Screw pumps belong to the family of dry compressing gas transfer pumps. 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.
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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.
Mechanical bearings support the rotors at both ends. The cantilever design 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.
Screw pumps can reach ultimate pressures of ~ 1e-3 mbar. The operating pressure range is 103 to 10-2 mbar. Various sizes with pumping speed ~ 60 - 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 m³/h
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.
Pumping technology continues to evolve, with more and more specialized pump systems available for specific applications. Centrifugal pumps have been the standard for many industries for many years. However, more and more operators are looking for screw pumps to handle certain materials.
Centrifugal pumps utilize an impeller design that is great for high-transfer applications and water-like fluids that need to be pumped through piping networks with variable flow rates. With their engineering principals, it’s easy to understand why so many operators depend on centrifugal pumps for a variety of common fluid processing applications. Yet, we all know that different pumps are often better for different jobs.
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Screw pumps are becoming increasingly popular in certain industries where standard centrifugal pumps just aren’t providing the desired flow rates and energy efficiency, or they are having trouble handling high-viscosity liquids and other materials that tend to hinder performance capabilities.
Twin screw pumps and triple screw pumps feature a positive displacement design that has proven to be more versatile and reliable for certain applications—especially in industries like chemical processing, petrochemicals and oil and gas, where viscosity is often a problem for centrifugal pumps.
Operators often lean on centrifugal pumps because of familiarity. These are the pumps they are used to working with and they find ways to adapt them for different performance requirements. They know how centrifugal pumps operate and have adapted their processes to pump what they need to pump. We often see oil processing plants who will blend or heat the crude oil in order to reduce viscosity before it goes through the centrifugal pump system. This is a common practice, but it may be an unnecessary step. What if you could eliminate the heating or blending stage by simply installing a pump system that will better handle the viscous raw fluids?
The truth is, centrifugal pumps rarely operate at their Best Efficiency Point (BEP) because they are being adapted, adjusted and used for applications for which they may not be ideally suited. Meanwhile, screw pumps might provide the desired performance for high-viscosity fluid transfer without having to make these other sacrifices.
Screw pumps contain multiple screws that engage with one another as rotated to form a sealed cavity within the pump casing. As the screw drives turn, the fluid shifts steadily and constantly through the pump. It creates a volumetrically consistent flow rate that is unaffected by pumping pressure or viscosity that might slow down a centrifugal pump.
Some screw pump advantages include:
DXP Pacific works with some of the world’s leading manufacturers of screw pumps. We have great screw pumps from companies like INOXPA and Circor IMO. These screw pumps can be utilized for a wide variety of fluid applications, so it is important to find the right pump for your specific fluid transfer needs.