Secondary pumps require a primary pump to initially ‘prime’ them for operation and/or to support their continuous operation. There are several factors which need to be considered for the correct combination or ‘matching’ of primary and secondary pumps to ensure safe and optimised performance.
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:
This would give a corresponding rise to ~1.5 mbar in the PP as shown below:
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