Filter cake
The residue of retained substances that forms on the surface of a filter during filtration is called filter cake . In the case of filtration with surface filters, it is essentially responsible for the cleaning performance.
Origin and meaning
During the filtration process, particles - usually solids - are retained by the filter medium and are deposited on it. With increasing duration, a clearly perceptible layer of particles forms - the filter cake, which leads to a significant improvement in cleaning performance, but also to an increase in flow resistance .
With increasing thickness of the filter cake, the particle separation takes place mainly through it. This can be explained by the fact that the distance between the individual particles of the cake is smaller than between the fibers of the actual filter. The efficiency of the filter cake increases particularly quickly if at least parts of the particles to be retained are fibrous.
On the other hand, the increasing flow resistance leads to an increasing power requirement to flow through the filter or, in the case of limited power, to a constant decrease in the volume flow through the filter. The pressure drop in the filter cake essentially determines the energy requirements of a dedusting system . In addition, the forces acting on the filter cake can lead to cake compression - accelerated compression of the filter cake and thus a faster increase in the flow resistance.
The filter cake must therefore be removed from the filter after some time, e.g. B. by backwashing or in the gas phase mostly by introducing a pressure pulse on the downstream side of the filter medium ( pressure surge method ). The method, which only takes a fraction of a second, leads to a sudden increase in the particle concentration in the clean gas during gas cleaning. That is why the state of cake filtration is aimed for as quickly as possible. It has been found that 60% to 90% of dust emissions occur during the cleaning phase.
If the filter cake becomes caked, it can no longer be removed; instead, the entire filter must be replaced. The following effects can lead to caking:
- in exhaust gas cleaning systems for more complex exhaust gas compositions, such as B. in waste incineration plants , by falling below the dew point of individual exhaust gas components
- in hot gas filtration
- due to sintering effects or the formation of a eutectic below the respective melting temperatures .
The filter must also be replaced if a filter cake should form during the deep filtration .
In membrane technology , ultra- and microfiltration is also very strongly influenced by the formation of filter cakes (top layers), also called fouling here . The minimization of this filter cake formation is a reason for the filtration with hollow fibers to use the energy-consuming tangential flow filtration instead of the cake filtration .
literature
- VDI 3677 Part 1: 2010-11 Filtering separators; Surface filters (Filtering separators; Surface filters). Beuth Verlag, Berlin. ( Summary online , table of contents )
Individual evidence
- ^ Nikolaj Albertovič Fuchs: The Mechanics of Aerosols . Pergamon Press, Oxford 1964, p. 213.
- ^ RG Dorman: Filtration . In: Charles Norman Davies: Aersosol Science . Academic Press, London and New York 1966, p. 219.
- ↑ Gunnar-Marcel Klein, Theo Schrooten, Tim Neuhaus, Rainer Kräbs: Energy-efficient jet pulse dedusting systems . Hazardous substances - cleanliness. Luft , 69 (2009) No. 5, pp. 199-204.
- ↑ a b c Eberhard Schmidt: On the compression of layers of dust deposited on filter media . Dust - cleanliness. Luft , 53 (1993) No. 10, pp. 369-376.
- ↑ Friedrich Löffler , Jörg Sievert: The periodic regeneration as a critical phase in the operation of bag filters with pressure surge cleaning . Dust - cleanliness. Luft, 48 (1988) No. 7/8, pp. 273-279.
- ↑ Joachim Binnig: State of the art in the dedusting of waste incineration plants . Hazardous substances - cleanliness. Luft, 69 (2009) No. 5, pp. 175-179.
- ↑ VDI 3677 sheet 3: 2012-11 Filtering separators; Hot gas filtration (filtering separators; high-temperature gas filtration). Beuth Verlag, Berlin. P. 31.
- ↑ VDI 3677 sheet 2: 2004-02 Filtering separators; Depth fiber filters (filtering separators). Beuth Verlag, Berlin. P. 68.
- ^ A b Robert Rautenbach : Membrane Process - Basics of Module and System Design . Springer, 1997, ISBN 3-540-61573-3 . Pp. 199-200.