You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
Formula 7-2<br />
K o of a Roots<br />
vacuum pump<br />
Formula 7-3<br />
K 0 laminar<br />
Formula 7-4<br />
K 0 molecular<br />
Formula 7-5<br />
S 1 for �p d > L R >> S R and thus<br />
K 0l =<br />
and the following applies in the molecular flow range due to<br />
S 0<br />
L R<br />
At laminar flow (high pressure), the compression ratio is limited by backflow through the gap<br />
between piston and housing. Since conductivity is proportional to mean pressure, the compression<br />
ratio will decline as pressure rises.<br />
In the molecular flow range, the return gas flow SR . p from the discharge side predominates<br />
v<br />
and limits the compression ratio toward low pressure. Because of this effect, the use of Roots<br />
pumps is restricted to pressures p a of less than 10 - 4 mbar.<br />
Volume flow rate (pumping speed)<br />
Roots pumps are equipped with overflow valves that allow maximum pressure differentials<br />
�p of between 30 and 60 mbar at the pumps. If a Roots pump is combined with a backing<br />
d<br />
pump, a distinction must be made between pressure ranges with the overflow valve open<br />
(S 1 ) and closed (S 2 ).<br />
p S + L v 0 R<br />
= K0 =<br />
p LR + S a<br />
R<br />
S 0 >> S r >> L R : K 0m =<br />
Since gas throughput is the same in both pumps, the following applies:<br />
S l =<br />
S v . p v<br />
p - �pd<br />
v<br />
As long as �p d
Change language