456 



HYDRAULICS AND ITS APPLICATIONS 



relative sidelong motion of the discs, -which part in the middle and allow 

 a portion of the jet to pass through to waste. 



The complication introduced by this device, together with the waste of 

 energy common to any such method of governing, form the chief draw- 

 hacks to the scheme. 



The table on the opposite page gives some details of typical Pelton 

 wheel installations of comparatively recent date. 



If desired, two or three jets may be arranged so as to play on a single 

 wheel, and the power obtained is then practically proportional to the 



Governor Rod 



Hydraulic 

 Cylinder C 



FIG. 207. Combined Needle and Deflecting Nozzle. 



number of jets. In such a case the sliding hood provides the most 

 convenient method of speed regulation. 



For heads above 400 feet, and for powers in single units up to about 

 2,000 B.H.P., the Pelton wheel is by far the most suitable type of prime 

 mover, while for units up to 15,000 H.P. and with heads ranging from 

 100 to 400 feet, it is for many purposes to be preferred to its only serious 

 rival, the inward radial flow or Francis turbine. In view of its combined 

 simplicity, efficiency, and ease of regulation, it is probably the most 

 perfect of all hydraulic prime movers, and this may be the more readily 

 granted when the difficult conditions under which it works are 

 remembered. Taking a jet of water to all intents and appearances as 

 rigid as a rod of glass, and, in virtue of its enormous velocity, possessing 

 almost infinite destructive possibilities ; dropping it almost without 

 splash into the tail-race divested of practically the whole of its kinetic 



