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Francis turbine: construction and working

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.

Describe the construction and working of a Francis turbine with a neat sketch. Mention the function of the draft tube.

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.

          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.

✓ Answer checked by Claude Sonnet 5.5

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