Tubular filtration element and method of making it

Abstract

A tubular element for removing particulate matter and bacteria of dimensions in excess of a predetermined value from a fluid flowing from the interior to the exterior of the filtration element has a first inner layer of a highly porous material that has a low coefficient of sliding friction, and a second layer of a microporous filtration material having pores of substantially uniform size. The element may include an outer support layer of a porous material that is not subject to shedding. In a preferred form, the inner layer is formed from a high wet strength paper, the microporous filtration layer has sufficiently small pores to remove all bacteria from the fluid, and the outer layer may be a fine porous plastic sheet material or a high wet strength, high porosity paper that is impregnated with the polymeric material that forms the microporous filtration material. The fine pore structure of the outer layer supports the fragile filtration layer. In a preferred form, both ends of the tubular element are impregnated with a suitable sealing material such as a polyurethane potting compound. The ends are preferably dipped in the sealing material to a predetermined depth. When cured, these end portions provide additional mechanical support for the tube ends and block the passage of particulate matter and bacteria past the filtration layer at its edge. A fabrication process for the filtration element that does not damage the fragile microporous filtration layer involves helically wrapping the inner layer on a fixed mandrel to act as a carrier, and helically wrapping an overlying layer of the microporous filtration material. If an outer layer is used this is wrapped on the microporous material. All layers move in unison along the longitudinal axis of the mandrel. Each layer is self-overlapping and at least the filtration layer is sealed in the area of the overlap.