The draft tube slows down the water leaving the runner and turns this velocity into suction under the runner.

Overview: Hydro Turbine design

The water leaves the runner still fast. The draft tube, a diverging cone, slows it down and turns this velocity into suction under the runner: on a low-head turbine, it is a large share of the efficiency.

Sizing the draft tube and reading its balance

• Inputs: length (m), outlet section (cm², rectangular outlet whose height is half its width), roughness (mm, material list).
• AutoSize draft tube: outlet section computed for the target velocity at the maximum site flow (2 m/s by default, adjustable in the target outlet velocity box) and a cone of 14° total angle, the length being adjusted accordingly.
• Results: inlet section, mean velocity, cone angle, equivalent outlet diameter; gain = (V inlet² − V outlet²) / 2g; losses of the cone (Idel'cik, and beyond 50° total angle, Borda sudden expansion), of roughness and of outlet kinetic energy; total loss.

A cone that is too open separates: the announced gain is no longer recovered. The outlet velocity is lost in the tailrace: the lower it is, the better the balance, at the cost of a longer draft tube.

Examples

Vertical cut: the water slows down in the draft tube cone (green to blue).

The piezometric graph: the draft tube pressure recovery under the runner.

Going further

The Hydro Turbine design reference page presents the complete method and a worked case. The other help pages of the window:

• Control panel
• Layout
• Spiral casing
• Distributor
• Runner
• Draft tube
• Specific speed
• Statistics
• Views and piezometric graph
• Fabrication kit
• Results, method, errors