Booster pumps (aka Roots blowers or intermediate stage vacuum pumps) fall into the class of dry, gas transfer pumps. As a dry pump, they do not introduce oil or water into the pumped gas stream. Gas transfer is accomplished by mechanically means, that is the transfer of pumped gas molecules, rather than their collection, as with, for example, cryogenic or turbomolecular pumps.
The words “Roots blower” are synonymous with these pumps and the reason they are also commonly referred to as booster pumps is that they are mounted at the inlet of a primary/backing pump (such as a rotary vane, claw pump, or screw pump). They “boost” the performance of the primary pump improving pumpdown speed (much as a relay race in which the baton is passed from one runner to the next). The combination of Root blower and primary pump provides roughly a seven-fold increase in pumping speed and a ten-fold increase in pressure, in comparison to a primary pump alone.