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.
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:
In this way, particles far smaller than the pore size can also be isolated.(Source: www.wikipedia.de)
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
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: