Fluid filter cartridge and method of its construction

DETAILED DESCRIPTION One aspect of the invention is a tubular, preferably single-ply, insert disposed in the annular region between concentric inner and outer filter elements of the above-described type of filter cartridge.
The insert has a side wall, perforations formed in the side wall to permit transverse flow through the insert from the filter elements to the annular region, and protrusions extending transversely from the side wall to permit longitudinal flow through the annular region; the side wall and the protrusions together form a spacer between the filter elements.
Another aspect of the invention is a method of constructing a fluid filter cartridge.
A first annular pleated filter element having a small diameter is placed inside of a second annular pleated filter element having a large diameter so the elements are concentrically arranged with an annular region therebetween.
Partial openings are cut in the side wall of a tubular insert, leaving flaps attached to the remainder of the side wall, and the flaps are permanently deformed away from the remainder of the side wall.
The insert is placed in the annular region.
The first and second elements and the insert are attached together to form a filter cartridge.
The described insert can be cheaply fabricated and provides a uniformly low, transverse, and axial flow resistance in the region between the filter elements.