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Filtration media, whether they are woven or non woven, are constructed from either natural or man made fibres. Today, the only natural fibres still used in limited applications are wool and cotton. Their primary benefit is in their ability to withstand higher temperatures.
The development of synthetic fibres, such as polyester, polypropylene, nylon, aramid, rayon, viscose and polyethylene, has all but eliminated the use of natural fibre media in liquid filtration. When we select a media for a specific application, its "fibre content" can be critical due to the fibres' ability to withstand specific chemical and thermal environments. Basic compatibilities are outlined in Thermal and Chemical Compatibilities.
Surface media are filtration structures which remove contaminate particles on the surface of the structure. They are generally two dimensional woven structures, and are only as deep as the diameter of the yarn from which they are woven. They will only trap particles that are larger than the window opening of the structure. Their advantage is that they can be woven with great precision, therefore offering exact window openings, providing entrapment of particles of a specific size. Their disadvantage, however, is that they do not offer high particle loading or "dirt holding capacity" because they have little depth. As a result, they tend to plug up or "blind off" more quickly than their "Depth Media" counterparts.
Not all surface media are woven. There are surface structures that are constructed mainly from matted fibres which are bonded together with heat or binding agents. They are commonly known as "spun bonded" or "point bonded" structures, and are primarily used in multi layer applications, as covers, or bypass and transfer layers due to their low inherent strength characteristics.
The most widely used surface media are woven structures using either multifilament or monofilament yarns made from polyester, polypropylene, and nylon.
Multifilament media are woven from strands or yarns which consist of many smaller strands or fibres that have been spun or twisted together. They have the advantage of being the most inexpensive woven media, and therefore produce a very low cost, disposable filter bag. An individual yarn may vary 50% or more in diameter and generally produce a woven structure that has a rectangular, rather than a square, window opening. Contaminate particles tend to get trapped within the fibres of the yarn making this media very difficult to clean and reuse. Despite these short comings, they are well suited to a great many filtration applications where high precision is not required, and low cost is key.
Monofilament media are woven from strands or yarns which have been extruded from a polymer, and therefore have a consistent diameter and a very smooth surface. The advantage of this type of yarn is that they weave a very precise filtration structure with consistent square window openings. The yarns smooth surface enables it to be cleaned and reused in many applications. Monofilaments offer precision rated media with high mechanical strength.
Depth media are filtration structures which remove contaminant particles both on the surface and within the depth of the media. They are typically of needled felt or melt blown construction in a three dimensional structure. This creates a tortuous path for particles to follow, often resulting in particulate of a size smaller than the actual pore openings being trapped within the structure.
The advantages are: high dirt holding capacity; higher void volume or pore volume; the capability of removing gelatinous particles, and particles smaller than the mean pore openings; most importantly, a long service life due to the three dimensional structure.
