588 HYDKAULICS AND ITS APPLICATIONS 



Since for maximum efficiency, the work done 



W Q a ( 1 tan a \ , 



U = - '2 { 1 T ^ ) f 00 ^ I" 38 - per second. 



g \ tan p ] 



TT ^ X 2 TT r 2 







tan a 



tan a 



( 



_ tan a f foot Jks. per second. (12) 



tan 



the work done is proportional to 

 An examination of equation (10) shows that the hydraulic efficiency dimin- 

 ishes with an increase 



in the ratio (= n,) 



except where l> 2 r 2 b 3 r 3 , 

 and therefore where 

 the velocity of flow is 

 the same at inlet and 

 outlet. Apart from this, 

 an increase in n, by 

 increasing the length 

 of wheel passages, re- 

 duces the efficiency by 

 increasing frictional 

 losses. This disadvan- 

 tage is, however, coun- 

 terbalanced by the in- 

 creased regulating effeci 

 of the centrifugal pn 



sure. It follows that as the efficiency of the governing mechanism 

 improved, it becomes advisable to reduce the value of n, and this explain 

 why the value adopted in practice has been gradually reduced of recei 

 years from 2 to as low as 1'25. Its usual value is about 1*5. 



The discharge angle y of the vanes (Fig. 256) may be determined 

 the consideration that, if w s = 0, 



UQ Us b s T 3 



FIG. 256. Vane Angles for Inward Radial Flow Turbine. 



