A dry vacuum pump assembly that includes a casting having an inner chamber, an inlet and an outlet communicating with the inner chamber, a pair of right and left handed screw rotors positioned in the casing, each screw rotor having a cross section formed by a Quimby curve, a circular arc, and a quasi-Archimedean spiral curve, the pair of screw rotors intermeshing with each other to pump a process gas pumped from the inlet to the outlet. The pump assembly further includes a nitrogen-supplying tube that communicates with the inner chamber at a position near the outlet in the casing, and an external pipe that connects the outlet to a scrubber or a trap which external pipe is straight and does not include a silencer.