The arrangement of the impellers and the method of providing water to seal the parts is shown in [Fig. 86]. A reservoir containing water is provided on the discharge side of the pump and a small pipe leads from this reservoir to the suction side of the pump. The vacuum lifts water from the reservoir and discharges same in a spray into the suction chamber. This water passes through the pump and is separated from the air in the discharge chamber to be returned to the suction chamber by the vacuum. This operation will start automatically as soon as any degree of vacuum is formed and will cease as soon as the pump is shut down.
FIG. 87. ROTARY PUMP ARRANGED WITH DOUBLE-THROW SWITCH FOR REVERSING PUMP.
Any of these rotary pumps having no valves can be changed to an air compressor by reversing the direction of rotation. This is adapted by the American Rotary Valve Company in connection with their wet separators to discharge the contents of the separator into the sewer, on all of their smaller-sized plants. [Fig. 87] shows one of these plants arranged with double-throw switch for reversing the electric motor used to operate the pump and also shows the arrangement of the rotary brush which is used to clean the screen in the wet separator, as has been explained in [Chapter VIII].
FIG. 88. POWER CONSUMPTION AND EFFICIENCY ROOT TYPE OF PUMP.
FIG. 89. THE ROTREX VACUUM PUMP, USED BY THE VACUUM ENGINEERING COMPANY.
The power consumption and efficiency of this type of pump are shown in [Fig. 88]. The watts per cubic foot of free air (Curve a-b) show a much lower consumption of power at the lower vacuum than any of the pumps already tested. This is probably due to the fact there is no internal friction. It will be noted that the power to operate at no vacuum is but 10 watts per cubic foot of free air, while all the others require from 24 to 34 cubic feet. This also results in the efficiency curve (c-e, [Fig. 88]) reaching its maximum value at a lower vacuum than in the case of the sliding vane pump ([Fig. 85]).
