Types of Draft Tubes

Though there are various types of draft tubes, but the important type of draft tubes are :

• Conical draft tube
• Simple elbow draft tube
• Elbow draft tube with circular inlet and rectangular outlet
• Moody’s spreading draft tube.

1. Earlier designs of draft tubes were of conical straight divergent type as shown in Figure A (a).
2. Taper angle of divergent portion of tube is of great importance.
3. If the taper angle, 20 is large it will cause the separation of flow from the walls of draft tube.
4. In case the taper angle is small, the length of draft tube needed will be long causing higher frictional losses.
5. Angle 8 ranges from 4° to 7º and the proportionate dimensions of the draft tube are shown in given Figure A.
6. It is fabricated out of mild steel plates. It has an efficiency upto 90% and it is employed for vertical shaft turbines.

(2) Simple elbow draft tube :

1. It has circular cross-section throughout from inlet to outlet as shown in Figure A (b) which is turned through 90°.
2. Elbow type draft tubes are used for Kaplan turbines.
3. These tubes reduce the depth and cost of excavation.
4. It is made of concrete with steel lining at inlet to resist cavitation.
5. Due to bend, such draft tubes have an efficiency upto 60%.

(3) Elbow draft tube :

1. Elbow draft tube is shown in Figure A (c). 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.
2. The horizontal portion of tube is gradually inclined upwards so that the water leaves the tube almost at tail race level.
3. In this case also, the cost and depth of excavation is reduced.
4. The efficiency of these tubes are in the range of 60% to 80%.
5. It is also made of concrete with steel lining near the runner.

1. It is similar to conical draft tube as shown in Figure A (d). It is provided with a solid central core which reduces the whirling action of water.
2. It has an efficiency upto 85% and it is used for vertical shaft turbines having large whirl component at exit of their runner.