582 



HYDRAULICS AND ITS APPLICATIONS . 



In the more modern form of screw pump the screw itself is similar to a 

 screw propeller, but usually carries about six blades. This is carried on 

 a horizontal or vertical axis and works with small radial clearance in a 

 cylindrical chamber through which flow takes place. In such a pump, 

 installed some few years ago in connection with the Chicago main drainage 

 scheme, the screw has a diameter of 13 feet 6 inches, and when rotating 

 at fifty-two revolutions per minute, has a capacity of 28,000 gallons per 

 minute against 3'5 feet head. 1 



Although both screw pumps and scoop wheels are capable of high 

 efficiencies they are cumbrous, and run at inconveniently low speeds, and 

 are in almost every case being replaced by the centrifugal pump. 



ART. 159. THE RECIPROCATING PUMP. 



The oldest type of reciprocating pump is the bucket pump illustrated 

 diagrammatically in Fig. 268. Originally devised 

 as a lift pump, it was fitted with a hollow bucket 

 or piston surmounted by a valve, and with a foot 

 valve to prevent escape of water on the down 

 stroke. 



On the up stroke of the pump a partial vacuum 

 is produced below the bucket, and the pressure 

 of the atmosphere acting on the free surface in 

 the supply reservoir produces a flow up the suc- 

 tion tube in virtue of this difference of pressure. 

 If li s = suction head in feet of water for any 



given position of the bucket. 

 ir a = atmospheric pressure (in feet of water). 

 Then, neglecting frictional losses and the effect 

 of acceleration, the pressure head on the under 

 side of bucket = ir a h_, feet. This has its mini- 

 mum theoretical value when -n a //, = 0, i.e 

 when an absolute vacuum is produced below the 

 bucket, and the maximum possible suction head 

 is thus equal to TT (I , or 34 feet approximately. 

 Practically, owing to the resistance of the suction valve, leakage at joints 

 and past the bucket, and to the liberation of dissolved air at low pressures, 

 this head is impossible of attainment, a suction head of 24 feet being only 

 maintained with difficulty, and the maximum value of the suction head, 



1 For a sketch of this pump see the Mechanical Engineer, March 20, 1908, p. 367. 



FIG. 2G8. Bucket or Lift 

 Pump. 



