CENTRIFUGAL PUMPS 



649 



gence, and thus the tendency to instability of flow, which is one of the 

 important factors in reducing the efficiency of the pump as compared with 

 the inward flow turbine. Since the relative velocity of flow increases 

 outwards, the passages would in all probability be most effective if designed 

 so as to be convergent outwards to suit this increased relative velocity. 

 This view is borne out by the results of a series of experiments carried out 

 by Mr. J. A. Smith, of Melbourne, 1 in which instantaneous photographs 

 of the flow through the impeller of a pump model while freely discharging 

 into the atmosphere, indicate that at a certain velocity the water tends to 

 leave the leading face of a passage as indicated in Fig. 321, which is 

 reproduced from his paper. If discharging under pressure it is evident 

 that the empty space shown in these passages would be occupied by dead 

 water and would be the 

 source of considerable loss 

 in eddy production. The 

 successive curves 1, 2, and 

 3, mark the boundaries of 

 the stream with increasing 

 velocities. 



In practice, the purpose 

 for which the pump is 

 designed determines the 



value of 7, which may FlG 321 



have any value from 15 



to 90, so that in general the water on leaving the vanes has a compara- 

 tively high absolute velocity. 



As is apparent from Fig. 320, at discharge, 



/ 3 cot y = ?/. 3 ?r g , 



while the relative velocity of water and vane is given by 



&'r = fa co sec y. 



Work done on Pump. The turning moment on the shaft, equivalent to 

 the change per second in the angular momentum of the water passing 

 through the wheel 



WQ 



it's 7 a ?r a r a } foot Ibs. 



Work done on water per second 



WQ , 



I vro r a 



?<'2 r 2 } w foot Ibs. 



1 See Engineering, December 5, 1902. This view is also borne out by the results of 

 experiments by C. B. Stewart, Bulletin of University of Wisconsin. No. 173, 1907. 



