Vacuum Technology Know How - Triumf

Pfeiffer Vacuum
Page 62
Formula 2-13
Holweck stage
pumping speed
Formula 2-14
Holweck stage
compression ratio
Vacuum Technology
The pumping speed S 0 of the Holweck stages is equal to:
Where b . h is the channel cross section and v . cos a the velocity component in the
channel direction.
The compression ratio increases exponentially as a function of channel length L and
velocity v . cos a [4]:
K 0 =
1
S 0 = . b . h . v . cos a
2
v . cos a . L
c – . g . h
mit 1 < g < 3
The values yielded by this formula are much too large, because backflow over the web from
the neighboring channel dramatically reduces the compression ratio, and this influence is not
taken into account in Formula 2-14.
In order to use small dry backing pumps, e.g. diaphragm pumps having ultimate pressures
of less than 5 mbar, turbopumps are today equipped with Holweck stages. These kinds of
pumps are termed turbo drag pumps. Since the Holweck stages require only low pumping
speeds due to the high pre-compression of the turbopump, the displacement channels and,
in particular, both the channel height as well as the clearances to the rotors can be kept
extremely small, thus still providing a molecular flow in the range of 1 mbar. At the same
time, this increases the compression ratios for nitrogen by the required factor of 10 3 . The
shift of the compression ratio curves to higher pressure by approximately two powers of
ten can be seen from Figure 2.22.
Compression ratio
10 12
10 11
10 10
10 9
10 8
10 7
10 6
10 5
10 4
10 3
10 2
10 1
Classical turbopump 520: Nitrogen
Classical turbopump 520: Hydrogen
Turbo drag pump: Hydrogen
Turbo drag pump: Nitrogen
10 - 3 10 - 2 10 - 1 10 0 10 1 10 2
mbar
Backing vacuum pressure
Figure 2.22: Compression ratios of pure turbopumps and turbo drag pumps
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