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Crossflow Turbine

Crossflow Turbine

Made of

WoodSteelIronWater

Brief description

A Crossflow turbine (also known as a Banki-Michell or Ossberger turbine) is an impulse water turbine where water flows transversely through a cylindrical runner fitted with curved blades. Uniquely, the water passes across the runner blades twice—first entering from the outside inward, and then exiting from the inside outward—extracting mechanical energy in both passes. Its simple design makes it easily manufacturable using basic workshop tools while offering high efficiency across varying water flow conditions.

Use / Function

  • Primary use: Generating rotary mechanical power and hydroelectricity from small-to-medium river heads.
  • Secondary uses: Direct mechanical driving of sawmills, grain mills, water pumps, and village micro-hydro power generators.
  • Scale: Small homestead or community scale (1 kW to 100 kW) up to small commercial installations, operating efficiently on hydraulic heads from 2 to 200 meters.

Operating principle

Double Impulse Crossflow Action:

  1. Guide Nozzle Acceleration: Water from a dam or penstock enters a rectangular inlet nozzle, which accelerates and directs the fluid jet at a specific angle (typically 16°) onto the outer rim of the cylindrical runner.
  2. First Pass (Outer to Inner): As water strikes the outer curved blades, it transfers about 70-80% of its kinetic energy to the runner as it moves inward toward the open central core of the cylindrical drum.
  3. Crossflow Phase: The water stream crosses the hollow center of the cylindrical runner without interfering with the main shaft.
  4. Second Pass (Inner to Outer): The water hits the blades on the opposite side of the runner from the inside moving outward, transferring the remaining 20-30% of its kinetic energy before discharging into the tailrace below.
  5. Self-Cleaning Action: The outward flow in the second pass naturally flushes away debris, leaves, and sediment trapped during the first pass.

How to create it

1. Runner Construction

  1. Cut two parallel circular side plates out of thick Steel plate or heavy Wood.
  2. Form 20 to 30 curved blades by cutting longitudinal sections from heavy steel pipe or bending sheet metal into circular arcs.
  3. Weld or bolt the blades uniformly around the perimeter between the two side plates.
  4. Mount a central shaft through the hubs of the side plates with heavy-duty Iron or steel bearings.

2. Guide Vane & Nozzle Housing

  1. Construct a rectangular welded steel or timber casing around the upper section of the runner.
  2. Install a hinged guide vane inside the nozzle to adjust the inlet water jet width according to seasonal river flow variations.

3. Installation and Drive Assembly

  1. Position the turbine housing securely over a concrete or stone tailrace discharge channel.
  2. Connect the turbine shaft to an Electric Generator, pump, or mill pulley using belts or gears.

Materials needed

  • Runner Blades & Plates: Structural Steel pipe/plate, or dense hard Wood reinforced with metal straps.
  • Shaft & Bearings: Iron or steel shafting with greaseable pillow block bearings.
  • Housing & Nozzle: Heavy steel sheet, timber, or masonry casing.
  • Working Fluid: Water from a stream or river penstock.

Variants and improvements

  • Banki-Michell Turbine: Early 20th-century standard design featuring curved blades made from split pipe sections and simple rectangular nozzle controls.
  • Divided Runner (Ossberger Type): Splitting the runner width into 1/3 and 2/3 sections with independent guide valves allows optimal efficiency at 1/3, 2/3, or full water flow rates.
  • Wooden Runner Micro-Hydro: Constructed using hard oak or teak blades for emergency off-grid power where steel fabrication is unavailable.

Limits and risks

  • Cavitation Risk: Operating the turbine below tailwater level without a draft tube can cause air locks or efficiency losses; draft tubes must be carefully vented.
  • Bearing Seal Degradation: Water leaks into main shaft bearings can cause rapid rusting or bearing failure if seals are not maintained.
  • Uncontrolled Overspeed: If the electrical or mechanical load is suddenly disconnected, the turbine runner can reach dangerous overspeed ratios (up to 1.8x normal speed).