Helically wrapped filter element

A tubular fluid filter element comprises at least two filter medium layers helically wrapped around a pervious, hollow tube, with the filter medium layers forming an overall graded pore structure, in the radial flow direction. Filter medium layers may be non-woven glass or polypropylene microfibrous ribbons of progressively varying pore size. Within each layer adjacent helical turns may abut or overlap by 25-75% of the ribbon width. Similarly helically wrapped layers of plastics mesh may be located between the inner tube and the innermost filter layer and between filter layers, to diffuse the fluid flow laterally of the layers.