CENTRIFUGAL PUMPS 



655 



vortex chamber. On adding this gain of head to that previously obtained 

 in passing through the wheel we have : 



Pv -PB + P3-P*_K W + /a 2 ) (1 - c 2 ) 



W 



-/3 2 cosec 2 y 



(8) 



= total gain of pressure head in pump. 



(e) Change of Pressure in Guide Vanes. In Stanton's experiments on 

 a pump fitted with guide vanes and radial impeller vanes, the guide 

 efficiency varied from 59 per cent, to 70 per cent., while with back curved 

 vanes this efficiency varied from 47 per cent, to 52 per cent. With a 

 third wheel of 12 inches diameter having curved vanes and working 

 under a head of 65 feet, 

 the guide passages had b 



an efficiency of 75 per 

 cent. 



In each case, with 

 curved vanes, the vane 

 angle at inlet was 15, 

 at outlet 30, guide angle 

 a = 3. Vane thickness 

 05 inch. Four guides 

 were fitted, and the num- 

 ber of impeller vanes was 

 varied from twelve to twenty-two without any appreciable effect on the 

 efficiency. 



An examination of Fig. 323 shows that the correct value of a is 

 given by c e 



tan a = - -: = 



FIG. 323. 



c e 



e d 



ad a e 



u s f s cot y 



Since the guide passages are rectangular in section the guide vanes 

 should diverge at an angle of about 11 (p. 87) for maximum efficiency. 

 Under these conditions, with a designed so as to prevent shock at entry 

 to the guide vanes as far as possible, about 75 per cent, of the kinetic 

 energy of discharge is converted into pressure energy. 



The total gain of pressure head in the pump is then equal to : 



- /a 2 cosec 



where k has a maximum value of about *75. 



