Draft Tube: Definition, Types, Function, Advantages, Application [With PDF]

Draft Tube is an important component of a reaction turbine.

The component is like a pipe in which area increasing gradually that connects the outlet of the runner to the tail-race.

There are two ends in which one end is connected to the runner outlet and the other end is submerged below the level of water in the tail-race.

It converts excess of kinetic energy into static pressure.

For making this component we used material which is cast steel and cemented concrete.

The efficiency of a Draft Tube:

The efficiency of the draft tube can be said as it is the ratio of actual conversion of kinetic energy into the pressure energy in the draft tube to the kinetic energy available at the inlet to the draft tube.

So this means: Actual conversion of kinetic energy into pressure energy / kinetic energy available at the inlet of the draft tube.

efficiency of a draft tube
V2= Fluids velocity at the inlet of the draft tube or at the outlet of the turbine, V3= Fluids velocity at the outlet of the draft tube, g= gravitational acceleration, hd= head losses in the draft tube

Here is a really short video on Draft Tube:

Types of Draft Tube:

Draft tubes are mainly classified in 4-types, and those are:

  • Simple Elbow draft tube
  • Elbow with a varying cross-section area
  • Moody Spreading tube
  • Conical Diffuser or Straight divergent

1. Simple Elbow draft tube:

This type is used for the low head.

The Efficiency of this tube is about 60 percent which is moderate.

The area of inlet and outlet are the same. A little bit the outlet section is changed.

simple elbow draft tube

2. Elbow with varying cross-section:

It is used in the Kaplan Turbine which I have covered in detail in my previous article, check out here.

In this tube the upper section is circular and the outlet is a rectangular section.

draft tube with varying cross section

3. Moody Spreading Tube:

This reducing the whirling speed of the water.

The efficiency around 88 percent which is good.

It has two passage one inlet and 2 outlets

it’s having central solid core where it distributes two parts of an outlet.

Moody Spreading draft tube

4. Conical Diffuser or Straight Divergent:

The cone angle less than 10 degrees if the angle is high cavitation will come. The cone angle is in diagram 4 when you draw a vertical line the angle made will be cone angle.

Having efficiency is about 90 percent.

Conical draft tube
By Adhish24 – CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=26387002

The function of a Draft Tube:

It reduces the velocity of the fluid and hence increases the pressure of the fluids to the atmospheric level at the tailrace.

Advantages of a Draft Tube:

This is one of the important components which is used to transform the water into energy.

This finds in the piping system, Dams where the turbine difficult to deliver the mechanical work.

Draft tubes Prevent the splashing water coming out of the runner and its guide water to the tailrace.

Applications of a draft Tube:

This is used to increase the pressure from low turbine exit pressure to the pressure of surrounding to which the fluid is rejected.

So this is the overview about the draft tube, I hope I am able to deliver my knowledge well, feel free to ask your doubt in the comment section else you can ask us directly on our community portal.

And don’t forget to share this stuff with your friend circle, and help them to learn more about the draft tube.

Till then keep learning with Learn Mechanical, see you in some another article, cheers, and enjoy the rest of your day.

Some Extra Resources for You

Kaplan Turbine
Francis Turbine
Turgo turbine

Reference (External Links):

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Anup Kumar Dey

A Mechanical Engineer with 19 years of working experience in various renowned MNCs.

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