A screw pump is a positive-displacement pump that use one or several screws to move fluid solids or liquids along the screw(s) axis.
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The screw pump is the oldest positive displacement pump.[1] The first records of a water screw, or screw pump, dates back to Ancient Egypt before the 3rd century BC.[1][2] The Egyptian screw, used to lift water from the Nile, was composed of tubes wound round a cylinder; as the entire unit rotates, water is lifted within the spiral tube to the higher elevation. A later screw pump design from Egypt had a spiral groove cut on the outside of a solid wooden cylinder and then the cylinder was covered by boards or sheets of metal closely covering the surfaces between the grooves.[1]
A cuneiform inscription of Assyrian king Sennacherib (704–681 BC) has been interpreted by Stephanie Dalley[3] to describe casting water screws in bronze some 350 years earlier. This is consistent with classical author Strabo, who describes the Hanging Gardens as watered by screws.[4]
The screw pump was later introduced from Egypt to Greece.[1] It was described by Archimedes,[5]: 242–251 on the occasion of his visit to Egypt, circa 234 BC.[6] This suggests that the apparatus was unknown to the Greeks before Hellenistic times.[5]: 242–251
Three principal forms exist; In its simplest form (the Archimedes' screw pump or 'water screw'), a single screw rotates in a cylindrical cavity, thereby gravitationally trapping some material on top of a section of the screw as if it was a scoop, and progressively moving the material along the screw's axle until it is discharged at the top. This ancient construction is still used in many low-tech applications, such as irrigation systems and in agricultural machinery for transporting grain and other solids. The second form works differently; it squeezes a trapped pocket of material against another screw. This form is what is typically referred to in modern times with the term 'screw pump'. The third form (the progressive cavity pump or eccentric screw pump) squeezes a trapped pocket of material against the cavity walls by spinning the screw eccentrically.
Like all positive-displacement pumps, all various kinds of screw pumps function by trapping a volume of material somehow, and then moving it. There are numerous ways to shape the screw or the cavity to accomplish this function, and the number of screws working together can be many. The term 'screw pump' refers generically to all of these types. However, this generalization can be a pitfall as it fails to recognize that the different ‘screw' configurations have different advantages and design considerations for each, which lead to the various kinds being suitable for very different use cases, material types, flow rates, and pressures.
One of the most common configurations of a screw pump is the three-spindle screw pump. Three screws press against each other to form pockets of the pumped liquid in the grooves of the screws. As the screws rotate in opposite directions, the pumped liquid moves along the screws' spindles. There is nothing magical about two, three or any number of screws; pockets are formed regardless. Three rather than two spindles are used because this allows the central screw to experience symmetrical pressure loading from all sides. This ensures that the central screw is not pushed sideways, will not be bent, and thus eliminates the need for radial bearings on the main axle to absorb radial forces. The two side screws can then be made as internally-hidden free-floating rollers, lubricated by the pumped liquid itself, thus eliminating the need for bearings on those axles. This is commonly desired because seals and bearings on machines are common sources of failure.
Three-spindle screw pumps are most often used for transport of viscous fluids with lubricating properties. They are suited for a variety of applications such as fuel-injection, oil burners, boosting, hydraulics, fuel, lubrication, circulating, feed, and to pump high-pressure viscous fluids in offshore and marine installations.
Compared to various other pumps, screw pumps have several advantages. The pumped fluid is moving axially without turbulence which eliminates foaming that would otherwise occur in viscous fluids. They are also able to pump fluids of higher viscosity without losing flow rate. Also, changes in the pressure difference have little impact on screw pumps compared to various other pumps. There is also very little back-drive on the power axle, and the output of the flow is typically very even and doesn't pulsate much.
The Screw pump transports liquid or semi-solid substances through the gap between the screw or spiral rotor and the shell. According to the different screw structures and working principles, Screw pump can be divided into single Screw pump, double Screw pump and three Screw pump.
1.Single Screw pump: a single Screw pump consists of a screw rotor and a fixed housing. When the screw rotates, the liquid is sucked in and pushed along the axial direction of the screw, thus achieving transportation. Single Screw pump is suitable for low to medium viscosity liquid and semi-solid substances, commonly used in sewage treatment, food processing, petrochemical and other fields.
2.Twin Screw pump: twin Screw pump consists of two symmetrically rotating screw rotors and a shell. The gap between the screws forms a sealing chamber, and the liquid is sucked in and transported through the gap between the rotor threads. Twin screw pump has high pressure stability and conveying capacity, and is suitable for media with high viscosity, low flow rate and solid particles, which are commonly used in chemical, petroleum and marine industries.
3.Three Screw pump: 3 Screw pump are composed of three screw rotors and a shell. Multiple sealing chambers are formed between the screws, through which liquid is sucked in and pushed forward. 3 Screw pump have higher pressure stability and transmission capacity, and are suitable for liquid media with high viscosity, high temperature and high pressure, which are commonly used in petroleum, chemical industry, power and other fields.
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The advantages and disadvantages of Screw pump
Advantages:
Wide application: Screw pump is suitable for a variety of media, including high viscosity, semi-solid substances and liquids containing solid particles. They are widely used in various industries, including chemical, petroleum, food processing, sewage treatment, papermaking, power, etc.
Efficient energy utilization: Screw pump has efficient conveying efficiency and stable pressure characteristics, which can realize energy conservation.
Low pulsation: the continuous conveying characteristic of Screw pump makes the flow pulsation low, which is conducive to the stability of some technological processes.
Can transport high temperature medium: some types of Screw pump can handle high temperature liquid. Screw pump is a good method for extracting heavy oil and sand bearing crude oil. The Volumetric efficiency of Screw pump increases with the increase of crude oil viscosity.
Self suction capacity: Screw pump has better suction capacity, so it works better in long pipes, oil with higher viscosity and air in the fluid.
Disadvantages:
High initial investment: the initial investment cost of Screw pump is relatively high, especially for the Screw pump with large capacity and high performance.
High maintenance requirements: Screw pump requires high maintenance and cleaning requirements, especially when conveying solid particles or corrosive media, the screw and pump body need to be cleaned and maintained regularly.
Sensitivity to liquid: Screw pump is sensitive to the viscosity and characteristics of liquid, and the change of liquid may affect the performance and output of the pump.
Overall, Screw pump has been widely used in many industries and application fields due to its wide applicability, efficient energy utilization and low pulsation. Although the price of Screw pump is higher than other types of pumps, the characteristics of Screw pump are more obvious. For applications involving the transportation of fuel, oil and other lubricants, Screw pump and gear pump are usually selected as pumping technologies. Since the fluctuation of fluid thickness affects the performance of centrifugal pump much more than that of positive displacement pump, Screw pump and gear pump are usually the most effective solutions for oil and fuel. When choosing to use Screw pump, factors such as specific application demand, budget and medium characteristics should be comprehensively considered.