Filter RatingFilters are rated in several ways—absolute,mean, and nominal. The absolute filtration ratingis the diameter in microns of the largest sphericalparticle that will pass through the filter under acertain test condition. This rating is an indicationof the largest opening in the filter element. Themean filtration rating is the measurement of theaverage size of the openings in the filter element.The nominal filtration rating is usually interpretedto mean the size of the smallest particles of which90 percent will be trapped in the filter at each passthrough the filter.Filter ElementsFilter elements generally may be divided intotwo classes—surface and depth. Surface filters aremade of closely woven fabric or treated paper witha uniform pore size. Fluid flows through the poresof the filter material and contaminants arestopped on the filter’s surface. This type ofelement is designed to prevent the passage of ahigh percentage of solids of a specific size. Depthfilters, on the other hand, are composed of layersof fabric or fibers which provide many tortuouspaths for the fluid to flow through. The pores orpassages must be larger than the rated size of thefilter if particles are to be retained in the depthof the media rather than on the surface.Consequently, there is a statistical probabilitythat a rather large particle may pass through adepth-type filter.Filter elements may be of the 5-micron, wovenmesh, micronic, porous metal, or magnetic type.The micronic and 5-micron elements havenoncleanable filter media and are disposed ofwhen they are removed. Porous metal, wovenmesh, and magnetic filter elements are usuallydesigned to be cleaned and reused.5-MICRON NONCLEANABLE FILTERELEMENTS.— The most common 5-micron filtermedium is composed of organic and inorganicfibers integrally bonded by epoxy resin and facedwith a metallic mesh upstream and downstreamfor protection and added mechanical strength.Filters of this type are not to be cleaned underany circumstances and will be marked Disposableor Noncleanable.Another 5-micron filter medium uses layersof very fine stainless-steel fibers drawn into arandom but controlled matrix. Filter elementsFigure 9-14.—Cross-section of a stainless steel hydraulic filterelement.of this material may be either cleanable ornoncleanable, depending upon their construction.WOVEN WIRE-MESH FILTER ELE-MENTS.— Filters of this type are made ofstainless steel and are generally rated as 15 or 25micron (absolute). Figure 9-14 shows a magnifiedcross section of a woven wire-mesh filter element.This type of filter is reusable.MICRONIC HYDRAULIC FILTER ELE-MENT.— The term micronic is derived from theword micron. It could be used to describe anyfilter element; however, through usage, this termhas become associated with a specific filter witha filtering element made of a specially treatedcellulose paper (fig. 9-15). The filter shown infigure 9-10 is a typical micronic hydraulic filter.This filter is designed to remove 99 percent of allparticles 10 to 20 microns in diameter or larger.Figure 9-15.—Micronic filter element.9-11&
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