The Roots type blower is a positive displacement lobe pump which operates by pumping, or moving, a fluid, usually air, but in some cases a process gas, using a pair of inter-meshing lobed rotors, which have a profile that looks not unlike the number 8.
When operating, the fluid is trapped in the outer pockets surrounding the lobes and is carried, or progressed, from one side of the machine to the other.
The lobe shaped rotors (usually two lobes, but it can be 3 or 4) work in tandem and move the fluid, usually air, from one side of the machine to the other. The lobes do not grab, or scoop the air out of the inlet pipe, but rather as they pass the inlet opening they cause a local low pressure area inside inlet port of the machine into which the fluid will then flow. As the rotors continue to turn the fluid that has entered the machine is trapped by the rotors and progressed from the inlet side to the outlet side.
On reaching the outlet side the fluid has no choice but to be pushed out into the discharge pipe.