Power Plant Engineering

11.12.2 Types of Draft Tubes
(1) Conical Draft Tube. This is known as tapered draft tube and used in all reaction turbines
where conditions permit. It is preferred for low specific speed and vertical shaft Francis turbine. The
maximum cone angle of this draft tube is limited to 8° (a = 4°) for the cause mentioned earlier. The
hydraulic efficiency of such type of draft tube is 90%.
In any event, the draft tube should be made as to secure a gradual reduction of velocity (uniform
decease towards the exit of draft tube) from the runner to the mouth. A form that is theoretically good is
“Trumpet Shaped”.

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POWER PLANT ENGINEERING
(2) Elbow Type Draft Tube. The elbow type draft tube is often preferred in most of the power
plants, where the setting of vertical draft tube does not permit enough room without excessive cost of
excavation. This offers an advantage in the cost of excavation ; specially in the rock.
If the tube is large in diameter ; it may be necessary to make the horizontal portion of some other
section than circular in order that the vertical dimension may not be too great. A common form of
section used is oval or rectangle.
If some other section instead of circular is used, then the tube is so made that the area increases at
a similar rate to what it would if it were circular. The other forms are generally used when the draft tubes
are moulded in concrete.
If the cross-section is gradually changed from circular at wheel discharge to rectangular or ellip-
tical at the mouth of the tube, there is considerable saving in the excavation cost. Such types of draft
tubes give better hydraulic efficiency than conical elbow type. The efficiency of this tube lies between
60 to 70%.
The horizontal portion of the elbow type draft tube is generally inclined upward to lead the water
gradually to the tailrace level and prevent the entry of air from the exit end.