662 



HYDEAULICS AND ITS APPLICATIONS 



with recurved vanes has a great advantage in virtue of its greater 



adaptability to suit varying conditions without serious loss of efficiency. 

 If, on installing a pump, the head is greater than that for which the 



vanes were designed, less than the rated quantity will be delivered, or in 



an extreme case the pump will not lift at all. Generally a slight increase 



in speed will rectify this. 



If, on the other hand, the designed head is greater than the actual, the 



delivery will be increased, and the engine or motor may be overloaded. 



This may be rectified either by reducing the speed or, and in general 



preferably, by slightly 

 throttling the discharge, 

 so as to increase the head 

 artificially. In either 

 case, the possibilities oi 

 perfect adjustment are 

 much greater with re- 

 curved vanes. 



These points are well 

 brought out by a com- 

 parison of Figs. 325 1 and 

 326, 1 which show the 

 characteristic curves from 

 a low-lift pump having 

 recurved vanes with an 

 angle y approximately 

 30, and a high-lift quad- 

 ruple pump with radial 

 vanes, the speed being 

 constant in both cases 



l^r 



Volume delivered 



FlG. 327. Curves showing the influence of the discharge 

 angle 7 on the volume delivered under constant head. 



Thus, in the pump with radial vanes, a 10 per cent, diminution in head 

 about the point of maximum efficiency is accompanied by a 23 per cent, 

 increase in B.H.P., while the same percentage decrease in the second 

 pump only involves a 1\ per cent, increase in B.H.P. 



On the other hand, the corresponding increase in the volume delivered 

 is 31'3 per cent, with radial vanes as against only 11/7 per cent, in the 

 other pump. A glance at the two curves shows that the efficiency falls 

 off much more rapidly with varying conditions when the vanes are radial. 



Fig. 327 indicates approximately the effect of a variation in y on the 

 speed (angular velocity) required to deliver a given volume of water, 



1 By courtesy of the Buffalo Forge Company. 



