therethrough due to the arrangement of the fibers of the mineral wool parallel to the longitudinal axis of cylinder 12 the exhaust gases flow through chamber b in flow paths which are also parallel to the longitudinal axis of the cylinder the hollow chamber c which is immediately downstream of chamber b with respect to the direction of flow of the exhaust gases is bounded at its right hand end with respect to the view in fig1 by the downstream face of the filter means or screen subassembly generally indicated at 40 which comprises a plurality of individual screens of different mesh size specifically indicated at 40a 40b 40c and 40d chamber c is bounded at its left hand end relative to the view in fig1 by the right hand or upstream face of a filter means or screen subassembly generally indicated at 42 which abuts against a hollow annular spacer ring 44 secured in a fixed axial position relative to cylinder 12 by a screw 46 which passes through the wall of cylinder 12 and through the rim of annular spacer ring 44 screen subassembly 42 comprises a plurality of abutting screens of different meshes indicated at 42a 42b 42c and 42d the bypass generally indicated at 50 is provided in conjunction with the chamber c and comprises a discharge or bypass port 52 which communicates with the interior of chamber c intermediate its length port 52 is connected to.