706 HYDEAULICS AND ITS APPLICATIONS 



(9) In starting the pumping the air should be admitted slowly. 

 Pumping will not commence immediately, but after several seconds, 

 perhaps even a minute, water will come with a rush. This is followed by 

 a lull, after which the operation becomes more uniform. The valve can 

 then be opened until continuous flow is obtained. 



For deep well pumping, the air lift pump is very suitab 1 ^, since it can 

 be used in any bore hole of sufficient diameter to admit the necessary pipe 

 lines, and can take advantage of the whole discharging capacity of 

 the hole. 



In addition, the absence of moving parts below the surface, the 

 possibility of installing the compressing plant at practically any distance 

 from the bore hole, and of pumping corrosive liquid or water carrying 

 solid matter in suspension, together with the certainty of operation, give 

 the system very obvious advantages, and in many instances greatly 

 outweigh the disadvantage of moderate efficiency. 



With hot liquids, too, the efficiency is augmented, since the volume of 

 air in the rising main is increased by the rise in the temperature, while 

 in many instances the aerating effect of the air is an advantage. 1 



ART. 190. HYDRAULIC AIR COMPRESSOR. 



By reversing the action of the air lift pump, and allowing water, under 

 a head h d , to flow down a vertical pipe of length (h d + /O, which has a 

 length h s submerged in the tail race (Fig. 341) a type of air compressor 

 is obtained which is fairly efficient, and has obtained some success from 

 its simplicity of construction. 



Water entering at the upper end of the down pipe induces a series of 

 small air jets through suitably placed openings, and, if the velocity is 

 sufficiently great, carries the entrained air to the bottom of the pipe, where 

 its pressure = 2*3 h s Ibs. per square inch approximately. The water is then 

 allowed to escape, while the air is collected in an air chamber surrounding 

 the falling main. 



The pressure to which the air may be compressed is thus independent 

 of the supply head and depends solely on h s . Since, however, the h< 

 required to maintain the required velocity of flow increases with h s , 

 limits the pressure attainable. 



1 Further information on this subject may be obtained from the following papers : 



" Proceedings Institute Civil Engineers," vol. 140, p. 323. 



" Proceedings Institute Civil Engineers," vol. 163, 1905-6, part I., p. 353. 



" British Association of Waterworks Engineers," 1903. 



Engineer, January 10, 1908, p. 26. , 



