438 



HYDRAULICS AND ITS APPLICATIONS 



having as profile the arc described with centre 0, and passing through M, 

 will give the required inclination at K and M, and approximately for all 

 points between. 



Regulation is performed by an adjustable sluice S, whose position is 

 regulated by some form of centrifugal or float governor. In order to 

 prevent loss by leakage past the vane tips the supply bed is extended to 

 form a circular lip closely fitting the wheel. This should be no longer 

 than is necessary to give contact with two vanes at once. If made too 

 long, the water on flowing down the vanes, instead of freely escaping with 

 relative velocity v r , meets this lip, its relative velocity is destroyed, and on 



FIG. 193. Poncelet Wheel. 



emerging from the lip it escapes with the forward velocity of the vane as 

 in the ordinary type of undershot wheel. 



In common with all impulse wheels, for efficient working the vanes 

 must not be submerged, and the level of the supply bed must be sufficiently 

 above that of the tail-race to obviate any flooding. This wheel is well 

 adapted for heads between 4 and 7 feet. Its speed is fairly high, and the 

 consequent fly-wheel effect renders it easy of regulation under variable 

 loads. 



Its part-gate efficiency is high if the head be kept fairly constant, but 

 with very variable heads, owing to the variation in velocity of approach, 

 its efficiency is not so good. 



