Vacuum Technology Know How - Triumf

Pfeiffer Vacuum
Formula 7-7
S against low pressure
Page 140
Formula 7-8
Pump-down time
Vacuum Technology
At low pressures, S R from Formula 7-4 is used and yields:
S = S 0 1 -
From Formula 2-5, it can be seen that S tends toward S 0 if K 0 >>
S 0
Using z. B. K = 40 und = 10, for example, yields S = 0.816 S .
0 0
S v
Consequently, the following should apply for rating a pumping station: ≤ 10 .
Because the overflow valves are set to pressure differentials of around 50 mbar, virtually
only the volume flow rate of the backing pump is effective for pressures of over 50 mbar. If
large vessels are to be evacuated to 100 mbar within a given period of time, for example, an
appropriately large backing pump must be selected.
Let us consider the example of a pumping station that should evacuate a vessel of V = 2 m³ to
5 . 10 -3 mbar in 10 minutes. To do this, we would select a backing pump that can evacuate
the vessel to 50 mbar in t = 5 minutes. The following applies at a constant volume flow rate:
1
V p 0
t 1 = ln
S p 1
Formula 7-8 yields the volume flow rate
p v
K 0 . p a
2,000 l 1,000 l
S v = ln = 20 .
300 s 50 s
We select a Hepta 100 with a pumping speed (volume flow rate) S of 100 m³/h as the
v
backing pump. Using the same formula, we estimate that the pumping speed of the Roots
pump will be 61 l / s = 220 m³ / h, and select an Okta 500 with a pumping speed S of 490 m³/h
0
and an overflow valve pressure differential �p of 53 mbar for the medium vacuum range.
d
From Table 7.1 below, we select the backing vacuum pressures on the basis of gap p , use v
the corresponding pumping speeds S for the Hepta 100 from Figure 2.10 and calculate the
v
throughput: Q = Sv . p . v
pv The compression ratio s calculated for an open overflow valve to a backing
vacuum pressure of 56 mbar. K can be found in Figure 2.14 for backing vacuum pressures of
0
153 mbar or less. There are two ways to calculate the pumping speed of the Roots pump:
+ �pd K = � pv S can be obtained from Formula 7-5: S = S 1 1 v . K or an open overflow valve, or S on the
� 2
S0 basis of Formula 2-5 for a closed overflow valve S = 2
1 S0 1 - +
K0 K . Sv
0
As the backing vacuum pressure nears pressure differential �p d , S 1 will be greater than S 2 .
The lesser of the two pumping speeds will always be the correct one, which we will
designate as S.
The inlet pressure is obtained on the basis of: p =
Q
Figure 7.1 shows the pumping
a
S
speed curve of this pumping station.
S 0
S v
.
S 0
S v
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