Direction of axial force
Thread Content
This post was last edited by chfpshk on 2010-4-27 at 11:54. In multi-stage centrifugal compressors, the unequal gas forces acting on both sides of each impeller result in a resultant force acting on the rotor in the direction of the low-pressure side; this resultant force is known as axial force. ? ? How to understand it? Is the axial force in the direction of the incoming flow? ? From the perspective of balancing the axial force with the balance disc, shouldn’t the axial force point in the direction of the incoming flow? ? Have I made a mistake? ? When a centrifugal compressor starts up, the rotor tends to shift to the other end. To maintain the rotor in its proper position, it is necessary to provide thrust support on both sides of the rotor. This is because at the time of startup, the gas pressure across each stage has not yet been established, and there is no pressure difference on either side of the balance disk. As long as gas is flowing, the rotor will shift in a direction opposite to the normal axial force; therefore, thrust support on both sides of the rotor is required to prevent accidents. Confused? Isn’t the direction of the axial force usually said to be in the direction of the incoming flow? ?
You haven’t understood it properly. In a centrifugal compressor, as the impeller rotates, a pressure similar to vacuum is created at the inlet, which is necessary to draw in air; therefore, the axial force acts in the direction of the inlet. During startup, since balance has not yet been established, the shaft will move axially as it rotates in order to achieve balance; therefore, thrust bearings are needed on both sides
First, two concepts need to be clarified: axial force and axial thrust. The axial force is caused by the pressure difference on both sides of the impeller; it acts in the direction of the lower pressure, that is, in the direction from which the air flows. The axial thrust is generated due to the influx of the medium during operation, as a result of the pressure exerted by the medium itself; the direction of this force is toward the high-pressure side, that is, in the same direction as the gas flow. The function of the thrust bearing is to prevent damage to the pump in these two situations, hence dual protection is required. Personal opinion, for reference only.
2# I have nothing to hide from you. [Boss, by axial force, we usually mean the force acting in the direction of the incoming flow, right? Then, as mentioned in the second point above, how do we distinguish between the primary and secondary thrust forces in a thrust bearing?]
It primarily balances axial forces and limits rotor movement radially.