488 



HYDRAULICS AND ITS APPLICATIONS 



(3) Any increase in the speed of rotation, by increasing the centrifugal 



pressure at the outside of the wheel 

 and at the outlet from the guide pas- 

 sages, tends to check the flow through 

 and the supply of energy to the wheel, 

 which now becomes to a certain ex- 

 tent self-regulating. 



(4) The centrifugal pressure of the 

 water also balances the pressure due 

 to a portion in practice generally 

 about one-half of the supply head, so 

 that only one-half of the pressure 

 head at entrance is absorbed in pro- 

 ducing velocity of influx, the other 

 half remaining as pressure head and 

 being gradually absorbed in its pas- 

 sage through the wheel. The velocity 

 of influx is thus never greater than 

 about *Jg h, and hydraulic friction 

 losses are thereby much reduced, while 

 the corresponding reduction in the 

 peripheral speed of the wheel enables 



this to be applied successfully to very high heads, up to about 500 feet. 



Since the mean velocity of flow in an inward flow turbine is less under 



FIG. 230. Francis Turbine with Inside 

 Cylinder Gate Regulation. 



Cylinder Gate 



FIG. 231. Outside Register Gate. 



a given head than in an outward flow, or axial flow machine, the size of 

 wheel is greater for a given power. As the size is usually increased axially, 

 the extra cost is, however, only small. 



