Page "Turbomolecular pump" Paragraph 11

At atmospheric pressure the mean free path of air is about 70 nm.

Turbomolecular blades cannot be built with anything close to such a small clearance, so this type of pump stalls if exhausted directly to the atmosphere.

Nonetheless Varian, Inc. since 2006 offers a pump where the last stages have blades optimized for zero flow and can pump against a pressure of one atmosphere.

Because the low pressure stages are limiting the flow, the high pressure stages can be fixed to zero flow.

Theoretically centrifugal pumps could be used, but it is more compact to use a circulating flow between hollow threads in the rotor and the stator.

In other cases the exhaust is connected to a backing pump, which produces a pressure low enough for the turbomolecular pump to work efficiently.

Typically, this pressure must be below 10 Pa with 1-2 Pa as common averages.