From Vacuum Technology: Calculations in Chemistry

3.3 HIGH/ULTRA-HIGH VACUUM PUMPS

It is now increasingly common to have high-vacuum systems based on turbomolecular pumps (TMPs), turbomolecular pumps + cryosurfaces, or cryopumps. Diffusion pumps were extensively used in the past and remain fairly widespread in laboratory applications. However, for industrial purposes their use appears to be restricted to systems requiring very high pumping speeds ( S 0= thousands of Ls -1), particularly where substantial amounts of particulates are handled.

High-vacuum gas-transfer pumps (diffusion pumps, TMPs) must be combined with suitable backing pumps to work satisfactorily, and due consideration must be given to the sizing and specification of the latter. Typical pumping speed vs pressure curves for high-vacuum and backing pumps are shown in Figure 3.8.


Figure 3.8: Backing-pressure requirements in the changeover from roughing- (backing) to high-vacuum pumps (Reproduced from W. Schwarz, in ref. (f) , p. 507; reprinted by permission of John Wiley & Sons Inc.)

In Figure 3.8, on the curve marked 'High-vacuum pump', it can be seen that the region of constant pumping speed ( S max) with inlet pressure leads to a region with declining pumping speed as the inlet pressure increases.

Further, on the curve marked 'Backing pump', at low enough inlet pressures, the pumping speed is low but increases to a constant level with increasing inlet pressure.

In the operation of high vacuum systems, initial pump-down involves the evacuation of the system from atmospheric pressure using a suitable roughing pump. Often, this is the same pump as that used...

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