The two-screw, low-pitch, screw pump consists of two screws of right-handed and left-handed threads mesh with close clearances, mounted on two parallel shafts. The driving shaft drives the other shaft through a set of herringbone timing gears. The gears maintain the clearances between the screws as they rotate and to promote quiet operation. The screws rotate in closely fitting duplex cylinders that have overlapping bores. All clearances are small, but there is no actual contact between the two screws or between the screws and the cylinder walls.
The complete assembly and the flow diagram are shown in Figure 9.28. Liquid is trapped at the outer end of each pair of screws. As the first space between the screw threads rotates away from the opposite screw, a one-turn, spiral-shaped quantity of liquid is enclosed when the end of the screw again meshes with the opposite screw. As the screw continues to rotate, the entrapped spiral turns of liquid slide along the cylinder toward the centre discharge space while the next slug is being entrapped. Each screw works in similar fashion, and each pair of screws discharges an equal quantity of liquid in opposed streams toward the centre, thus eliminating hydraulic thrust. The removal of liquid from the suction end by the screws produces a reduction in pressure, which draws liquid through the suction line.
Figure 9.28 Two-screw, Low-pitch, Screw Pump
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