A heat treatment furnace is pumped from atmosphere. The mechanical pump carries the first part of the pump-down, where gas volume is large, starts are frequent, and the gas may be warm and humid.
An oil-free rotary vane pump can handle roughing and low-vacuum holding directly. Where volatiles or moisture are present, fit condensation and filtration ahead of the pump and drain condensate regularly. Pump-down time to the target rough pressure follows from furnace volume and effective pumping speed; an undersized line bore lengthens it clearly.
Vacuum drying, degassing and impregnation behave similarly: a large volume of atmospheric gas at the start, then small amounts of water or solvent vapour. When selecting, give the working pressure, the capacity and the target pump-down time in one pass.