very much in a draft
An example pumping sequence goes like this:
A vacuum chamber filled with air at room pressure is to be pumped down into really low pressures:
Turn on the first pump, it pulls air and the pressure drops, through viscous flow and a bit into molecular flow, he pressure drop slows and then stops. The pump can't remove molecules any faster than they're leaking into the chamber. Turn on the second pump. It could probably viscous flow as it starts up, but as it's blades spin faster and faster (think jet engine) ....... then ion pump.......
https://uspas.fnal.gov/materials/15ODU/Session4_1_MechanicalPumps.pdf
https://uspas.fnal.gov/materials/15ODU/Session4_2_IonPumps.pdf