Screw pumps belong to the family of dry compressing gas transfer pumps. (Learn more about the origins of dry pumps here) They are positive-displacement pumps that use two screw shaped intermeshing rotors to move gas along the screw’s axis. They are frequently used in industrial vacuum applications, often in combination with roots blowers and as oil-free roughing pumps in high and ultrahigh vacuum systems.
Working principle: How do screw pumps work?
Screw pumps operate using two counter-rotating screw rotors which are engineered so that they rotate “towards each other”. This traps the gas in the space between the “screws” of their rotors. As the screws rotate, this trapped volume decreases which not only compresses the gas but moves it towards the exhaust.
Fig. 1 schematic of a classic screw pump (without motor)
Fig. 2 single ended
Fig. 3 double ended
Fig. 4 cantilever design
In the first cases (Fig 2, 3), mechanical bearings support the rotors at both ends. The cantilever design (Fig.4) solution supports the rotors at the high pressure end and the rotors can be cooled internally. A motor drives the two rotors via a gear. Gear and the bearings are lubricated but separated from the pumping mechanism ('vacuum generator') via shaft seals or labyrinth seals, thus the compression is oil-free. The rotors have no mechanical contact between each other and the pump housing, resulting in zero mechanical wear. To keep the installed electrical power low, modern pumps are driven by an electronic frequency converter and rotate slower at pressures ranges near atmospheric pressure. Some versions use so-called blow-off valves instead to keep the rotating speed constant also at high pressures. Cooling is normally done by water.
The performance of screw pumps
Screw pumps can reach ultimate pressures of ~ 1e-3 mbar. The operating pressure range is 103 to10-2 mbar. Various sizes with pumping speed ~ 60 - 1200 m³/h are available. The pumping speed in the vacuum range < 10 - 50 mbar can be enhanced by placing a roots blower on top. These combinations, often in one single pump housing, are available up to 9000 m³/h
Advantages & Disadvantages of screw pumps
- high robustness
- high tolerance against water vapour and particles/dust
- scales to customer requirements
- very high pumping speeds
- no contamination of the medium being pumped
- frictionless rotation - rotor wear is eliminated
- highly efficient due to internal compression
- operational costs and maintenance requirements are relatively low
- frequency converter operation – easily optimised for process requirements – leads to high energy efficiency
- a gas ballast needed to pump light gasses
- worse ultimate pressure and lower pumping speed for light gasses (Helium and Hydrogen) if without gas ballast
- cannot be scaled down to small pumping speeds below 50 m³/h; below approx. 100 m³/h multistage roots or scroll pumps are used
Screw pumps have become the standard solution in almost every industrial vacuum process. Vacuum furnaces for brazing or sintering, metallurgical systems, even steel degassing plants use the advantage of dust resistance and long service intervals. In food processing, food drying, food packaging and even freeze dryers tend to use oil free screw pumps to avoid the contamination of pump oil by water or debris from the process. Large scale coating like architectural glass coaters use screw pumps as roughing pumps for the high vacuum pumps. Screw pumps also are the ideal choice for regeneration of larger cyropumps.
Screw pumps are today also the standard primary pump in large scale scientific experiments such as big storage rings, gravitational wave detectors or space simulation chambers. One example is the KATRIN experiment which is the world's largest Ultra High Vacuum (UHV) chamber, where the initial evacuation is done by screw pumps.
Picture 1 KATRIN UHV vessel during transport
Source: Karlsruhe Institute of Technology
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