ABSTRACT

Helical grooved molecular drag pumps are used in the thin film industries as either components of compound molecular pumps or independent pumps. The flow on a rotor of the molecular drag pump varies from viscous to slip to free molecule flow according to the decrease in pressure, and the flow becomes turbulent in a very high-pressure range. The purpose of our study is to obtain the basic data useful for design by clarifying the relationship between the rotor geometry and the pumping performance. In the analysis of the pumping performance, the flow going through the grooves and the ridges are treated separately in the first step. Then, they are connected by the continuity condition of the flow rate normal for the groove-ridge interface. The pressure gradient that is discontinuous at the groove-ridge interface is smoothed by Boon and Tal’s Narrow groove theory. The validity of the theory was examined by experiments. They involve the measurement of the pressure ratio of the discharge to the suction pressure and the pressure difference between the suction and discharge sides under a zero-flow condition, and the relationship between the pressure ratio or difference and the flow rate under constant discharge pressures. More precise verification of the theory was performed by the measurement of the pressure distribution along the axis of the rotor. The proposed theory satisfactorily predicts the measured pumping performance from free molecule to viscous flow regime, but the theory needs modification so as to include the inlet effect in the turbulent flow regime.