Draft Tube

A draft tube is a diverging tube fitted to the exit of a water turbine runner that recovers the kinetic energy still present in the water as it leaves the runner, converting that kinetic energy into pressure energy by slowing the flow as it widens. This recovery lets a reaction turbine, particularly one operating on a low head, avoid backflow from the tailrace and run more efficiently, while also allowing the turbine itself to be mounted above the tailrace rather than submerged in it, which simplifies inspection and reduces the excavation a powerhouse needs. The draft tube is a standard fitting on reaction turbines such as the Francis and Kaplan turbine designs rather than a standalone machine, but it is engineered and studied as its own distinct hydraulic component.