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.
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.
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.
Efficiency 88%
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.
Efficiency 90%
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.