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Filters – a definition
Even though their areas of application are so vast, very few people actually know exactly how filters work. What does "filtration" actually mean? The word - derived from the French "filtrer" or the Italian "filtrare", meaning „to strain" - originally meant "to run through felt". Before the invention of paper this was the finest method of filtration, and is still the state of the art in some sectors even today. Filtration is a mechanical process that serves to separate or clean a medium, a suspension (particles in water) or an aerosol (particles in a gas). The terms filtration and filtering are synonymous here. Filters, especially those used in industrial applications, are highly complex and sophisticated products; this becomes very clear when we take a closer look at just how they function.
The motivating force in filtration is the difference in pressure in the transport medium - for instance a gas or a liquid - before and after the filter. The medium is thus sucked through the filter (negative pressure) or pressed through it (overpressure).
There are several different types of filtration, depending on where exactly the process is taking place. If filtration occurs on the surface of a filter, particles settle there and gradually form a so-called filter cake, which itself functions as an ever thicker filter. This procedure is thus referred to as surface filtration, or cake filtration. To maintain the filtration effect, the filter cake has to be removed at regular intervals. In this way not only the solids (filter cake) are preserved but also the cleaned transport medium. If this separation process takes place inside the filter it is referred to as depth filtration. The particles are retained inside the filter, and the medium can be passed on. Since the solids can only be removed from the filter with difficulty, this procedure is mainly found in cases where only the cleaned medium is to be used later on. If the filter lies crosswise to the flow, this is referred to as cross-flow filtration. The shear forces prevent a filter cake from forming, and the surface is cleaned. Cross-flow filtration is mainly used for filtering liquids in the chemicals, foodstuffs and pharmaceutical industries.
Properties of Filters
The actual filtration effect can be achieved on the basis of various different physical properties. Several filters function like a sieve, retaining solids from a stream or gas or liquid because of the different size of the particles or pores. Particles larger than the pores cannot penetrate the filter. This is referred to as the sieve effect. The filtration effect can also take place by utilizing the inertia of different particles. If the particles are too inert, they cannot follow the flow in the medium and meet the filter material past which, for instance, gas or liquid is flowing. Furthermore, complex physical properties are used, including:
- Brownian motion (thermal agitation of particles in liquids)
- Electrostatics in gas streams
- The barrier effect with filter cakes (even though particles flow past the filter material, they cannot pass through it because of its geometrical expansion)
- Thermophoresis (movement of particles resulting from a temperature change inside a liquid)
In this way, particles far smaller than the pore size can also be isolated.
Today, for development of ever better filters, computer simulations are now being used in calculations of the filtration effect. The Fraunhofer Institute for Industrial Mathematics (ITWM) has created software representing a virtual nonwoven on-screen which can calculate effects such as filter efficiency, pressure loss, service life and other important filtration parameters. This enables the virtual material design of filter media.
As already mentioned earlier, dynamic developments are leading to continually new shapes and types of filter. In mid-2010, for instance, a German nonwovens manufacturer - headquartered in Emsdetten, North Rhine-Westphalia - co-developed a nonwoven with a Swiss partner that would absorb only oil and nothing else. The fibers have been mixed with a substance that separates oil and water. An initial field test will show whether the new nonwoven can help to limit the damage caused by the "Deepwater Horizon" oil spill.
Areas of application for filter media
The most diverse materials can be used as filter media: nonwovens, felts, tissues and knitted and warp-knitted materials made from synthetic fiber, ceramic fiber, glass fiber, microfiber, nanofiber and also metals. Fillings, porous solids or paper are also used for filtering, however. Filter can also undergo chemical or physical treatment respectively. In a stream of gas, small particles can be filtered out electrostatically. This method utilizes nonwoven filters where the fibers are electrically polarized or charged during the actual production process. Depending on the application, breathing protection filters also contain activated carbon. Its large inner surface absorbs complex gas molecules. Microfilters/fine-dust filters (e.g. HEPA filters, filter membranes) keep microbes at bay in medicine and in water preparation. Ceramic filter elements consist of open-pore ceramic. In foundries, ceramic filters are used in the purification of molten metals. Water filters are also partly made from ceramic. Diesel particulate filters can sharply reduce a vehicle‘s particle emissions. Alongside nonwoven filters, open-pore sintered-metal filter elements are also used in microelectronics production for filtration of gases and liquids. Additional filters for protection are referred to as police filters. The use of certain filters is often prescribed by norms such as DIN 2000 (norm for central drinking-water supply) or the Euro norm for vehicles which led to the introduction of environmental stickers.
The supply of clean air and pure water are just two reasons why the filtration industry as a whole has excellent future prospects. The global market is currently estimated to have a volume of more than 220 billion US Dollars. The benefits of nonwovens as filter media are obvious:
- Removal of a broad range of bacteria, viruses, metals and minerals from contaminated water
- Even and consistent structure - also across large surfaces
- High tensile strength and tear resistance properties
- Resistant to chemicals
- High retention
- High air permeability
- Excellent abrasion resistance
- Flame-retardant effect
- Can absorb oils and fats