Matching primary and secondary vacuum pumps

The consequences of a wrong selection can be serious, ranging from a ‘stalled’ diffusion pump (and major oil contamination) to over-heating of a turbomolecular pump.

The following describes these requirements illustrated with common examples.

A primary vacuum pump (PP) is one which exhausts to atmospheric pressure. These include oil-sealed rotary vane (OSRV), diaphragm, scroll, multi-stage roots, piston, screw and liquid-ring pumps.

Secondary pumps (SP) require initial evacuation by primary and sometimes other secondary pumps to a required pressure before operation. For example, oil diffusion pumps (ODP), turbomolecular pumps (TMP), vapour boosters (VB), mechanical boosters (MB), ion getter pumps (IGP), titanium sublimation pumps (TSP), non-evaporable getters (NEG), cryogenic, molecular drag and regenerative pumps.

In some cases, supporting backing pumps are required for continuous operation; this is the case for ODP, TMP and VB pumps.

To match a PP and SP, there are several things to consider:

•  Initial pump-down time by the PP to a point where the SP ‘takes over’ the pumping process. This is especially important if the SP starts at the same time as the PP and a given pressure must be reached in a given time to prevent the SP from ‘timing’ out.
• Initial ‘surge’ of gas flow and pressure spike when the SP starts. This is illustrated by the figure below where an nXDS15i scroll is the PP and an nEXT300D is the secondary TMP; the spike in throughput corresponds to the rise in pressure when the SP (nEXT300D) ‘takes-over’ the pumping.

This would give a corresponding rise to ~1.5 mbar in the PP as shown below: 

• Maximum Backing Pressure (MBP) / Critical Backing Pressure (CBP) should not be exceeded. This puts a limit on the maximum gas flow. The PP must have sufficient speed performance at the required backing pressure rather than assuming the peak/nominal speed of the PP.
• The MBP value may be quoted by the manufacturer for zero flow rather than with flow.
• Cleanliness (is a dry pump required?).
• Does the PP need to be intergratable with the SP?
• What is the typical residual atmosphere of the PP – particularly when combined with TMPs with respect to compression? This is important since for example the relatively low compression ratio of a TMP for H₂ may limit its achievable ultimate pressure if the H₂ partial pressure in the backing pump is significant.