--- title: "Francis turbine: construction and working" subject: "Fluid Machines" chapter: 3 key: "semester_5/fluid_machines:francis-turbine-working-components" source: https://www.bctengineer.com/q/semester_5/fluid_machines/francis-turbine-working-components site: BCT Engineering --- # Francis turbine: construction and working Fluid Machines · Chapter 3: Water Turbine - Asked in: year not given, 6 marks - Video: [FRANCIS TURBINE | HOW FRANCIS TURBINE WORKS | PARTS OF FRANCIS TURBINE](https://www.youtube.com/watch?v=kxEyC4GKyic) — Mechanical Nexus - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/fluid_machines/francis-turbine-working-components - Whole chapter: https://www.bctengineer.com/md/semester_5/fluid_machines/chapter-3 ## Question Describe the construction and working of a Francis turbine with a neat sketch. Mention the function of the draft tube. ## Short answer A Francis turbine is a radial‑inward reaction turbine for medium heads. Water enters via a spiral casing and adjustable guide vanes that impart whirl; then flows through the runner where pressure drops and angular momentum change generates torque. The draft tube converts kinetic energy at runner exit to pressure, allows the runner to be set above tailrace, and recovers head. ## Answer A **Francis turbine** is an inward radial (mixed flow in modern designs) **reaction turbine** used for medium heads (about 40 to 600 m) and medium discharge. Part of the head is converted to velocity in the guide vanes and the rest in the runner, so the pressure falls from inlet to outlet of the runner and the runner is full of water. ```text Penstock | +------v-------+ Spiral casing | ___ ___ | (volute) | / \ / \ | | | R U N | <-- guide vanes | \__/ \__/ | (wicket gates) +------+-------+ | Shaft up | runner ====v==== Draft tube (diverging) \ / \___/ Tailrace ``` #### Main parts 1. **Spiral (scroll) casing:** its cross-section decreases along the flow so that water enters the guide vanes with uniform velocity all around. 2. **Stay ring and stay vanes:** carry the load of the casing and guide water to the guide vanes. 3. **Guide (wicket) vanes:** adjustable vanes that give water the correct angle at runner inlet and regulate discharge by turning about their pivots (operated by the governor). 4. **Runner:** a wheel with 16 to 24 curved blades fixed between a crown and a band. Water enters radially inward and leaves axially. 5. **Draft tube:** an expanding pipe connecting the runner exit to the tailrace. #### Working Water from the penstock enters the casing, passes the guide vanes with high whirl velocity, and moves inward through the runner blades. The change of angular momentum, together with the drop of pressure in the runner (reaction), exerts torque on the shaft. The water leaves nearly axially with little whirl. #### Function of draft tube - Allows the runner to be placed above the tailwater level, for easy inspection, without losing head. - Converts the kinetic energy at the runner exit into pressure energy by gradually increasing the area, thus lowering the pressure at the runner exit below atmospheric and increasing the effective head across the runner. - Discharges water to the tailrace below the tailwater level, so the whole head between the runner and tailrace is used. Efficiency of a Francis turbine is about 90 to 94 % at the design point. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Francis turbine is: A. Inward radial (mixed) reaction turbine for medium head B. Tangential impulse C. Axial impulse D. Axial reaction low-head Answer: A. Head is converted partly in guide vanes and partly in the runner; the runner is full of water. (Key point: Type) 2. Its cross-section decreases along the flow so that: A. Pressure rises B. Flow stops C. Water leaves axially D. Water enters guide vanes with uniform velocity all round Answer: D. The decreasing area keeps velocity uniform around the guide vanes. (Key point: Spiral casing) 3. Guide vanes: A. Are fixed and carry casing load B. Brake the wheel C. Hold the shaft D. Are adjustable, give correct entry angle and regulate discharge Answer: D. Wicket gates pivot under the governor to regulate discharge. (Key point: Guide vanes) 4. The draft tube lowers the runner exit pressure below atmospheric by: A. Closing gates B. Reducing area C. Adding pump D. Converting exit kinetic energy to pressure with gradually increasing area Answer: D. This raises the effective head across the runner. (Key point: Draft tube) ## Related in Fluid Machines - [Draft tube: function and types](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Adraft-tube-types-function) — Fluid Machines, chapter 3 - [Kaplan turbine construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Akaplan-turbine-working) — Fluid Machines, chapter 3 - [Francis turbine velocity triangles and power](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Afrancis-turbine-numerical) — Fluid Machines, chapter 3 - [Cross-flow (Banki-Mitchell) turbine](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Across-flow-turbine-notes) — Fluid Machines, chapter 3 ## Related in other subjects - [Why draft tube is used in Francis turbine](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Adraft-tube-francis) — Hydro Power, chapter 7 - [Draft tube function and turbine selection criteria](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Adraft-tube-turbine-selection) — Hydro Power, chapter 7 - [Francis overall efficiency with draft tube](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afrancis-draft-tube-efficiency-32mw) — Hydropower Engineering, chapter 7 - [Max height of straight draft tube (Francis)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afrancis-draft-tube-max-height-11760kw) — Hydropower Engineering, chapter 7