Self-supporting filter units

摘要: A filter unit is made of two portions (14) rigid corrugated material secured together along lands (50, 52) extending parallel with the corrugations and, if desired, intermediate them, and also (54) formed one end corrugations, thus closing passages when are together. Suitable fastenings adhesive bonding, metallic fasteners, e.g. eyelets, or stitching. Further flanges (56) may be provided for facilitating a connection suitable suction other air-drawing source; such further out-flanged connection. An apparatus forming sheet whereby from continuous supply progressively between sets blade members (102), being moved sequentially alternately (as sets) into engagement material. The preferably heated in order to treat render it rigid. comprise severing means (112) by which cut (14).