PELTON WHEEL 447 



of the buckets, r the pitch circle 'radius, and t the thickness or diameter 

 of the jet, n is given very approximately by the relationship 



If H = r -j- s, so that s is that portion of the bucket projecting beyond 

 the pitch circle, we have 



Giving s a value ranging from '60 t in the case of a wheel of less than 

 8 feet diameter to '565 t where the wheel is upwards of 6 feet in diameter, 

 values of n in close accord with modern practice are obtained. Generally, 



/r 



values of n given by the formula n = k V ~p where k ranges from 7*0 



to 8 - as the wheel diameter decreases from 6 feet to 3 feet, will be found 

 to give results which are sufficiently near for all practical purposes. The 

 theoretical value of s thus being obtained, a little additional overlap is 

 usually given to allow for any slight variation in the axial position of 

 the jet. 



Speed Regulation. Since the efficiency of a Pelton wheel, or other 

 impulse wheel, depends on the maintenance of the correct ratio of 

 peripheral velocity of bucket and velocity of jet, if high efficiency is to be 

 expected at all loads the method of governing must be such as to keep 

 the latter velocity as nearly as possible constant. Where this is the 

 case, there is no reason, except for the greater proportional effect of 

 windage and mechanical friction at part loads, why the efficiency should 

 not be independent of the load. Where, however, the jet velocity is 

 variable, the efficiency falls off considerably as this departs from its 

 theoretically correct value, and for this reason the impulse wheel, while 

 giving excellent part-load efficiencies under a constant supply head, is 

 unfitted for situations in which the percentage variation of head is likely 

 to be great. Since this is more likely to be the case under a low supply 

 head, it affords one reason why the impulse wheel is not in general 

 advisable under such conditions. 



The Speed Regulation of a Pelton wheel is usually performed in one of 

 four ways. 



