Draft Tube:
- A draft tube is a pipe of gradually increasing diameter which connects the exit of runner of a turbine to the tail race.
- Therefore, it discharges the water from the runner to tail race.
- In case of Pelton wheel, the available head is converted into kinetic energy before entry to runner buckets and the turbine operate under atmospheric pressure conditions.
- The velocity of water leaving at turbine exit is small, therefore, the exit of runner is kept above the tail race level.
- Whereas in case of reaction turbines, only the part of available head is converted into kinetic energy before entry to runner and rest of energy is in the form of pressure energy.
- This pressure energy is gradually converted into runner, thus the velocity leaving the runner is at high velocity.
Function:
- The function of this tube are:
- It permits the negative head to be established at outlet of runner, hence it increases the net head available to turbine.
- To convert the kinetic energy of water at exit of runner into pressure energy so that useful head at runner exit is increased.
Important Types of Draft Tubes:
- Conical
- Simple elbow
- Elbow draft tube with circular inlet and rectangular outlet
- Moody’s spreading draft tube
1. Conical draft tube:
- Earlier designs of these were of conical straight divergent.
- Taper angle of divergent portion of tube is of great importance.
- If the taper angle is large it will cause the separation of flow from the walls of draft tube.
- In case the taper angle is small, the length of tube needed will be long causing higher frictional losses. It may also lead to cavitation problem.
- Angle ranges from 4 to 7 and the proportionate dimensions of the draft tube.
- It is fabricated out of mild steel plates.
- It has an efficiency up to 90% and it is employed for vertical shaft turbines.
2. Simple elbow draft tube:
- It has circular cross-section throughout from inlet to outlet which is turned through 90°.
- Elbow type draft tubes are used for Kaplan turbines.
- These tubes reduce the depth and cost of excavation.
- It is made of concrete with steel lining at inlet to resist cavitations.
- Due to bend, such draft tubes have an efficiency upto 60%.
3. Elbow draft tube:
- It is circular in cross-section at inlet in its vertical leg which turns into rectangular cross-section in horizontal portion of tube upto outlet.
- The horizontal portion of tube is gradually inclined upwards so that the water leaves the tube almost at tail race level.
- In this case also, the cost and depth of excavation is reduced.
- The efficiency of these tubes are in the range of 60% to 80%.
- It is also made of concrete with steel lining near the runner.
4. Moody’s spreading draft tube:
- This tube similar to conical draft tube.
- It is provided with a solid central core which reduces the whirling action of water.
- It has an efficiency up to 85% and it is used for vertical shaft turbines having large whirl component at exit of their runner.