CENTRIFUGAL PUMPS 



The efficiency 17 ' of the pump is thus given by 



fc + 1) + /a 2 cosec 2 y (A; 1) 2 k u 3 / 3 cot y 



665 



(21) 

 2 ?/3 (^3 /a cot y) 



Giving A; the value '75 and taking / 3 = J 3 , we get the following 

 values of 17' : 



Here again, any large reduction in the value of y, by necessitating an 

 increased speed of rotation, is likely to reduce rather than increase the 

 overall efficiency. 



For the guides to be efficient it is essential that their angle of 

 divergence be about 11; that the vane angle a be so designed as to take 

 the water without shock, on leaving the wheel ; that a sufficient number 

 of guide vanes be used to efficiently direct the mass of water, four being 

 about the minimum ; that the areas of the guide passages at the exit 

 from the wheel should be proportioned so as to keep the velocity at 

 entrance, the same as on leaving the wheel, and that all guide curves 

 should be smooth and gradual. 



Since the whole object of the guides is to neutralise the evil effects of 

 a high velocity of discharge, their relative effect will be greatest where 

 this velocity is greatest, i.e., with radial pump vanes when delivering 

 against a high head, and will be least with recurved vanes and a low 

 head. For this reason, and since large percentage variations of head are 

 more likely to occur where this is low, the diffuser ring is seldom fitted 

 for heads under about 80 feet. 



The gain in overall efficiency attainable by its use naturally varies 

 considerably, but may be taken as between 10 per cent, and 20 per cent. 



ART. 177. COMPOUND MULTIPLE CHAMBER PUMP. 



Where a number " n " of impellers mounted on the same shaft are used 

 in series so as to form a compound high-lift pump, the impeller diameters, 

 and vane angles are made the same for each chamber, so that each 



TTt 



impeller gives a total head = feet. If, in the foregoing theory then, 



