TIP OF THE MONTH 06

QUESTION: Wide-range turbo-molecular pumps are cutting-edge, aren't they? So why is the old stuff with poor fore-vacuum compatibility still around?

ANSWER: Although wide-range turbo-molecular pumps, also known as hybrid turbopumps, have good fore-vacuum compatibility, in selected applications they can have disadvantages compared with classic turbopumps without a Holweck or Gaede stage.

BACKGROUND: To begin with, all turbo-molecular pumps operate at very low pressures in a fine or high vacuum. This type of pump requires a backing pump connected in series which is able to compress against atmosphere. The final pressure or process pressure of this backing pump must be below the maximum fore-vacuum pressure shown in the catalogue (= maximum exhaust pressure) of the turbopump in continuous operation. Wide-range turbo-molecular pumps allow operation at relatively high fore-vacuum pressures from around 1 to 10 mbar. Classic turbo-molecular pumps on the other hand need fore-vacuum pressures of around 0.01 to 0.1 mbar. As a result, the wide-range pumps are often combined with diaphragm pumps, while classic turbo-molecular pumps are used with rotary vane pumps or high-quality oil-free pumps.

In the series connection mentioned above, the turbo-molecular pump acts as a flow resistance when pumping down from atmospheric pressure if a bypass is not used. A bypass is avoided in vacuum systems whenever possible owing to the cost of the three valves that are normally needed and the additional installation components.

The clearances between rotor and stator are bigger in classic turbo-molecular pumps than in wide-range TMPs or pumps with just one molecular stage. This means that classic turbo-molecular pumps offer less flow resistance when pumping down from atmospheric pressure, and make for a faster pumping process. The flow resistance varies from one model to another, and can result in significant differences in the pump-down time of a vacuum system. This can have a huge impact on the productivity of a vacuum system with frequent load-lock cycles, for example.

The smaller gap clearances also make wide-range molecular stages more sensitive to the intake of particles from the vacuum process. This can make classic turbo-molecular pumps significantly more robust, e.g. for processes in the glass working industry or for coating processes.

At adixen:

• classic turbo-molecular pumps are called "ATP"
• simple molecular pumps are called "MDP"
• hybrid turbo-molecular pumps are called "ATH"

For pumps with active magnetic bearings, an "M" (magnetic bearing) or "MT" (magnetic bearing and temperature control) is added to the type designation. If you are not sure which type of pump you should use - talk to us!

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