CENTRIFUGAL PUMPS 659 



Effect of a Variation in y. Equation (12) indicates that as y is diminished 

 the efficiency is increased, and this is in general borne out by the results 

 of experiment. As previously pointed out, however, this gain in theo- 

 retical efficiency is to a certain extent counterbalanced by the fact that 

 since a diminution in y necessitates an increased speed of rotation 

 for pumping against a given head, this involves increased frictional 

 losses. 



Thus experiments by Parsons l on two 14 inch impellers, one having 

 y = 90 and the other y 25 (approx.), showed that the second was 

 about 1*16 times as efficient as the first. In each case / 3 = } V 2 g H', 

 and an examination of the table on p. 658 shows that the theoretical 

 manometric efficiencies in the two cases would be approximately '49 and 

 63, the ratio of these being T28. 



Again, since : 



it follows that the necessary peripheral speed increases with the working 

 head, so that, because of increased frictional losses at these high speed?, 

 it might be inferred that the higher efficiencies are to be expected with 

 comparatively low working heads a view which, in the ordinary type of 

 single impeller pump, is borne out in practice. The minimum per- 

 missible value of y increases with the working head, and while for heads 

 of about 10 feet it may be as low as 15, it increases to about 25 with 

 30 feet head, and for heads of upwards of 60 feet is not generally less 

 than 35. In the modern types of high-lift pump, however, 

 great care is taken to polish every part of the impeller so as to 

 reduce friction losses to a minimum, and under such circumstances y may 

 be reduced to as low as 20 against heads of from 90 to 120 feet with 

 excellent results as regards efficiency. 



Although a high efficiency is to be aimed at in the design of a pump, 

 this is not the only factor which may affect the most suitable value of y. 



From (15) we have 



/ = sin y V uf - 2 g H', (16) 



and if & 3 feet is the width of the impeller at the discharging periphery, 

 and n the number of vanes each of effective thickness t feet, the area of 

 the wheel passages at the periphery is 



A a = 2 TT r a bs n t b 3 cosec y, 

 while Q = / 3 A 3 = A 3 sin y V~M? 2 g H'. (17) 



1 " P roc. Inst. C. E.," Vol. xlvii., 1876-77, p. 267. 



U U 2 



