A strainer element (4, 32) for connection to a collection header (3, 35), the strainer element (4, 32) comprising: a perimeter frame; a first fluid permeable screen and a second fluid permeable screen supported on the frame, characterized in that: said permeable screens have a plurality of corrugations formed of a parallel plurality of peaks and valleys, contact at alternating peaks maintaining the fluid permeable screens in opposed spaced relationship for defining fluid flow channels therebetween through which fluid passing through either one of the fluid permeable screens can be conducted towards the collection header (3, 35); and said fluid flow channels communicate with said header (3, 35) through said perimeter frame, wherein each one of the fluid permeable screens are a perforated metal screen or metal mesh, wherein said fluid flow channels are configured to communicate with said header (3, 35) through openings in a marginal side edge of said perimeter frame; wherein first ends of the first and second fluid permeable screens are fixed to the marginal side edge of said perimeter frame, and a cross-section of said marginal side edge is configured to fit slots in the collection header (3, 35); wherein the frame comprises, at the end of the strainer element (4, 32) for connection to the collection header (3, 35), perforated metal caps to seal the ends of the corrugations, wherein the caps include openings through which flow from the fluid passage can communicate with the header (3, 35); wherein the strainer element (4, 32) is configured for fluid flow to enter through the first fluid permeable screen and the second fluid permeable screen, and pass through the fluid flow channels formed by the corrugations which are in fluid communication with the header (3, 35) through the caps. a perimeter frame; a first fluid permeable screen and a second fluid permeable screen supported on the frame, characterized in that: said permeable screens have a plurality of corrugations formed of a parallel plurality of peaks and valleys, contact at alternating peaks maintaining the fluid permeable screens in opposed spaced relationship for defining fluid flow channels therebetween through which fluid passing through either one of the fluid permeable screens can be conducted towards the collection header (3, 35); and said fluid flow channels communicate with said header (3, 35) through said perimeter frame, said permeable screens have a plurality of corrugations formed of a parallel plurality of peaks and valleys, contact at alternating peaks maintaining the fluid permeable screens in opposed spaced relationship for defining fluid flow channels therebetween through which fluid passing through either one of the fluid permeable screens can be conducted towards the collection header (3, 35); and said fluid flow channels communicate with said header (3, 35) through said perimeter frame, wherein each one of the fluid permeable screens are a perforated metal screen or metal mesh, wherein said fluid flow channels are configured to communicate with said header (3, 35) through openings in a marginal side edge of said perimeter frame; wherein first ends of the first and second fluid permeable screens are fixed to the marginal side edge of said perimeter frame, and a cross-section of said marginal side edge is configured to fit slots in the collection header (3, 35); wherein the frame comprises, at the end of the strainer element (4, 32) for connection to the collection header (3, 35), perforated metal caps to seal the ends of the corrugations, wherein the caps include openings through which flow from the fluid passage can communicate with the header (3, 35); wherein the strainer element (4, 32) is configured for fluid flow to enter through the first fluid permeable screen and the second fluid permeable screen, and pass through the fluid flow channels formed by the corrugations which are in fluid communication with the header (3, 35) through the caps.