Rotary displacement pumps use rotation to move fluid and eliminate air from lines. Gear, screw, and moving vane pumps are the three main types, each with their own advantages and disadvantages. Tight clearance is required for efficient operation.
Rotary displacement pumps are pumps that move fluid using the principles of rotation. The vacuum created by the rotation of the pump captures and sucks the liquid. Rotary pumps are very efficient because they naturally remove air from lines, eliminating the need to manually purge air from lines.
Rotary pumps also have their weaknesses. Due to the nature of the pump, the clearance between the rotating pump and the outer rim must be very tight, requiring the pumps to rotate at a slow and constant speed. If the pumps are operated at high speeds, the fluids will cause erosion, just like ocean waves polish stones or erode rocks into sand. Pumps that experience such erosion eventually show signs of enlarged clearances, which allow liquid to slip through and detract from pump efficiency.
Rotary displacement pumps can be grouped into three main types. Gear pumps are the simplest type, consisting of two gears side by side with the teeth interlocking. The gears move away from each other, creating a current that traps the fluid between the gear teeth and the outer casing, eventually releasing the fluid onto the discharge side of the pump as the teeth mesh and spin again. Many small tines maintain a constant flow of fluid, while fewer large tines create a tendency for the pump to discharge fluid in short pulsating jets.
Screw pumps are a more complicated type of rotary pump, featuring two screws with opposite threads, i.e. one screw turns clockwise and the other counterclockwise. The screws are each mounted on shafts that run parallel to each other; the shafts also have gears on them that mesh with each other to rotate the shafts together and keep everything in place. The rotation of the screws, and consequently of the shafts on which they are mounted, draws the fluid through the pump. As with other forms of rotary pumps, there is minimal clearance between the moving parts and the pump body.
Moving vane pumps are the third type of rotary pump, consisting of a cylindrical rotor enclosed in a similarly shaped housing. As the rotor turns, the vanes trap the fluid between the rotor and casing, drawing the fluid through the pump.
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