OVERSHOT WATER-WHEEL 



429 



point of the wheel, by enclosing the discharge side of the wheel below A 

 in a closely fitting shrouding, with as small a clearance as possible 

 (Fig. 185 A), and by this means the efficiency may be increased. This 

 method has the further advantage that the relative velocity of discharge 

 is now increased by the head of water in the buckets, and as the direction 

 of this motion is opposite to that of the wheel, the absolute velocity of 

 discharge, and therefore 

 the kinetic energy rejected, 

 is reduced. 



Effect of Varying the 

 Tail-Race Level. If the 

 tail-race level rises so that 

 the wheel is partially sub- 

 merged, considerable re- 

 sistance to rotation is 

 caused if the vanes move 

 in the opposite direction 

 to that of flow in the tail- 

 race. By reversing the 

 direction of inflow, how- 

 ever (Fig. 186), the direc- 

 tion of the tail-race flow 

 becomes the same as that 

 of the buckets, and these 

 may now become partially 

 submerged without serious 

 loss of efficiency. 



Further advantage may 

 be taken of the fall by ar- 

 ranging a masonry breast- 

 work A (Fig. 187) to fit the wheel closely, the tail-race level being now 

 slightly below the level at which each bucket finally empties itself. The 

 compartment B is kept dry either by draining into a sump where practica- 

 ble, or by means of a small pump driven by the wheel. 



To avoid running at inconveniently low speeds either of two expedients 

 may be adopted. The first is to run the wheel at a speed from 50 to 

 100 per cent, in excess of that giving maximum efficiency, when, since 

 velocities are small, the amount of energy wasted by shock at entrance is 

 riot considerable. 



The second expedient consists in giving hi a larger value, and in taking 



FIG. 186. Overshot Wheel with Reversed Inflow. 



