To size a crossflow turbine, take the jet speed as 0.98 times the square root of 2 × 9.81 × head, set the runner rim speed at half the jet speed, and make the nozzle area equal to the flow divided by the jet speed. For the carrier’s site of 5 meters of head and 30 L/s this gives a 9.7 m/s jet, a 30 cm runner turning about 309 RPM, a nozzle area of 3,090 mm², and about 1,030 watts at 70% efficiency.
Part of Micro-Hydro Turbine
A crossflow (Banki) turbine passes a sheet of water through a drum-shaped runner twice; it tolerates changing flow and can be welded up from curved sheet metal.
One Runner Covers Many Sites, So The Arithmetic Matters
The carrier calls the crossflow the most forgiving and easiest turbine to build, usable from 3 to 20 meters, at 65 to 80% efficiency and 200 to 500 RPM. The Crossflow Turbine page has the fabrication steps, quotes 75 to 88% efficiency and a head range from 1.5 meters upward. The two efficiency ranges disagree, so this page uses 70%, which lies inside the carrier’s range, and does not average them.
Method: Size The Runner And Nozzle From The Site
Step 1: Jet speed. v = 0.98 × √(2 × 9.81 × H). At H = 5 m, 2 × 9.81 × 5 = 98.1 and its root is 9.905, so v = 0.98 × 9.905 = 9.71 m/s.
Step 2: Rim speed. Use half the jet speed, the same rule Turbine Design gives for runners: u = 9.71 ÷ 2 = 4.85 m/s. That half is carried over from the Pelton case from my recall of crossflow practice; confirm it with a loaded test run.
Step 3: Runner speed. For D = 0.30 m, n = 4.85 ÷ (π × 0.30) × 60 = 309 RPM, within the carrier’s 200-500 RPM. The Crossflow Turbine page’s formula D ≈ 0.8 × √H × 60 ÷ n gives n = 0.8 × 2.236 × 60 ÷ 0.30 = 358 RPM for the same runner, about 15% higher; a few hundred RPM is fine either way.
Step 4: Nozzle area. Area = flow ÷ jet speed = 0.030 ÷ 9.71 = 0.00309 m², which is 3,090 mm². The nozzle is a slot as wide as the runner, so thickness = area ÷ runner width: a 0.25 m runner needs a 12.4 mm slot, a 0.15 m runner a 20.6 mm slot.
Step 5: Power. Hydraulic power = 9.81 × 30 L/s × 5 m = 1,471 W. At 70%: 1,471 × 0.70 = 1,030 W, in line with the carrier’s 30 × 5 × 7 = 1,050 W.
Step 6: Drive. 1,500 ÷ 309 = 4.85, a single belt stage with sheave diameters in that ratio (Generator Coupling (Belt vs Direct)).
Part Flow Is Where The Crossflow Earns Its Place
The Crossflow Turbine page says a crossflow stays efficient down to about 25% of design flow because the nozzle gate narrows. For 30 L/s that is 7.5 L/s, a dry-season value. Narrowing the slot cuts the flow while the jet speed, set by the head, stays near 9.7 m/s. How much power a part-flow run gives is a measured quantity here, not a calculated one.
Diagnosing A Crossflow That Underperforms
| Symptom | Likely cause | Check |
|---|---|---|
| Shaking at speed | uneven blade spacing or an unbalanced runner | spin by hand on two level supports; the heavy side sinks |
| Power well below 1,030 W | head overstated, or the nozzle leaks past its seals | re-measure head with the hose and gauge; look for spray beside the slot |
| Slow, heavy turning | tailwater high enough to drown the runner | the tailrace must carry the full flow without backing up |
| Cracks at blade roots | fatigue from flexing thin blades | thicker strip; support long blades with a centre disc |
Common Mistakes
- Reading the head from the pipe length — only vertical height counts (Head Measurement (Elevation Drop)).
- A shaft too light for the runner — a long runner on a thin shaft flexes and rubs.
- Rim speed far from half the jet speed — a loaded generator drags the runner below it, and output falls.
Safety
An open drum with steel edges, spinning at 300 RPM
The runner is exposed through the nozzle opening and the open housing. Fit a guard over the belt and runner, close the nozzle gate to stop the machine, and wait for the runner to coast down before opening the housing. The gate must be within reach of the person who is at risk.
What’s Next
- Learn next: Tailrace & Environmental Flow to keep the runner out of its own exhaust
- Combine with: Generator & Electrical for the load the runner drives