Effect of the Orbital Movement Speed of a Scroll on Leakage in a Scroll Vacuum Pump

The gas leakage through slot channels in an oil-free scroll vacuum pump is considered. It is shown that a profile channel between scrolls makes the determining contribution to the gas leakage. Two variants of gas flow through channels with moving walls are considered. In the first case, when a rotor rotates about its geometrical axis at a speed directed from inlet to outlet, the interaction between gas molecules and a moving wall increases a conductance in this direction. It is the operating principle of molecular pumps. In the second case, when a rotor rotates around the center which does not coincide with geometrical center of the rotor head, rotation has an opposite effect: a leakage from outlet to inlet increases. It is shown that a similar situation is observed when a moving scroll executes an orbital movement relative to a fixed scroll in a scroll vacuum pump. Correspondingly, a displacement of the contact point in a profile channel of the scroll pump increases the backward leakage. A leakage through the moving profile channel of the scroll pump is calculated for the molecular and viscous gas flow regimes. The conductance in the molecular flow regime may increase by a factor of 10-15 in comparison to the channel with fixed walls. In the viscous flow regime, even with critical pressure ratios, an increase in the wall speed up to 50 m/s results in the mass leakage growth by 15-45 %. When gas flow regimes are subcritical, the effect of wall speed is more significant.

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