376 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.
(3) Hydracone or Moody Draft Tube. This type of draft tube is also known as bell-mouth draft
tube. The whirl component of the water is large when the turbine works at part load conditions. Turbine
runs at high speed under low head conditions.
The high speed runner under low head has high whirling component. As water enters the draft
tube from the high speed runner, it whirls in the direction in which the runner rotates. The axis of
rotation is the vertical axis of the draft tube. The vortex tends to remain in the same plane because of its
gyroscopic properties and therefore, will not follow the center line of the tube. This causes eddies and
whirls in the curved portion of the draft tube and only a portion of the discharged area at the exit may be
effective. Actually water flows back into the part of the draft tube and causes serious eddies and losses.
The central cone arrangement of hydracone draft tube reduces the whirl action of the discharged
water and increases the efficiency of this draft tube to 85%.
Two different hydracone type draft tubes are shown in Fig. 11.28(e) and Fig.11.28(f). The flow
coming out of the high speed runner is whirling flow and can be considered as free vortex flow. The
pressure is minimum at the center of the free vortex, therefore, the cavitation may start by the liberation
of air and vaporization of water at the center of the draft tube immediately underneath the runner if the
velocity of whirl is large. To prevent the cavitation of runner under above-mentioned condition, the high
speed runners are used in low head plants, and the central cone of draft tube has been extended right
upto the tube as far as possible as shown in Fig. 11.28 (g). The tube shown in Fig. 11.28 (e) is used for
low speed turbines where the whirl velocity is not high.