An analytical model is presented, useful in practice, for calculating the compression curves for turbomolecular pumps, Wide Range turbomolecular pumps and Roots blowers.
It is demonstrated that the model, primarily proposed for calculating and systematically analysing the compression curves for turbomolecular pumps [*], can be applied to Wide Range turbomolecular pumps and Roots blowers as well.
The model is based on an ordinary differential equation for the pressure as a function of position inside the pump. Solving the differential equation makes it possible both to calculate the compression curves for a finite gas throughput (Q > 0) and for zero gas throughput. A hypothesis is posed holding that the compression curve for zero gas throughput can be derived from a compression curve for a finite gas throughput, e.g., Q = 1 sccm in case of turbomolecular pumps.
For each type of vacuum pump mentioned above the comparison with experimental data shows that the model provides an excellent qualitative and quantitative description of the observed phenomena in the whole relevant pressure regime, i.e., perfect agreement over more than five pressure decades.
[*] G. Voss in "Vakuum in Forschung und Praxis", 17 (2005), Nr. 6, p. 324 ff.
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