Vacuum Technology Know How - Triumf

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The turbopump will be selected on the basis of the required gas throughput. The maximum
permissible gas throughputs for various gases are specified for the respective pumps in the
catalog, with the throughput curves of turbopumps and backing pumps being used in this
connection (Figure 7.4). The throughput must be the same for both pumps, because the
same gas flow will pass through both pumps successively: S =
Q
.
v
The following rule of thumb applies for the backing pump: If the maximum gas throughput of
the turbopump is attained, the pumping speed of the backing pump must be selected high
enough so that only one half of the critical backing pressure will be utilized.
The volume flow rate at the process chamber is throttled to the required level by means
of either RPM or a regulating valve. It is frequently not possible to employ regulation as a
function of RPM, as it takes too long to set the desired pressure via RPM.
M
5
Figure 7.5: Vacuum system with pressure and throughput regulation
Example:
Let us consider a system in accordance with Figure 7.5.
Q = 20 mbar . l / s gas throughput
p = 0.05 mbar process pressure
a
1
4 M
This results in a volume flow rate S of 400 l / s. We select a HiPace 2300 as the turbopump (2)
and a Uno 120 as the backing pump (3). With this backing pump, we can attain a backing
vacuum pressure of 0.8 mbar at a gas throughput of 20 mbar . 1 / s, i.e. a little less than one
half of the critical backing pressure of 1.8 bar.
2
3
p a
p v
1) Process chamber
2) Turbomolecular pump
3) Rotary vane pump
4) Butterfly valves
(pressure-regulated)
5) Gas flow regulator
Page 149
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