What is Draft Tube ? Describe its Functions and Classifications
In hydraulic turbines, the component draft tube is very significant. In impulse turbines, the flow energy of the fluid is converted to kinetic energy by means of the nozzle. In Reaction Turbines, part of the flow energy is converted to K.E but remaining energy is still the pressure energy. Draft tubes are used in Reaction turbines to convert K.E into useful pressure energy. If you have any doubt about the reaction and impulse turbine please go through this link.
What is Draft Tube? Where is it used (Functions of Draft Tube)?
I already mentioned earlier that draft tubes are an important component of reaction turbine. It is a conduit of a gradually increasing area located under the runner of the reaction turbine. The main purpose here is to convert the kinetic energy of the fluid into static pressure energy by decelerating the velocity at the runner exit. Cast steel or cemented concrete is used for the manufacturing of draft tube.
In reaction turbines like Kaplan or Francis, both pressure and kinetic energy are used to make the rotor run. At the exit of the runner of these turbines, there is a negative pressure developed which is less than the atmospheric pressure. So to improve the work done this kinetic energy is converted to pressure energy again by the means of the draft tube. And the water also moves out to tailrace.
Different types of Draft Tubes
Simple Elbow Draft Tube
Low head. This is simple in design. The incoming side and outgoing side has the same cross-section. Efficiency 60%
Elbow with varying cross section
Inlet circular cross-section outlet rectangular cross-section. Generally used in Kaplan Turbines. Efficiency 70%
Moody Spreading Draft Tube.
It reduces the whirling action of water. It has one inlet and two inlets.
Conical Diffuser or Divergent Draft Tube
The cone angle is less than 10 degrees. If the cone angle is more then there is a chance of cavitation.
Why Are Draft Tubes Not used in Impulse Turbines?
In an Impulse turbine, the Kinetic energy is used to rotate the blades. Here no device is needed to convert the kinetic energy to pressure energy because there is no loss of pressure. The inlet and outlet pressure remains the same. Example of impulse turbine is Pelton wheel. In Pelton wheel, the velocity of water is used to rotate the blade.
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