36 at fixing positions 61 in the form of point impressions spaced apart from one another uniformly in the peripheral direction and has a flange at the bottom with which it engages around the two base plates 46 46 u2032 like a clamp a u shaped filter unit 50 is arranged directly beneath the upper end wall of the filter housing 56 between the two generator stages 20 30 the filter unit 50 extends from one side wall of the filter housing 56 up to the other side wall transversely through the filter housing 56 as can be seen in fig2 the two u limbs project downwardly from the upper end of the filter housing 56 and can end above the axial center of the two generator stages 20 30 the can however also extend along the side wall of the filter housing 56 up to the base plates 46 46 u2032 the remaining free space between the two generator stages 20 30 beneath the filter unit forms an outflow passage 52 in both pressure housings 26 36 the outflow openings 28 38 opening into the outflow passage 52 are provided which lie in a lower region of the pressure housings 26 36 and are thus located beneath the filter unit 50 a steel band.