2.3.2 Molecular conductance
Now let us also consider the conductance 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 each of the two 90° pipe bends, i. e. a total length of $l$ = 2.47 m. In accordance with Chapter 1, Formula 1-30, the piping resistance is:
$L_{Rm}=\frac{\bar{c} \cdot \pi \cdot d^3}{12 \cdot l}=$ 1.71 · 10-2 m3 s-1
where $\bar{c}$= 471 m s-1 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 with the following formula:
$S_{eff}=\frac{S_v \cdot L_{Rm}}{S_v + L_{Rm}}=$1.09 · 10-2 m3 s-1 mit $S_v$=2.97 · 10-2 m3 s-1
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 the pump and the vacuum chamber. This applies in particular for turbopumps that should ideally be flanged directly to the vacuum chamber.
