A filter element is the central component of any filter, where the actual process of filtration takes place. Generally speaking, filter elements are divided into two classes: surface and depth. Surface filters are made from closely woven fabric or treated paper and feature uniform pore sizes. Fluid flows through the pores of the filter while debris and contaminants are stopped at the filter surface. Surface filters are designed to prevent the passage of a high percentage of solids of a given size.
The second type of filter element, depth filters, comprise many layers of fabric or fibers that provide twisting and winding pores for the fluid to pass through. Depth filters are unique in that they retain particles within the many layers of the filter, rather than just on its surface. To achieve this, the pores must be larger than the rated size of the filter. Just as important as using a filter is ensuring that the filter is being used correctly. An incorrectly used filter element will be rendered useless and provide a false sense of security while critical parts of a machine are being irreversibly damaged. This means selecting the right class of filter is of paramount importance.
The main components of any filter are the design of the filter element itself, and the type of media used in the element. The liner must be structurally sturdy in order to withstand varying pressure, while remaining porous enough to allow flow. The top seal must also be leak free to ensure a good seal throughout the service life of the filter element. Standard seals are made of a material called BunaN, but if a filter will be exposed to fluids such as phosphate, water glycol, water/oil emulsions, or others, a fluoroelastomer or fluorel seal may be required.
Filter elements collect particles in four main ways: inertial, impaction, diffusion, interception, and sieving. Inertial collection captures large, heavy particles suspended in a flow stream. As the fluid changes direction to enter the fibrous filter, the particles continue in the same direction (hence, ‘inertia’) and collide with the filter where they are trapped and held. The second method of capture is diffusion. Diffusion works on the smallest particles that are not held in place by the viscosity of the fluid and diffuse within the flow stream.
The third form of particle capture, interception, works on mid-size particles that are neither large enough to have inertia or small enough to diffuse within the flow stream. This medium-sized debris follows the flow stream as it bends through the fibrous space. Particles are intercepted when they come into contact with the fiber. Sieving, the fourth method of capture, is the most common mechanism in hydraulic filtration. Sieving is done when the filtered particles are too large to fit between the pores of the filter fibers.
Many types of contamination can be present in fluids, making filtering a critical aspect of the proper operation of machines. Types of contaminants can include solid, liquid, gaseous, dirt, sand, and other fibers. Many contaminants are caused simply by normal manufacturing, assembly, and testing processes. High levels of debris are also common in hydraulic fluid, and can also enter the systems via reservoir vents if not filtered properly.
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