Crossflow Turbine
Part of Hydro Generator
A simple, fabricable turbine design where water jets pass through a cylindrical runner twice, suitable for medium-head streams and small-scale power generation.
Why This Matters
The crossflow turbine (also called a Banki-Michell or Ossberger turbine) occupies an ideal position for post-collapse hydro power: it works over a wide range of head (1.5 to 200 meters) and flow conditions, it can be built from steel plate and bar stock with basic welding equipment, it’s self-cleaning (debris passes through rather than jamming), and it can be fabricated by a skilled metalworker without specialized turbine manufacturing equipment.
Unlike Pelton and Francis turbines that require precision casting and machining of complex curved buckets, the crossflow turbine uses simpler geometry: curved steel blades bent from flat stock and welded to end discs. The manufacturing is within reach of a serious workshop with an angle grinder, welder, and basic steel-working tools.
Crossflow turbines were invented in the late 19th century and have been used extensively in developing-country micro-hydro projects throughout the 20th century precisely because they can be locally manufactured. This manufacturing history means there is good documentation, standard designs available, and a track record of successful field fabrication.
Operating Principle
Water enters the turbine through a nozzle shaped like a section of cylinder, directed tangentially onto the runner from the outside. The runner is a cylinder with curved blades running parallel to the axis. Water passes through the blade ring on the entry side (first pass), crosses the open interior of the runner, and exits through the blade ring on the opposite side (second pass), transferring energy to the blades twice.
First pass: Water strikes the curved blades at the outer circumference and is partially deflected, transferring momentum. The blade curvature turns the water inward.
Second pass: Water crosses the open runner interior and strikes the blades from inside, being redirected outward and transferring remaining momentum before exiting.
Efficiency of a well-made crossflow turbine: 75-88%. This compares favorably with Pelton wheels (80-90% for precision designs) and exceeds water wheels (50-75%).
The wide range of acceptable head (1.5m to 200m) results from being able to change nozzle dimensions for different flow and head combinations while keeping the same runner. Unlike many turbine types, a crossflow turbine maintains good efficiency even at partial flow (down to about 25% of design flow) because the nozzle width can be adjusted.
Runner Design and Fabrication
The runner consists of two circular end plates with curved blades welded between them.
Runner diameter: Standardized design formulas give runner diameter as a function of head and speed. A practical approximation for medium-head applications (3-20m):
D (meters) ≈ 0.8 × √(H) / n × 60
Where H = head in meters, n = rotational speed in rpm. For a 10m head installation targeting 600 rpm: D ≈ 0.8 × √10 / (600/60) = 0.8 × 3.16 / 10 = 0.25m (25 cm diameter runner). Scale up for more power.
Blade number: 24-30 curved blades is standard. More blades improve efficiency slightly but complicate fabrication.
Blade curvature: Each blade is a circular arc with entry angle 30° from tangent at the outer rim and exit angle 90°. The blade curves from approximately 30° at entry to 90° at the inner end. This curvature can be achieved by bending flat steel strip over a template.
Fabrication sequence:
- Cut two circular end discs from 8-10mm steel plate
- Mark 24-30 equally-spaced radial lines on each disc — these are the blade attachment points
- Cut blades from 4-6mm steel strip, cut to the correct arc length and width
- Bend each blade to the correct curvature using a bending jig (a curved wooden form works)
- Weld blades to end discs: tack weld first, check alignment, then full weld
- Mount on shaft and true up (balance and check runout) on a lathe or by spinning in v-blocks
Critical dimension: Blade-to-blade gap must be consistent around the runner. Uneven spacing causes vibration and efficiency loss. Use a spacing jig to position each blade before welding.
Nozzle Design
The nozzle directs water tangentially into the runner at the correct angle and velocity. It’s a critical component — a poorly designed nozzle wastes most of the head before the water even reaches the runner.
Nozzle shape: Converging rectangular cross-section, narrowing from the penstock diameter down to the nozzle throat. The throat width equals the runner face width; the height is determined by the design flow rate and velocity.
Entry angle: Water should enter the runner at 30° from the tangent (matching the blade entry angle). A guide vane or shaped nozzle exit directs the jet at this angle.
Velocity at nozzle: For maximum efficiency, jet velocity = 0.98 × √(2gH) (Torricelli’s theorem with a 0.98 velocity coefficient for a good nozzle). For 10m head: V = 0.98 × √(2 × 9.81 × 10) = 0.98 × 14.0 = 13.7 m/s.
Flow control: A sliding gate or pivoting plate in the nozzle controls flow and therefore power output. A simple adjustment mechanism (screw or lever) allows the operator to match power output to load.
Nozzle materials: Weld-fabricated steel with smooth interior, lined with hard rubber or abrasion-resistant coating if sediment load is high. The jet impacts the nozzle walls at high velocity, causing erosion if water carries grit.
Installation and Civil Works
The crossflow turbine requires a relatively small civil installation compared to water wheels:
Penstock: A pipe delivering pressurized water from the forebay (upstream reservoir or diversion point) to the turbine. For a crossflow turbine, the penstock ends at the nozzle — its diameter should be 2-3 times the nozzle throat width to avoid excessive velocity losses.
Turbine housing: An open or lightly covered structure supporting the turbine shaft bearings and directing exhaust water to the tailrace. The runner runs partially open to air (not submerged), so the housing doesn’t need to be watertight — just strong enough to support loads.
Tailrace: The channel directing discharged water away from the turbine. Must be sized to carry the full flow without backing up and drowning the runner.
Bearing arrangement: Two pillow-block bearings (one on each side of the runner) are sufficient. The shaft is horizontal. Use self-aligning bearings to tolerate minor misalignment. Belt drive or direct coupling to generator on one shaft extension.
A properly built crossflow turbine running on 10m head with 50 liters per second flow can produce approximately 3-4 kW of mechanical power — enough to charge a village battery bank or run several kilowatts of AC loads through an inverter.