What is "draft tube"

Draft \Draft\, a.

1. Pertaining to, or used for, drawing or pulling (as vehicles, loads, etc.). Same as Draught; as, a draft horse.

2. Relating to, or characterized by, a draft, or current of air. Same as Draught.

   Note: The forms draft and draught, in the senses above-given, are both in approved use.

   Draft box, Draft engine, Draft horse, Draft net, Draft ox, Draft tube. Same as Draught box, Draught engine, etc. See under Draught.

In power turbines like reaction turbines, Kaplan turbines, or impulse turbines, a diffuser tube is installed at the exit of the runner, known as draft tube. In an impulse turbine the available head is high and there is no significant effect on the efficiency if the turbine is placed a couple of meters above the tail race. But in the case of reaction turbines, if the net head is low and if the turbine is installed above the tail race, there can be appreciable loss in available pressure head. If the pressure at the exit of the turbine is lower than the pressure of fluid in the tail race, a back flow of liquid into the turbine can result in significant damage. By placing a diffusing pipe at the exit of the runner, the pressure head is increased by decreasing the exit velocity, and both the overall efficiency and the output of the turbine can be improved.

A draft tube at the end of the turbine increases the pressure of the exiting fluid at the expense of its velocity. This means that the turbine can reduce pressure to a higher extent without fear of back flow from tail race. The Draft tube gives an advantage of placing the turbine above the tail race so that any required inspections can be made easily. Moreover, it also converts the wasteful kinetic energy at the exit of the runner into the useful pressure energy.