Oil-sealed rotary vane pumps are very frequently used as roughing pumps. The principle of operation is very simple and shown here:

   (a) gas from the vacuum system is expanded into the pump

   and (b) the gas is pushed through the pump exhaust.

The operation principle of this pump makes it obvious why there are two stages on the vacuum system: the rotary vane pump will work fine when the pressure is high enough to ensure a gas flow through the pump but hopeless when the mean free path of the molecules

is very long.