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What are the differences between these various types of turbo-pumps (Part 1)?


QUESTION: Why so many different types of turbo-pump in the catalogue? Surely, pumping speed and final pressure are the only really important criteria, aren't they?

ANSWER: Not at all! The decisive factor in the choice of any vacuum pump should always be the application. The biggest mistakes are always made by not choosing the right high vacuum pump for the job. This can be in terms of conductance losses or it can mean putting too much emphasis on catalogue values while neglecting practical requirements.

BACKGROUND: What is actually meant by "turbo-molecular pumps"? Turbo-molecular pumps are mechanical vacuum pumps for the fine and high vacuum range through to ultra-high vacuum. The main design types are molecular pumps, turbo-molecular pumps and hybrid pumps. These pumps use the movement of gas particles for their pumping effect (gas kinetics). As the speed of the gas atoms and molecules is several hundred metres per second, the rotors of these pumps must have high speeds so that their surfaces achieve comparable speeds.

When individual molecules hit a wall, they do not immediately bounce off again like a snooker ball, but they linger on the surface for a certain time. So, if the surface is moving sufficiently fast, the particles will be carried along with it. When they leave the surface again, they pick up the speed of the surface as well and so fly away from it in the direction of pumping. This is the operating principle of the molecular pump. Molecular pumps are characterised by

  • Very high critical backing pressure
  • Final pressure in the area of 10-6 mbar
  • High compression ratios particularly for heavy gases
  • Excellent air ingress stability
  • Small distances between rotor and stator
  • Low pumping speeds

Molecular pumps are used in applications with limited conductance for example, or for frequent cycling operation. The robustness of a purely molecular pump in pressure swing flushing or in commercial helium leak detection equipment is hard to beat!

A turbo-molecular pump is based on the same physical principles as the molecular pump, but consists of a number of pairs of rotors and stators arranged alternately with blade rings set in the opposite direction. Turbo-molecular pumps are characterised by

  • Lower critical backing pressure than molecular pumps
  • Final pressure in the area of 10-9 mbar or lower
  • Greater distances between rotor and stator than molecular pumps
  • High pumping speeds

Turbo-molecular pumps have established themselves as the work horses of high vacuum technology and, with pumping speeds of between 30 and more than 3000 l/s, can be used for multiple applications.

Particular designs of turbo-molecular pumps combine the advantages of both principles on one pump shaft with a single motor. These are called hybrid or wide-range turbo-molecular pumps. In these, the turbo-molecular pump part is located on the high vacuum side, the molecular part on the booster pump side. This construction produces the optimum combination of the high critical backing pressure of the molecular pump with the high pumping speeds of the turbo-molecular pump.

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, the type description is supplemented by "M" (magnetic bearing) or "MT" (magnetic bearing and temperature control). If you are not sure which type of pump you should use - talk to us!

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