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Draft tube

In Power Turbines like Reaction Or Kaplan Or Impulse), a diffuser tube is installed at the exit of the runner, known as Draft Tube.〔Elementary Draft Tube Theory (http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/machine/chapter_7/7_7.html)〕 In an Impulse turbine the available head is considerably high and there is no significant effect on the efficiency if the turbine is placed a couple of meters above the tailrace. But in the case of Reaction turbines, if the net head is low and if the turbine is installed above the tailrace, 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 tailrace, 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.〔Valan Arasu A , “Turbo Machines”, Vikas Publishing House, Chapter 9, Page 402〕
== Efficiency ==

It is defined as the ratio of the actual conversion of kinetic energy into pressure energy in the draft tube to the kinetic energy available at the draft tube inlet
ɳ = Difference of kinetic energy between inlet and outlet-tube losses/Kinetic Energy at the inlet
ɳdt = :$\frac ^2\right)-2gh_\right\}$
V2 = Fluids velocity at inlet of draft tube or at the outlet of turbine
V3 = Fluids velocity at outlet of draft tube
g= gravitational acceleration
hd = head losses in draft tube
Draft Tube allows turbine to be placed above the tail race and simultaneously allows it to operate at the same efficiency if it was placed at the tail race〔Valan Arasu A , “Turbo Machines”, Vikas Publishing House, Chapter 9, Page 403〕

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