PELTON WHEEL 



441 



.a. 



to a wheel developing 375 B.H.P. under 860 feet head, and having a 2-inch 

 jet on a 35-inch pitch circle, showed an efficiency about 6 per cent, greater 

 with bucket b than with bucket a. 1 



In practice an efficiency of unity is impossible of attainment for several 

 reasons. 



(1) In order that the discharge from one shall clear the back of the 

 following bucket, the jet cannot be deflected through the full 180, the 

 actual deflection usually 



being 160. Thus ki- 

 netic energy is rejected 

 in virtue of the motion 

 of the water parallel to 

 the axis of the wheel at 

 discharge. To prevent 

 this loss becoming large, 

 the buckets should not 

 be spaced too closely to- 

 gether. 



(2) The relative velo- 

 city of water and bucket 

 at discharge is less than 

 at the point of impact 

 because of skin friction, 

 while windage causes 

 the actual velocity of 

 impact to be less than 

 that theoretically equiv- 

 alent to the head at 

 the nozzle. Both these 

 causes have the effect of 



reducing the pressure on the bucket, and to obviate this loss as far as 

 possible the wetted surface of the buckets should be a minimum, and 

 therefore the number of buckets should be as small as is consistent with 

 continuous impact, while they should be made no larger than is necessary 

 to give the required change of direction with easy curves and without shock. 

 Also the surface should be as smooth and well finished as possible. 



To reduce windage the jet should be circular in section, since this gives 

 the minimum perimeter per unit area of cross section, while it has the 

 further advantage of being the most stable form of jet. Other forms 



1 " Proceedings Inst. Civil Engineers," 1907, vol. 170, p. 51. 



FIG. 197. 



