EXAMPLES 461 



The coefficient of velocity increased from '971 to '989 as the nozzle 

 area was increased, the coefficient of discharge diminishing at the same 

 time from '965 to '838, and the efficiency increasing from '958 to "986. 

 Fig. 209 shows the shape of needle and tip used in these experiments. 



EXAMPLES. 



(1) A Pelton wheel working under an effective head of 2,100 feet is 

 36" diameter and is supplied through a single J" nozzle. Determine the 

 necessary number of revolutions of the wheel for approximately maxi- 

 mum efficiency and the probable horse-power, assuming an efficiency of 

 83 per cent. 



( 1,060 revolutions per minute. 

 Answer. 



p 



(2) A Pelton wheel develops 140 B.H.P. under a head of 98 feet. The 

 wheel is 20' 0" in diameter, and is supplied by two nozzles. Determine 

 the number of revolutions per minute and the necessary nozzle diameter, 

 if the efficiency is 80 per cent. 



f 35 revolutions per minute. 

 Answer. \ . 



I Diam. = 4'd ins. 



(3) Show that the efficiency of a Pelton wheel is theoretically equal to 



1 -*> 



Where r = mean radius of bucket circle. 

 N = revolutions per minute. 

 h = effective head at nozzle. 

 C v coefficient of velocity at nozzle. 

 k ratio of relative velocity at exit from and entrance to 



buckets. 

 a = total angle through which jet is deflected. 



