LAND DRAINAGE BY MEANS OF PUMPS. 37 



capacity by 50 per cent, but at this overload the efficiency of the 

 pnmp is but a small part of its best figure. 



Rotary displacement pumps and plunger pumps have occasionally 

 been used for pumping drainage water, but they are not adapted to 

 this use. They may have a very high mechanical efficiency, but 

 their cost more than offsets this advantage. Scoop wheels have been 

 extensively used in England and Holland, and to some extent in this 

 country in Louisiana. They may be made to have a higher efficiency 

 than centrifugal pumps, especially for very low lifts, but they are not 

 adapted to locations where the head is subject to wide fluctuations. 

 A full description of the construction and operation of these scoop 

 wheels has been given by W. W. Wheeler. 1 Another publication 2 

 describes a recent large and sucessful installation in Holland, and 

 gives the results of tests. 



Recently large screw pumps have been installed for the pumping 

 of drainage water from districts in Louisiana. One district of about 

 9,000 acres has installed and is operating two such pumps, each 

 having a diameter of discharge pipe of 78 inches. The city of New 

 Orleans recently contracted for and is now installing 12 such pumps 

 for the disposal of drainage water; these have diameters of discharge 

 of 12 feet. The impeller in the screw pump is a segment of a screw 

 and is somewhat similar to a boat propeller, except that it has many 

 more blades. On the discharge side diffusion vanes take the water 

 from the blades of the impeller and change its partly spiral motion 

 to one which is parallel with the axis of the discharge pipe. It is 

 expected that such a pump will operate more satisfactorily under 

 changing heads than will a centrifugal pump, and for very large 

 installations it is cheaper than a centrifugal pump of equal capacity. 



SOURCES OF POWER. 



Steam, gas, and electricity are all used as sources of power for 

 pumping plants. In Illinois soft coal is abundant and cheap, though 

 in some places crude oil or natural gas may be found more available. 

 A suction gas producer with a gas engine has been tried, but such a 

 plant requires hard coal or coke for fuel, which greatly increases the 

 expense. Furthermore, such a type of plant is not adapted to inter- 

 mittent working. Electricity is by far the most convenient source 

 of power where it is available, but in general it will be considerably 

 more expensive than using coal to produce steam. Its convenience 

 will perhaps offsel its cost , especially for a small plant. In the latter 

 part of tins bulletin, under the heading "Amount and cost of pump? 

 Lug," data from a number <>f districts are given which show the com- 



1 The Drainage oi Pen and Low Land ■ by Gravitation and steam Power. London and New York, 

 1888. 

 « engineering News, 63 (1910), No. 2d, p. 681. 



