Cross-flow (Banki-Mitchell) turbine

The cross-flow turbine (Banki-Michell or Ossberger turbine) is a partial-admission impulse-reaction turbine with a drum-shaped runner, used in small hydro plants.

        water in
          |  nozzle with
          v  guide vane
        +-----+
       /   ___ \
      |  /     \ |   <- 1st pass: outside to inside
      | |  ( )  ||
      |  \_____/ |   <- 2nd pass: inside to outside
       \_________/
           |
        tailrace

Construction

• A cylindrical runner made of two end discs joined by 20 to 30 curved blades (flat strips bent to arc form, about 30 mm to 50 mm apart on rim).
• A rectangular nozzle of the width of the runner, with an adjustable guide vane (flow regulator), directs water at about 16 degrees to the tangent.
• The casing leads water to the tailrace. Head range is about 2 to 100 m.

Working

Water from the nozzle strikes the blades near the rim and flows inward through the runner (first pass, about 70 to 80 % of the power), crosses the empty space inside the drum and passes through the blades outward a second time (about 20 to 30 %). It thus crosses the runner twice, hence the name. The pressure at entry and exit is nearly atmospheric.

Advantages and applications

• Simple, cheap, easy to build locally and repair; suitable for micro and mini hydro (up to a few MW).
• Flat efficiency curve (about 80 % over a wide load range), by splitting the runner into sections of different width (1/3 and 2/3 as in Ossberger design).
• Self-cleaning, since the leaves and sand fall off in the second pass.
• Used for low to medium heads and widely used in rural electrification schemes.
• Disadvantage: peak efficiency (about 80 to 87 %) is lower than Pelton or Francis turbines.