RECIPROCATING PUMPS 



581 



woude for the purpose of maintaining the level of the Amsterdam Canal. 

 The plant is required to lift over a range of heads varying from *5 foot to 

 8 feet, and to discharge 1,500 cubic feet per second against a normal head 

 of 1'8 feet at 4'5 revolutions per minute. 



Under normal conditions the vane angle at entrance is 24, and at 

 exit 48. The diameter is 28 feet, the diameter of the circle to which 

 the vanes are tangent being 5*5 feet. An important feature of the 

 installation is the movable breast float B which is fitted in front of each 

 wheel. This breast is pivoted on a horizontal axis at its lower end, and 

 is free to take up a natural and unrestrained position in the current of 

 discharging water. It is constructed of heavy planking and angle iron, 

 and the comparatively high efficiency of the plant is attributed largely to 

 the action of this float. The overall efficiency varies from 40 per cent, 

 under 1*6 feet head to 74 per cent, under 6'6 feet head. 



ART. 158. THE SCREW PUMP OR ARCHIMEDEAN SCREW 

 has in the past been largely used in Holland, and works with advantage 

 against heads not exceeding 10 feet, having an efficiency equal to that of 

 the scoop wheel. The 

 pump consists of an 

 inclined shaft carrying 

 one or more helices of 

 considerable diameter, 

 which rotate with small 

 clearance in a closely - 

 fitting tube or open semi- 

 circular channel con- 

 necting head and tail- 

 water. 



The angle of inclina- 

 tion of the shaft to the 

 horizontal is so arranged 

 as to be less than that of the helical surface, so that water on being 

 admitted to the bottom of the tube always tends to run down this 

 surface. Thus (Fig. 267), a particle of water admitted at P tends to 

 flow to the point Q. By a rotation of the axis portions of the helix from 

 Q to R successively adopt the same position as Q relative to the axis. As 

 they do so the water tends to flow into these positions and is thus passed 

 along the screw, finally emerging at the top into the head-water. The 

 most advantageous angle for the helix is found to be between 30 and 40. 



FIG. 267. Archimedean Screw. 



