702 HYDRAULICS AND ITS APPLICATIONS 



In every case provision must be made for distributing the air evenly 

 and in small bubbles among the water in the lift tube, since experiment 

 shows that the system is then much more effective than where the air 

 bubbles are so large as to fill the tube. The diameter of the bubbles at 

 their initiation should be about J inch. Some such arrangement of foot 

 box, as shown in Fig. 840 c, is essential for efficient working. Comparative 

 tests show that this type, in which air enters all around the circum- 

 ference of the lift tube gives efficiencies 20 per cent, greater than that 

 shown in Fig. 340 d, in which the air is supplied in a single central jet. 



Theory of the Air Lift Pump. 



Let V w = volume of water raised in cubic feet per second. 



r. /M . = mean volume of air used, in cubic feet per second, 



during its passage through the rising main. 

 V a = volume used in cubic feet ( per second, at atmospheric 



pressure, p a . 

 ' pi = pressure at base of rising main. 



Then, assuming isothermal expansion of the air in the lift tube,we have : 



y 



The mean specific gravity of the mixture in the tube = 



r,,.+ iv 



. . Head producing flow = h s (1i d + h s ) _T feet of water, 



w V wi 



/Y _i_ Y \ 

 = ^ s \ ~~ W F " / (^ (l ~^~ '''*) ^ ee ^ ^ m i x ^ ure m 



the lift tube, 

 Y 

 = Y^ h s h d feet of mixture. 



Equating this to the sum of the friction head h ft and the kinetic 

 head h m we have, on reduction : 

 y y ^ct + h f + h v 



. T7 _ T7 Pi - ^ . /l d + /l / + fe ^ 

 'Pa 



34 log, 



*Pa 



This relation enables the volume of free air per cubic foot of water to 



