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.