PELTON WHEEL 



443 



Change of tangential momentum per second, per Ib. 



= - { v COS a u 2 ?V cos y}' 



9 

 Work done per Ib. of water per second 



= - { v cos a u 2*Y cos 7 } ft- ^bs. 



/ 



.-. Efficiency = r- { v cos a u 2 r r cos y } ft. Ibs. 



O ft 



The loss due to friction and eddies in buckets = 

 where iv r = V v 2 + u 2 2 v u cos a (Fig. 199). 



2V 



ft. Ibs. per Ib. 



The loss due to rejection of kinetic energy = -- ft. Ibs. per Ib. 



*L Cl 



where vf = u 2 -{- 2 r r 2 + 2 it 2 v r cos 7- 



Experiments indicate that in the average wheel 2 # r may be as low as 

 from *5 to *6 times iv r . It is certain, however, that in a well-designed 

 bucket, having a ratio of bucket width to jet diameter not less than about 

 3*3, this ratio is greater, and approximates to '75. 



Taking a = 10 ; y = 160 ; u = '46 v ; 2 v r = '75 iv r ; this makes 

 jv r = '55 v ; 2 ^V = '41 v ; and the hydraulic efficiency = '84, a value 

 agreeing closely with the results of the best modern practice. 



