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The gas flow through a vacuum component can be continuous, transitional, or molecular depending on the pressure range and geometries involved. Dilute gas flows in molecular and transitional regimes were simulated using the direct simulation Monte Carlo method. The computational model takes as data combination of pressure, temperature of gas, type of gas, and geometry of the vacuum components. The results of the computations are gasflow, local velocities, and molecular density distribution. The model was first tested and validated for several simple geometries, such as circular and rectangular finite length tubes with static and moving walls, for different flow conditions. The Couette flow problem is treated in all the transition regime, from the molecular regime to the beginning of the viscous regime. The model was found to agree well with other published results, and allowed us to better understand flow phenomena for the different regimes from the molecular case to the viscous case. This first study was conducted to develop, in the future, a model that we can adapt to more complicated geometries of vacuum components, such as stages of turbomolecular or molecular pumps, to predict the flow rate from molecular to transition flow regimes. © 1999 American Vacuum Society.