Speed is not increased for additional depth, because more runners are added, and this compounding of the runners increases the efficiency of the pump.
A ball bearing is placed over the stuffing-box to carry the entire weight of all the movable parts of the pump, and also the column of water. In deep wells cone roller bearings are used in place of the ball bearings.
The pumps are made to fit all sizes of wells and of any desired capacity. Runners of various pitches are made for the different sizes in order to suit the supply of water or the power available. If, after testing, the supply of water in the well is found to be limited, the runners are changed to raise the amount of water due to a given horse-power, then runners can be furnished with a pitch suited to lifting that particular amount of water.
Fig. 457.
For example, if one runner at a given speed, gives 10 pounds pressure per square inch, then two runners would give 20 pounds; three, 30 pounds, and so on. For this reason water may be elevated higher above the discharge with this pump than with a centrifugal, for it would require a higher rate of speed to lift a given amount of water 20 feet with one runner, than to lift the same amount 5 feet. Hence the advantage of compounding the runners as the lift is increased. The compounding of runners is one of the main features of success and efficiency of this pump.
Fig. 458.
Where the water is beyond the suction limit this pump can be used to raise the water to the surface, discharging into the suction of the force pump. In this manner, whatever surplus of power the propeller pump might have in raising the water to the surface, would be utilized in helping the water through the force pump.