According to the invention, a vacuum pump is disclosed comprising a
screw-rotor pump having a compression section (8) and an expansion
section (7), and wherein a discharge (10) from the compression section
communicates with at least one ejector (1) for discharge of compressed
gas through the ejector, and wherein the expansion section (7) is
connectable via a first valve means (5), to a drive-gas source (P) for
operating the screw,-rotor pump and the ejector in parallel. Also, a
method for providing sub-pressure to an industrial process is disclosed
wherein at least one ejector (1) is used initially to reduce the pressure
to a predetermined lower level, from where the pressure is further
reduced by means of a screw-rotor pump (7, 8) that is arranged to operate
through, and in parallel with the ejector.