Abstract: A water vapor resistant filter comprising a permeable molded material, nonwoven fabric or felt comprising polyimide fiber having a repeating unit represented by formula (I) is described. The permeable molded material, nonwoven fabric or felt is covered with a condensed polycyclic polynucleus aromatic resin, a melting fluorine resin, a solvent-soluble cold setting fluorine resin or a fluorine elastomer: ##STR1## X, R and n are defined in the specification. A water vapor resistant antistatic filter is also described, which comprises a permeable molded material, nonwoven fabric or felt comprising polyimide fiber having a repeating unit represented by the above-described formula (I). The permeable molded material, nonwoven fabric or felt is covered with an electroconductive material and a condensed polycyclic polynucleus aromatic resin, a solvent-soluble cold setting fluorine resin or a fluorine elastomer. Processes for producing these filters are also described.

Abstract: A filter element, including a pair of filter plates that are each made of a nonwoven fabric or felt of a polymeric material shaped into a plate having a corrugated cross section and which are positioned in such a way that the grooves in one filter plate are opposed to those in the other filter plate, is reinforced with reinforcements that are placed across the width of the filter element. The reinforcements and the filter element are secured at their respective ends by means of bolts. The reinforcements are similar in cross-sectional shape so that they can circumscribe the corrugated cross section of the filter plates such that a pair of the reinforcements can hold the filter element therebetween.