A fluid displacement apparatus includes a stator section with a rotor therein. The stator section includes a cylindrical casing, a helically-convoluted chamber section within the cylindrical casing, and a rigid sleeve within the cylindrical casing and separate from the helically-convoluted chamber section. The rigid sleeve includes a circular internal bore. The rotor is rotatably disposed within the cylindrical casing. The rotor includes a helically-lobed section disposed within the helically-convoluted chamber section, and a circular cylinder section disposed within the rigid sleeve. The circular cylinder section provides a fluid passageway between the rigid sleeve and the circular cylinder section. Side loads from the rotor are distributed along a contact line at any point of rotation of the circular cylinder section within the rigid sleeve.

A vacuum booster assembly includes, but is not limited to, a booster housing defining a booster chamber and including a gas inlet and a gas outlet; a first rotor positioned within the booster chamber and adapted for rotation therein, the first rotor including a first shaft and at least two lobes defining a first lobe profile; and a second rotor positioned within the booster chamber and adapted for rotation therein, the second rotor including a second shaft and at least two lobes defining a second lobe profile, wherein the first and second rotors are formed from a metal having a coefficient of thermal expansion from about 1 (10.sup.6 in/in*K) to about 13 (10.sup.6 in/in*K), and wherein at least one of the outer surface of the first rotor, the outer surface of the second rotor, or the booster chamber includes a coating.

A vacuum booster assembly includes, but is not limited to, a booster housing defining a booster chamber and including a gas inlet and a gas outlet; a first rotor positioned within the booster chamber and adapted for rotation therein, the first rotor including a first shaft and at least two lobes defining a first lobe profile; and a second rotor positioned within the booster chamber and adapted for rotation therein, the second rotor including a second shaft and at least two lobes defining a second lobe profile, wherein the first and second rotors are formed from a metal having a coefficient of thermal expansion from about 1 (10.sup.6 in/in*K) to about 13 (10.sup.6 in/in*K), and wherein at least one of the outer surface of the first rotor, the outer surface of the second rotor, or the booster chamber includes a coating.