the cold discharge disc the tapering of the conduits tends to compensate for the fact that the mass flow rate decreases in each in the direction of the flow and contributes therefore to the maintenance of a constant rate of flow through the discharge orifices on each side and to the reduction of fluctuations and losses in both flows the characterization of baffle 10 as 34 substantially helicoidal 34 is intended to describe a configuration similar to a helix but with such departures from an exact helix and with such variations of thickness along its length as may be deemed desirable to produce a better flow on each side baffle 10 may also perform the structural function of supporting body 1 whether or not this body houses the bearing assembly which supports shaft 2 fig3 shows an arrangement in which the centerline of the prerotation channel follows a path which is a modified helix or spiral components shown in fig3 corresponding to similar components in fig1 and 2 have been given the same reference numbers with the subscript b in the arrangement shown in fig4 enshrouding wall 15 corresponds to enshrouding wall 5 of the arrangement shown in fig1 but is part of the rotor.