CENTRIFUGAL PUMPS 651 



the suction reading will be ( H s + H/ a + ^- ) feet, and the delivery 



\ ^ 9 / 



reading will be H d -f H^ - + ~ - if the section of the pipe is 



A ^9 



increased after passing the manometer, and will be H d -f- H fd if it 

 remains of the same cross section. In either case if v s = v d the " mano- 

 metric head " will be given by H -f- II j -\- -=- foot = H m . 



* 9 



It is worth noting that this is greater than the head //' obtained by 

 adding the dead lift H and the friction head II f . In the average pump v 

 may range up to 8 feet per second, so that in a low lift pump the term 



should not be neglected. 



^j (I 



Efficiency of Pump. Manometric Efficiency. Neglecting losses due to 

 disc friction, mechanical friction, and slip, we have seen that the work 



done on the water per Ib. is - foot Ibs. The ratio of the energy H m 



/ 



obtained from the pump per Ib. of water, to - l is sometimes termed the 



hydraulic efficiency, but more correctly the theoretical manometric 

 efficiency if. 



~ Ua u<a ~ Ua (a fa cot y) 

 Hydraulic Efficiency. The work actually done on the water per Ib. is 



given by - - -j- TT/^I where L h represents the total loss of energy in 



foot Ibs. expended in overcoming hydraulic resistances in the pump itself, 

 and the ratio 



r, = 



h 



is more correctly termed the hydraulic efficiency. 



Actual Working Efficiency. The actual working efficiency is the ratio 

 of the energy obtained from the pump to the work done on the pump 

 shaft per Ib. of water. If N be the number of revolutions per second ; Q 

 the volume per second in cubic feet ; and T the turning moment on the 

 shaft in feet and Ibs., 



_ H m 



" 2 TT N T "" w 3 % Lh+L m 



9 W Q 



where L h and L m are respectively the hydraulic and mechanical losses in 



