Shelco, of Newington, Conn., introduces its Nominal Rated Pleated filter cartridges. With more square feet of filtering media, the cartridges offer more dirt-holding capacity and longer change-out intervals than any other cartridges on the market. Made of economical polyester, polypropylene or cellulose media, they are the best alternative to string wounds, resin-boned or spun cartridges. The MicroSentry Pleated Cartridges are made of 100 percent synthetic fibers to eliminate any contamination from binders or additives.