Creating Vacuum – Turbo Molecular Pumps

In Physics, a “Vacuum” is defined as the absence of matter in a control volume. Generally, total vacuum is an ideal extreme condition. Therefore, in reality we experience partial vacuum where ambient pressure is different from zero but much lower than the ambient value.

Depending on the pressure we can have different degrees of vacuum, ranging from low vacuum (at 1×105 to 3×103 Pa) to extremely high vacuum (at pressures <10-10 Pa). For the purpose of comparison, space vacuum might present pressures down to ~10-14 Pa in the interstellar regions.

Vacuum is needed in research and several industrial sectors for a wide range of different applications and purposes. The main way to create vacuum is by first using primary vacuum pumps -machines that relying on the general principles of viscous fluid dynamics.

With the decrease of pressure, the distance the gas/fluid molecules will travel before they collide with each other (also called mean free path or MFP) increases. When MFP increases, it reaches a level where gas molecules are no longer interacting with each other, and the laws of continuum fluid mechanics are no longer valid.

At 1 bar, the MFP of the molecules is ~70 nm whereas in a high vacuum the MFP might increase from 10cm up to 1Km. In these conditions, we use the so-called “secondary vacuum pumps”. Though the principle of pumping a gas or fluid at very low pressure ranges is different from conventional pumps, some of them resemble the operation and design of turbopumps and are called turbo molecular pumps.

vacuum-pump
Figure 1: Schematic of a turbomolecular vacuum pump – from Wikimedia Commons

Turbo molecular pumps, introduced in 1958, are drag axial pumps (or momentum transfer pumps) used in high vacuum to pump fluid from pressures below 102 Pa. Similarly to conventional compressors, they consist of multiple stages made of a rotor and stator component. They operate transferring impulses from the rapidly moving blades to the gas molecules and pushing them towards the outlet while increasing the pressure to the one at the inlet of the backing pump.

When the molecules of the vacuum chamber enter the first stage, they are hit by the rotor blade surfaces (thin metal plates with almost no “aerofoil” features) which propels them in the stator section hitting the stator plates and moving them through the following stages. The plates’ orientation increases through the stages to adapt to the pressure variation, with some designs showing an increase of near 90 degree angles. These pumps work in parallel with primary pumps downstream (or backing pumps), which are used to bring the fluid to ambient pressure before discharging it.

holweck
Figure 2: Schematic of a Holweck stage– from Pfeiffer Vacuum

Turbomolecular pumps can work if the molecules hit by  the rotor blades reach the stator fins without colliding with other molecules. This can be achieved by reducing the gap of rotor and stator plates to be smaller than the MFP, however for manufacturing limitations, this gap is in the order 1mm. For this reason, a turbo molecular pump can work effectively when the MFP at the exhaust is of the same order of magnitude, hence exhaust pressures lower than 10Pa. Very low clearances also influence the design of the last stages of the pump. In fact, some designs may present a last stage showing a helical type channel, which is called a Holweck stage design.

To create the directed motion of the gas molecules, the rotor blade tips should move at a very high speed, so the operating rotational speeds for these machines vary from 30000 to 90000 rpm. This may create high stresses and heat due to friction in the bearing system and so consequently some exotic solutions, such as magnetic levitation bearings,  can be used with increased costs of the single unit.

A further drawback is that bigger molecules are pumped more efficiently, whereas smaller ones (such as hydrogen or helium) are more difficult to pump and remove to create a higher level vacuum. This aspect can however be alleviated by the integration of Holwech stages which increases the effectiveness of dragging smaller molecules.

 

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