Pfeiffer Vacuum

7.3.2 Molecular conductivity

Now let us also consider the conductivity of the same piping in the molecular flow range. The piping has a diameter of 0.07 m and a length of 2 m. The elongated length of 0.235 m each of the two 90° pipe bends, i.e. a total length of 2.47 m is taken into consideration [20]. In accordance with Formula 1-22, the piping resistance is
LRm = 0.017123 m3/s

where c = 471 m/s for air at T = 293 K. The orifice conductivity of the pipe inlet has already been taken into account.

The effective volume flow rate is obtained in accordance with the following formula:
Seff = 1.0864 · 10-2 m3/s where Sv= 2.9722 · 10-2 m3/s.

In the molecular flow range, the pumping speed of the backing pump would be reduced to nearly one third! In this range, it is absolutely necessary to pay strict attention to short runs and large piping cross sections between pump and recipient. This applies in particular with respect to turbopumps that are ideally flanged directly to the recipient.

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