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Pfeiffer <strong>Vacuum</strong><br />
Page 60<br />
<strong>Vacuum</strong> <strong>Technology</strong><br />
Specific pumping speed S A<br />
6<br />
l / (s . cm 2 )<br />
5<br />
4<br />
3<br />
2<br />
1<br />
0<br />
0 50 100 150 200 250 300 350 400 450 500 m / s<br />
Figure 2.18: Specific turbopump pumping speeds<br />
Pumping speed S<br />
120<br />
%<br />
100<br />
80<br />
60<br />
40<br />
20<br />
H 2<br />
He<br />
CH 4<br />
N 2<br />
Ar<br />
= HiPace 80<br />
= HiPace 300<br />
= HiPace 700<br />
= HiPace 1500<br />
= HiPace 2300<br />
Mean blade velocity V<br />
0<br />
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150<br />
Molecular weight M<br />
Figure 2.19: Pumping speed as a function of molecular weight<br />
The pumping speeds (l / s) thus determined still tell nothing about the values for light gases,<br />
e.g. for hydrogen. Pump stages having differing blade angles are normally used in a turbopump<br />
to optimize the maximum pumping speed for hydrogen. This produces pumps with<br />
sufficient compression ratios for both hydrogen (approximately 1,000) and nitrogen, which<br />
should be 10 9 due to the high partial pressure in the air. In the case of pure turbomolecular<br />
pumps, backing-vacuum pressures of approximately 10 - 2 mbar are required due to their<br />
molecular flow.<br />
www.pfeiffer-vacuum.net
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