Screw pumps are rotary, positive displacement pumps that can have one or more screws to transfer high or low viscosity fluids along an axis.  A classic example of screw pumps is the Archimedes screw pump that is still used in irrigation and agricultural applications. 

 

Although progressive cavity pumps can be referred to as a single screw pumps, typically screw pumps have two or more intermeshing screws rotating axially clockwise or counterclockwise.  Each screw thread is matched to carry a specific volume of fluid.  Like gear pumps, screw pumps may include a stationary screw with a rotating screw or screws.  Fluid is transferred through successive contact between the housing and the screw flights from one thread to the next.  Geometries can vary.  Screw pumps provide a specific volume with each cycle and can be dependable in metering applications. 

 

The geometries of the single or multiple screws and the drive speed will affect the pumping action required.  The capacity of screw pumps can be calculated based on the dimensions of the pump, the dimensions of the surface of the screws, and the rotational speed of the rotor since a specific volume is transferred with each revolution.  In applications where multiple rotors are used, the load is divided between a number of rotating screws.  The casing acts as the stator when two or more rotors are used.  Based upon the needs of the application, timed or untimed rotors may be chosen. Untimed rotors are simpler in design. 

 

The combination of factors relating to the drive speed, flow, and the characteristics of the fluid transferred may affect the flow rate and volume fed through each cavity. In water and wastewater treatment applications, a less viscous solution will require a lower power drive compared to untreated sewage, excess sludge, or concentrated slurries, which may require a higher power motor.  The viscosity of the fluid transferred and the lift required may affect the speed and power required.  Indicators of pump malfunction include decrease in flow rate or increased noise. The efficiency of screw pumps requires that each rotor turns at a rate that allows each cavity to fill completely in order to work at full capacity. 


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