THE AIR LIFT PUMP 701 



In general, the depth of submersion h s (Fig. 340) is made from (1*5 to 2) 

 times the lift h d , so that the total length of lift pipe = (2'5 to 3) h d . As 

 the depth of immersion is reduced, the relative air consumption increases 

 and the efficiency diminishes. As will be shown later, a further increase 

 in h g tends to more efficient working, and is advisable where this may be 

 obtained without great expense in deepening the bore hole. 



To keep down frictional losses, the velocity in the lift tube, calculated 

 on the volume of water discharged, should not exceed 5 feet per second. 

 The efficiency of the system, as calculated from the ratio of the work done 

 in lifting water through a height h d , to the indicated work in the air 

 compressor engine cylinder, is generally between 25 and 30 per cent. this 

 allowing for a compressor efficiency of 75 per cent. but under favourable 

 conditions may rise to 45 per cent. 



There are three methods of arranging the pipe lines in a well or 

 bore hole. 



(1) The central air tube system (Fig. 340 a), in which the air pipe is 

 suspended in the centre of the lift tube. 



(2) The annular air tube system, in which the space between the lift 

 tube and the bore hole is used as the air line (Fig. 340 I). 



(3) The side-by-side system, in which the air and lift tubes are carried 

 down the well side by side (Fig. 340 c). 



The first of these systems has the disadvantage that the hydraulic mean 

 depth of the water passage in the lift tube is reduced by the air tube, being 



y ~ *"-f Rr D-d ' 



_ 

 2 TT (R + r) 2 4 



where D and d are the internal diameter of the lift tube and the external 

 diameter of the air tube respectively. 1 



This leads to increased frictional losses and so to diminished efficiency. 

 The system is, however, very suitable for application to a small bore hole 

 of suitable dimensions, since the only additional expense is the provision 

 of the comparatively small air pipe, while any alteration in the length of 

 this, to suit different conditions of working, is a simple matter. In 

 general this will be found more advantageous than the second system, 

 the chief advantage of the latter lying in its possibilities of more effective 

 air distribution. 



Where the well or bore hole is of large diameter, the side-by-side system 

 has many advantages in virtue of its accessibility, simplicity, and flexibility. 



1 With a plain tube of the same sectional area the hydraulic mean depth is given by -j 1 

 where A = V TP^d*. 



