LAND DRAINAGE BY MEANS OF PUMPS. 39 



quantity pumped must decrease. Along the upper Mississippi and 

 the Illinois River the greatest demand on the pumping plant usually 

 occurs at the time when the head pumped against is greatest. If the 

 speed of the pump be fixed to give full capacity at the greatest head, 

 at lower heads the discharge of the pump will be greatly increased 

 and the efficiency much lowered. As the average lift will be much 

 lower than the maximum, usually scarcely half, the yearly efficiency 

 of the pump will be small and the consumption of power large. By 

 far the better method is to connect the pump and the motor by 

 means of a belt with three sizes of pulleys or with silent chain drive 

 with two or three sizes of sprocket wheels. The pump speed can 

 then be altered to suit the head. In Louisiana, where the fixed- 

 speed ratio is used, the greatest demand on the pumping plant occurs 

 at the lowest head, and as the head increases the amount of water to 

 be pumped greatly decreases; consequently the capacity of the pump 

 fits the need. 



AUXILIARIES. 



The steam plant will require the usual auxiliaries. A surface 

 condenser will furnish pure feed water, but some form of jet con- 

 denser is simpler and cheaper. For priming the pumps a steam 

 ejector is the simplest arrangement, though an air pump may be 

 used. Separators and traps, filters and heaters, should be used as 

 needed for safe and economical operation. Piping and boilers 

 should be well covered, to reduce as much as possible the amount of 

 radiation and condensation of steam. 



In large plants boilers, engines, and pumps should all be divided 

 into two or more equal-sized units, so arranged that they can, when 

 necessary, be operated independently, and the auxiliaries should be 

 installed in duplicate. 



CALCULATION OF THE SIZE OF PUMPING PLANT. 



To illustrate the method of determining the size of machinery to 

 be installed in a pumping plant, the calculation will be made for a 

 district of 10,000 acres. To remove a depth of water of one-fourth 

 inch over the whole district in 24 hours will require a capacity of 

 105 cubic feet per second. For such a district it will be best to use 

 two separate pumps, which should be identical in size. A 30-inch 

 pump will bo rated at about 49 cubic feet per second, a 32-inch 

 pump at about 56 cubic feet per second, and a 36-inch pump at 

 about 71 cubic feet per second. In this case it would bo best to use 

 two 32-inch pumps, which would seem to have a slight margin of 

 excess capacity. However, since pumps of this stylo so frequently 

 fail to have the capacity assumed, they should be taken only under 

 the guaranty of the manufacturer and subjected to a thorough test 

 after they are installed. The pumps will need to bo run at about 



