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What is Draft Tube ? its function, types and advantages

In a Hydroelectric Power Plant, the draft tube plays an important role in its functioning. It helps in the efficient and smooth discharge of the water from the reaction turbine. If you have any doubts regarding hydroelectric power plant don't worry we've got you covered. 

What is Draft Tube ? (Function of Draft Tube)

A draft tube is a tube of increasing cross sectional area which is used to carry the water from the exit of the reaction turbine to the tailrace. The smaller end of the draft tube is connected to the exit of the turbine, whereas the other part is submerged into the tailrace. 

The main function of the draft tube is to convert the Kinetic energy of water into pressure energy.

The draft tube brings the pressure of the water exiting the turbine back to atmospheric pressure levels by reducing the velocity of the water at the tailrace.

Cast steel and cemented concrete are the materials used to built the draft tube. The draft tube must be strong enough to withstand the high pressure and velocity of the water.

Draft Tube efficiency

The efficiency of the draft tube is stated 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.

The efficiency of a draft tube depends upon how much of the kinetic energy of water is converted into pressure energy. The more the conversion of energy, the more will be the efficiency of the draft tube.

Types of Draft Tubes

The various types of draft tubes used are :
  • Conical Draft tube.
  • Simple elbow draft tube.
  • Elbow draft tube with rectangular outlet.
  • Moody spreading draft tube.

Conical Draft tube

This type of draft tube has a shape similar to the shape of a frustum of a cone. It is mostly used in Francis turbine. The angle of the cone is limited to 8 degrees and it has a maximum efficiency of 90%.
Conical draft tube

Simple Elbow Draft tube

It has an increasing circular cross-section. The simple elbow draft tube is bent with the inlet and outlet at right angles.

 It has efficiency less than that of conical draft tube. It has maximum efficiency of about 60%. It is used for horizontal shaft Francis turbine.

Elbow draft tube with rectangular outlet

Its similar to the simple elbow draft tube, but has circular cross sectional inlet and rectangular cross section at outlet. This helps in increasing the efficiency of the draft tube. 
Elbow draft tube

Moody spreading draft tube.

It is a draft tube which has one inlet and two outlets as shown in the figure below. It is mostly used in Kaplan turbine

The efficiency of draft tube is around 85-88%.
Moody draft tube diagram

Advantages of using draft tube

The advantages of using draft tube are:

  • The net head of the turbine is increased as the hight between exit of the turbine and tailrace is increased due to usage of draft tube.
  • The use of draft tube considerably reduces the amount of kinetic energy available at the tailrace.
  • The conversion of kinetic energy into pressure energy results in negative pressure head at the exit of the turbine, which helps in increasing the overall efficiency of the turbine.
  • More work is obtained from the reaction turbine due to the draft tube.
  • The draft tube guides the water in a proper manner and thus prevents splashing of water.

That's all about Draft Tube. If you like this post or have any suggestions do let us know in the comments we would love to hear it from you.

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