Eccentric Screw Pump

eccentric screw pump

The eccentric screw pump is a screw pump with only one screw in the pump. Usually the screw of the eccentric screw pump is called the rotor, and the screw bushing is called the stator. The rotor driven by the external power source and the stator are engaged to form a sealed cavity that separates the suction cavity and the discharge cavity, so that the pump can work effectively.

eccentric screw pump

Working principle

The working principle of the eccentric screw pump is vividly used as an analogy. The screw and the conveyed medium can be regarded as the relative movement of the screw and the nut. When the screw rotates, if the limit screw cannot move axially, the nut will move axially along the screw. The screw mentioned here can be regarded as the screw that rotates in the screw pump, and the medium filled with the spiral groove is a liquid nut. However, the mechanism made only according to the above principles cannot lift the conveyed medium from the suction cavity of the pump to the discharge cavity due to the communication from the discharge cavity of the pump to the intake cavity. The lines of contact (ie, the lines of engagement) that engage in the holes of the sleeve act as spacers. This sealed cavity actually acts like a valve in a reciprocating pump. In this way, when the screw rotates, the liquid nut, the medium in the sealing cavity, can move along the axial direction from the suction cavity to the discharge cavity with the sealing cavity. 2 times), effectively draining the medium out of the pump.

The reason why the word “effective” is emphasized here is that, as mentioned above, it is impossible to completely seal the sealed cavity, and there will always be some medium leaking from the discharge cavity back to the suction cavity. Obviously, the pump work is only meaningful when the leaked part of the medium is relatively small. The less the leaked medium, the higher the volumetric efficiency ηv of the pump. Therefore, the volumetric efficiency ηv is an extremely important indicator to measure the performance of the screw pump.

For some types of screw pumps, the sealing chamber cannot completely separate the discharge chamber and the suction chamber of the pump in theory, but only within a certain speed and discharge pressure range. If it can operate normally when transporting a certain medium and still have relatively high volumetric efficiency, then this kind of screw pump still has value. This is because people can study the screw pump composed of different screw numbers, different screw head numbers, and various profiles or various profile combinations based on this theory. The screw pump products are mainly divided into single screw pumps, screw thick slurry pumps, vertical screw pumps, corrosion-resistant screw pumps, etc. When the screw bushing hole meshes with the sealed cavity of different spatial shapes, even if it is theoretically impossible to discharge the cavity It is completely separated from the suction cavity, that is, it cannot be completely sealed, but as long as it can operate normally when a certain medium is transported within a certain performance parameter range and achieve a certain volumetric efficiency, such a screw pump has practical value and can exist. . It can be seen that the profile line that constitutes the screw helical surface, that is, the tooth curve of the helical surface, is the core technology of the screw pump. It is the emergence of various types of new screw pumps that have continuously expanded the application range of screw pumps, and screw pumps have become an emerging industrial pump with a wide range of applications.

According to the above, the working principle of the screw pump can be described as follows: when the screw of the screw pump is driven by the external power to rotate, the suction end of the screw groove is periodically opened, and the spiral concave causes the volume V of the suction cavity. The opening of the groove gradually increases to V+ΔV. According to the Boyle-Marriott law, the pressure Ps in the suction cavity is reduced to

Ps=P’s V/V+ΔV

In the formula, P’s – the pressure in the suction cavity before the spiral groove is opened.

That is to say, due to the rotation of the screw, a vacuum is formed in the suction chamber, and a pressure difference is generated between the pressure on the free surface of the medium and the pressure Ps in the suction chamber. Under the action of the free surface pressure at the inlet end of the pump, the medium enters the suction cavity that forms a vacuum, and then fills the spiral groove formed by the meshing of the screw surfaces in the screw bushing hole, that is, the medium enters the suction cavity that has been opened at one end, in the sealed cavity. Then, with the rotation of the screw, the opening of the sealing cavity is closed by the helical protruding part of the meshing screw, and the medium that effectively separates the suction cavity and the discharge cavity from the sealing cavity enters the sealing cavity and moves toward the discharge cavity along the axis of the screw. Move and discharge straight out of the pump.

It should be noted that the structure of some screw pumps has no screw bushing, which is integrated with the pump body, and the sealing cavity is formed by the meshing of the holes in the pump body with the helical segments.

For the screw pump to operate normally, it is necessary to make the screw meshing with each other rotate synchronously. For the pair of helical surfaces of the driving screw and the driven screw to form a tooth profile, the tooth profile of the cross-section obeys the meshing law of the gears. The driving screw can transmit the rotation to the driven screw without resorting to the gear meshing law for a special transmission force. The screw can transmit the rotation to the driven screw without the need for special force-transmitting gears and other parts to achieve synchronous rotation; and for the non-sealed screw pump whose helical surface of the screw does not obey the gear meshing law, the driving screw must pass through the synchronous gear, etc. The parts transmit torque to the screw to achieve synchronous rotation.

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