12.8.8 Draft tubes
The draft tube is a conduit discharging water from the runner to the tail
race, and has a twofold purpose: (a) to recover as much as possible of the
velocity energy of the water leaving the runner, thus increasing the
dynamic draft head; (b) to utilize the vertical distance between the turbine
exit and the tailwater level, called the static draft head (Fig. 12.15). The
most common draft tube is the elbow type (Fig. 12.15), which minimizes
the depth of the substructure. Compared with the vertical type (Fig. 12.16)
516 HYDROELECTRIC POWER DEVELOPMENT
Fig. 12.15 Elbow-type draft tube
Fig. 12.16 Vertical draft tube