Inside of any filter assembly resides the material intended to trap undesirable particle contamination as hydraulic fluid passes through the filter element. Although filter elements often look similar at first glance, the construction and quality of each can vary widely. The two types of filter media construction, surface media and depth media, offer advantages and disadvantages, and which option is used depends on the application.
Surface media is created by weaving or layering metal, paper or synthetic fibres to create a porous surface where the openings are an engineered, relatively consistent size. The woven steel mesh of a suction strainer is a typical example of a surface type filter media. The construction of the mesh leaves precise gaps between the layers of wire, and in this case, is described by the number of layers per square inch. A finer mesh is reflected in a higher number, and also results in finer filtration, meaning the micron size of the openings is smaller. Table 1, at right, shows the comparison between microns and mesh number.
The initial advantage of surface media filtration is they offer low pressure drop due to their relatively large nominal hole sizes. Fluid passes freely through surface media, allowing all particles to pass except those larger than the hole size, which get trapped. As the surface media becomes clogged with particles over time, surface media quickly loses its advantage of low pressure drop. As fewer and fewer open pores allow fluid and particles to pass, pressure drop increases exponentially, and the element needs cleaning or replacing.
Therein lies the second advantage of surface media, especially with strainers; they can be cleaned and replaced. During regular servicing, a suction strainer can be cleaned with a solvent bath and then replaced before enough particles are trapped to cause excessive backpressure.
A depth media uses multiple layers of fibre or foam so that particles can be trapped within the depth of the filter, rather than just on the surface. Because a depth type filter element can be created with multiple layers of increasingly small pore sizes, they provide a dual benefit of trapping many particles of various sizes, and are capable of very high dirt holding capacity before pressure drop increases. The disadvantage is they experience higher pressure drop right out of the gate, especially they’re a lower micron rating.
Depth media is more costly, and is typically given an “absolute” rating, meaning they trap nearly every single particle larger than the filter size its rated for. This is important to consider, since pump, valve and component manufacturers provide minimum requirements of filtration, which must be met using filters with absolute ratings.
A depth media filter element is used most often as the primary filter for contamination removal and offers finer filtration—much finer—than any surface media can offer. A 350 mesh strainer offers an average pore size of 40 microns, and it’s nearly impossible to get much finer than this with surface media. A depth media element using a quality synthetic material can provide absolute filtration down to 3 microns or finer.
Surface media elements for hydraulic machinery are used nearly exclusively with suction strainers, and depth media elements are the standard choice for return-line, pressure and kidney-loop filtration. Although some manufacturers make crossover product—depth media for suction strainers, for example—these products are rare.