and disposed within the external gap formed between the screen and the cylindrical support frame or plate 26 the support frame 26 may be constructed from a rolled cylindrical plate having relatively large holes or other shaped openings these openings would be staggered so that they do not line up vertically in construction the thin screening plate may be attached by electron beam or resistance welding or the like to the series of support rods the screenplate 20 and rod 25 assembly can be rolled to the required curvature and assembled to the support frame 26 by means of attaching the rods to the support frame to facilitate this a vertical strip 27 is provided to locate the screen fig7 shows a fourth alternative embodiment wherein the principle employed is that wire 30 is wound spirally around a cylindrical screenplate 20 such as to resist bursting and or buckling forces in a radial direction the pitch of the spiral being selected to suit the permissible unsupported span of the perforated plate the outer strips 35 provide axial stiffness together with torsional stiffness in both directions to resist buckling and twisting the spirally reinforced assembly may be manufactured as follows using a mandrel to ensure accurate dimensions the screenplate is formed into a cylinder around the mandrel and seam welded then the top mounting flange 22 and bottom mounting ring not shown are weld assembled to the cylinder next the reinforcing wire may be wound spirally around the apertured length of the screen cylinder and attached by resistance welding.