2.8.1.2 Holweck stage operating principle
A Holweck stage is a multi-stage Gaede's type molecular pump [10] having a helical pump channel. Due to the rotation of the rotor, gas molecules entering the pump channel receive a stimulus velocity in the direction of the channel. Backflow losses occur through gaps between the webs that separate the Holweck channels from one another and the rotor. The gap widths must be kept small to minimize backflow. Cylindrical sleeves (1) that rotate about helical channels in the stator (2) are used as Holweck stages. Arranging stators both outside as well as inside the rotor enables two Holweck stages to be easily integrated within one and the same pump.
Figure 2.21:
Operating principle of a Holweck stage
The pumping speed S0 of the Holweck stages is equal to:
Formula 2-13:
Holweck stage pumping speed
Where b · h is the channel cross section and v · cosα the velocity component in the channel direction.
The compression ratio increases exponentially as a function of channel length L and velocity v · cosα [4]:
Formula 2-14:
Holweck stage compression ratio
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 103. The shift of the compression ratio curves to higher pressure by approximately two powers of ten can be seen from Figure 2.22.
Figure 2.22:
Compression ratios of pure turbopumps and turbo drag pumps
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