A Roots Blower refers to a specific design of positive displacement
pump with two lobed impellers with a 
roughly figure of 8 shape which
are mechanically linked via gears such that they rotate in opposite
directions. The figure of 8 shapes allows the impellers to be close
to but never in contact with both each other and the pump walls at
every position of their rotation. The close tolerances of the pump
allow the impellers to move without a lubricant if required.
The term derives from the Roots brothers who invented the rotary lobe pump more than a century ago. Still in business today, Roots™ vacuum pumps are a diverse line of pumps manufactured by Roots division of industrial giant Dresser Industries.
Roots blowers are typically used in applications where a large volume of gas must be moved across a relatively small pressure differential. This includes low vacuum applications, with the roots blower acting alone, or use as part of a high vacuum system, in combination with other pumps. Roots blowers are also used as superchargers.
Roots blowers are also used in reverse to measure the flow of gases
or liquids, for example, in gas meters.
Technical considerations
Because rotary lobe pumps need to maintain a clearance between the
lobes, a single stage roots blower can only pump gas across a limited
pressure differential. If the pump is used outside its specification,
the compression of the gas generates so much heat that the lobes expand
to the point that they jam, damaging the pump.
Roots pumps are capable of pumping large volumes but as they only
achieve moderate compression, it is not uncommon to see multiple roots
blower stages, frequently with heat exchangers (intercoolers) in between
to cool down the gas. The lack of oil on the pumping surfaces allows
the pumps to work in environments where contamination control is important.
The high pumping rate for hydrocarbons also allows the roots pump to
provide an effective isolation between oiled pumps, such as rotary
compression pumps, and the vacuum chamber.
Comparative advantages
The roots pump is never used as a stand alone pump in high vacuum applications but its high pumping speed can be used to reduce the end pressure and increase the pumping speed of other vacuum pump systems in the medium to low vacuum range.
