CENTRIFUGAL PUMPS 657 



to those of A, but are not extended to the outer periphery of the return 

 chamber. In design D the opening on the radial line is only one-half 

 that in A, B, and C. The return passages are similar to those in C. 

 The head and efficiency curves are given for speeds of 1,000, 1,500 and 

 2,000 revolutions per minute, and show that design D is much inferior 

 due to throttling at the throat of the guide passages. B is superior to 

 both A and C in generation of head, and superior to A in efficiency 

 especially at low speeds, but is inferior to C in efficiency except at high 

 speeds. At these speeds great loss of energy results on the eddying 

 which takes place in the collecting chamber of C which is prevented by 

 the guides forming the continuous passages of design B. Design B is 

 most usually adopted as best meeting average conditions. Design G, 

 which approaches more nearly to the conditions of a single chamber 

 pump, gives considerably higher maximum efficiency at speeds of 1,500 

 and 2,000 revs., due to less power being absorbed by friction against the 

 walls of the guide passages, and also to tangential motion imparted to the 

 water as it enters the subsequent impeller by the tangential curvature of 

 the return passages. 



ART. 176. MANOMETRIC EFFICIENCY UNDER DIFFERENT WORKING 



CONDITIONS. 



If there were no losses in the pump other than those already considered, 

 the expressions just obtained would give the manometric head H m . 

 Actually, frictional losses and leakage in the impeller itself along with 

 losses due to shock at entrance to impeller vanes and to guide vanes at any 

 other than normal speed and discharge, make the true manometric head 

 less than that calculated. The ratio of the calculated manometric head 



neglecting these losses to the head - - is termed the theoretical 



<7 



manometric efficiency and will be denoted by r?'. 



If the sum of the dead lift, H, and of the friction loss H f in suction and 

 delivery pipes be called //', the total gain of pressure in the pump must 



v* 

 be 77' + f - feet, where v is the velocity of flow along the discharge pipe. 



A 



Thus, neglecting hydraulic losses in the impeller : 



'"3 2 +/2 2 /a 2 cosec 2 y j increase in pressure ) _ H , _^ feet /g\ 



2 y 1 after leaving impeller! " 2 g 



and we have : 



, "a 2 +/2 2 -/3 2 cosec 2 y + k vf 



H.A. U U 



