heat exchangers and cooler, so heavier, costlier and slower to start; requires cooling water; leakage of working fluid must be prevented. | Point | Open cycle | Closed cycle | |---|---|---| | Working fluid | Fresh air each cycle | Same fluid recirculated | | Heat supply | Internal combustion | External heat exchanger | | Heat rejection | To atmosphere | In pre-cooler | | Fuel | Clean gas/oil only | Any fuel, nuclear | | Size/weight | Small, light | Large, heavy | | Blade fouling | Yes | No | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which sequence describes the ideal Brayton cycle? A. Isothermal compression, constant-volume heating, isothermal expansion, constant-volume cooling B. Isentropic compression, constant-pressure heating, isentropic expansion, constant-pressure cooling C. Isentropic compression, constant-volume heating, isentropic expansion, constant-volume cooling D. Isentropic compression, isothermal heating, isentropic expansion, isothermal cooling Answer: B. Brayton consists of two isentropic and two constant-pressure processes. (Key point: Processes of Brayton cycle) 2. Which statement describes the open Brayton cycle? A. Same working fluid is recirculated and cooled in a pre-cooler B. Heat is supplied externally through a heat exchanger C. Fresh air is burnt with fuel internally and exhausted to the atmosphere D. Only helium can be used as working fluid Answer: C. In the open cycle, atmospheric air is compressed, burnt directly with fuel, expanded and exhausted; heat rejection occurs in the atmosphere. (Key point: Open cycle features) 3. Why can a closed Brayton cycle use coal or nuclear heat and avoid blade fouling? A. The fuel is burnt inside the turbine B. The working fluid is clean and heated externally in a heat exchanger C. It operates at lower pressure than atmosphere D. It has no compressor Answer: B. Heat is supplied externally to a recirculating clean fluid, so combustion products never touch the blades and any fuel may be used. 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