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Introduction to Filtration
Filters are manufactured and classified in many different ways. Depth Filter, Pleated Filter, Membrane Filter, Absolute Filter, Nominal Filter are a few of the names you will hear. What is a Charge Modified, Solvent Cast, Track Etched, Melt Blown, Sintered or String Wound Filter?
Sound confusing? It really isn't, because a filter will be either absolute rated or it will be nominally rated. Your application will determine which one you choose.
Nominally Rated Filters
Nominally Rated Filters are not 100% efficient at the stated particle size. For example, a nominal filter might be rated at 10 micron rating with a 75% efficiency. This means approximately 75% of all particles that are 10 microns or larger will be trapped by the filter. Because of variations in the filter's pore sizes, there will be some pores in the structure that are larger than 10 microns. Although these pores comprise only 25% of the filter's internal structure, they will allow larger size particles to pass through the filter media.
Nominally rated filters typically will be rated in the 1 to 200 micron range. They are widely used in laboratory, industrial, commercial, and residential applications. Some examples are:
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String Wound Filters: Polypropylene, cotton, nylon, rayon or similar yarn is wound around a support core (plastic, tinned steel or stainless steel). String wound filters are inexpensive, and their major advantage is the wide variety of construction materials, offering almost universal acceptance in any application. Most filter medias require trace amounts of chemical additives which may require pre-rinsing in sensitive applications. At higher operating pressures, another disadvantage is a tendency to form "channels" through the media which can cause a bypass situation. This channel effect, however, does prevent the unloading of previously accumulated particles.
Spun Bonded Polypropylene Filters: A melt blown process is used to create a solid matrix depth filter which traps particles throughout the cross-section of the filter element. Their graded density pore structure traps larger particles in the outer layers and smaller particles in the inner layer. Only pure polypropylene is used in the manufacturing process.
Advantage: Rinse up is typically not required.
Disadvantage: Not as wide a chemical compatibility as wound filters, and operating temperature is limited to about 160 deg F (71 deg C) maximum.
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| Pleated Depth Filters: Typically made with polypropylene or glass microfiber, the media is pleated to increase the effective filter surface area. These filters offer very high retention efficiencies in the 95% range and have excellent dirt loading and dirt holding capacities. The pleated surfaces give these filters a higher surface area which translates into more retention capacity at lower differential pressures. These filters are available in micron ratings down to 0.2 micron. |
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Nominally rated filters are often used as pre-filters to protect expensive absolute filters. They are also used to filter make up water for beverages, food products, electroplating solutions, edible oils, boiler feed water and chemical filtration. Reverse osmosis or DI (de-ionization) systems will also benefit from pre-filtration. Whenever removing the majority of particles is desired, nominally rated filters will be a cost-effective tool.
Absolute Rated Filters
By definition, Absolute Filters will remove particles larger than its specified micron size at very nearly 100% efficiency. The filter's efficiency can also be expressed as a Beta ratio, where:
Beta ratio = (dirty side particles / clean side particles)
For example, an absolute filter that removes 99.98% of 1 micron and larger particles would be considered to have a Beta of 5000 at 1.0 micron (10000 dirty side / 2 dirty side).
Absolute rated filters are made from membrane media such as polycarbonate film, nylon, polyethersulfone, polypropylene, or PTFE (such as Teflon). Micron ratings are available in the range from 0.05 micron to 40 micron, but the most common applications are in the 0.2 - 1.0 micron range. Because of the very tight filtration at high efficiency, O-ring seals are recommended to prevent fluid (and contaminant) bypass of the filter element.
Typical application might include pharmaceuticals, dialysis water, point of use chemicals, ultrapure gases, semiconductor, or any application where even trace contamination is unacceptable.
Absolute filters also find use in "sieving" operations, such as in magnetic media applications. In these cases, smaller (desirable) particles are part of the product, and are able to pass through the uniformly sized pores. Larger (undesirable) particles are trapped by the filter.
In summary, choosing a filter for an application is very critical. Users should be able to distinguish the differences that characterize nominal and absolute rated filters, and know when to select the proper one. If you are not certain, your filtration specialist at AIM Filtration will be pleased to guide you through the process.
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