Surface filter
A surface filter is used to separate particles from flowing fluids . In contrast to the depth filter , the actual separation effect is not caused by the filter medium, but by the filter cake that - with the exception of cross-flow filtration - forms on the surface of the filter.
With a surface filter z. B. dust particles (and associated pollutants such as dioxins / furans ) separated from flue gases , but also usable products such as flour from a carrier gas flow. The degree of separation of the particles is usually well over 99%.
Surface filtration includes u. a. the following filter forms or principles:
-
Cake filtration or dead-end filtration
- Bag filters (most common use, detailed in the remainder of this article)
- Filter press or mass filter
- Cross-flow filtration .
Filter elements in surface filtration can be in the form of hoses, candles, bags, cartridges or bulk layers.
Filter media
Filter media are porous flat or body structures. The main differentiator is their flexibility. They are therefore usually stored in
- flexible filter media,
- rigid filter media and
- Bulk layers
distinguished.
The flexible filter media include textile filters. They are preferably made from synthetic fibers, but also from metallic or inorganic fibers. Due to their chemical, mechanical and thermal properties, natural fibers have been largely displaced.
Rigid filter media are characterized by high rigidity and inherent strength. They usually consist of fibers or grains. The base material made of plastic, metal or ceramic is usually sintered . Granular ceramic filter elements with expanded clay or silicon carbide as the base material can be used for hot gas filtration .
Grainy, often oxidic material, the type and grain size distribution of which can be adapted with regard to the separation task, is used to build up bulk layers. In contrast to the aforementioned filter media, bulk bed filters are only a loose composite of granular material through which the fluid flows.
Designed as a bag filter
Wilhelm Beth from Lübeck is considered to be the inventor of the bag filter .
function
Bag filters are used in filter houses - consisting of several chambers - through which a gas stream flows that contains exhaust gases or a valuable material . In each chamber there are several rows of cylindrical filter hoses, which are stabilized by means of embedded support baskets or rings. The particle-laden gas generally flows through the filter bags from the outside to the inside, as a result of which a layer of dust ( filter cake ) builds up on the surface of the filter medium , which itself acts as a highly effective filter with increasing thickness.
However, the build-up of the dust layer causes an increase in the differential pressure and thus an increase in the energy consumed in the hose flow. For this reason, the hose must be cleaned regularly (in practice this is done either after a certain time has elapsed or when a defined differential pressure is reached). The cleaning is usually done by injecting compressed air into the hoses, whereby the dust layer is detached and falls down into a collecting funnel. This is emptied via a rotary valve .
In the case of hot gas filtration in particular , a layer of dust remaining on the filter medium is desirable in order to act as an auxiliary filter layer to protect the medium and to achieve a high degree of separation.
The cleaning of the filter bags is an energy and emission-intensive process: If the periods between the individual cleaning processes are too short, stable operation cannot be achieved, the filter medium is exposed to excessive mechanical loads and the emission-reducing effect of the filter cake is impaired: 60% to 90% of the dust emissions occur during the cleaning phase.
To remove gaseous pollutants from the gas stream, sorbents (e.g. activated carbon , sodium hydrogen carbonate or calcium hydroxide ) can be blown in before the filter , the particles of which also settle in the filter cake. When flowing through the filter cake, the pollutant gas concentration is then reduced by adsorption or chemical reaction with the sorbents. With a correspondingly high sorbent concentration, this happens to a large extent in the entrained flow before reaching the filter.
Filter medium
Needle felts are usually used as the filter medium at low temperatures (below 250 ° C) . In some cases, fine fabrics are also used, but with a decreasing tendency due to the lower separation efficiency of bare tissue. For a better deposition can inflow side an ePTFE - membrane to the fabric or the needle felt to be applied. For filter media with an ePTFE membrane, glass fabrics are often used as a carrier in addition to needle felts .
Rigid ceramic (grain or fiber ceramics ) or metallic (e.g. sintered metals , see sintered filter) filter media are used in particular for high temperature applications (above 250 ° C) , in this case the term filter cartridge is used instead of filter hose . To reduce pollutants (e.g. dioxin ), such media can also be coated with catalysts .
application
Bag filters are used in industry, for example, for flue gas cleaning , especially in power plants , cement works and waste incineration plants . Another application is the separation of grinding dust . In mills , the flour is often pneumatically conveyed , here bag filters are used to recover the flour from the gas flow. In crematoriums , bag filters with catalytically activated needle felt are used to reduce polychlorinated dibenzodioxins and dibenzofurans .
disposal
Depending on the application, the filter cake and the filter material should be treated. If the retained dust is not or does not contain a product, it may have to be disposed of as hazardous waste . This depends on the filtered dust, the separated pollutant gases and any sorbent added .
literature
- Friedrich Löffler : Dust separation . Georg Thieme Verlag, Stuttgart 1988, ISBN 3-13-712201-5 .
- Friedrich Löffler, Hans Dietrich, Werner Flatt: Dust separation with bag filters and pocket filters . Springer, Berlin 1998, ISBN 3-540-67062-9 .
- VDI guideline 3677. Sheet 1: Filtering separators - surface filters . In: Association of German Engineers (ed.): VDI / DIN manual for keeping the air clean. Volume 6: Exhaust gas cleaning - dust technology . November 2010.
- VDI guideline 3926. Sheet 1: Testing of filter media for cleaning filters - standard testing for the comparative evaluation of cleanable filter media . In: Association of German Engineers (ed.): VDI / DIN manual for keeping the air clean. Volume 6: Exhaust gas cleaning - dust technology . October 2004.
Individual evidence
- ↑ VDI 3677 sheet 1: 2010-11 Filtering separators; Surface filters (Filtering separators; Surface filters). Beuth Verlag, Berlin, p. 28.
- ↑ VDI 3677 sheet 3: 2012-11 Filtering separators; Hot gas filtration (filtering separators; high-temperature gas filtration) . Beuth Verlag, Berlin. Pp. 15-18.
- ↑ Rüdiger blessing Busch: era - factories in Lübeck. Lübeck 1993, ISBN 3-7950-0114-5 , chapter: Beth and Dräger - No factory without an idea.
- ↑ VDI 3677 sheet 3: 2012-11 Filtering separators; Hot gas filtration (filtering separators; high-temperature gas filtration). Beuth Verlag, Berlin, p. 37.
- ↑ Eberhard Schmidt , Bertold Weiß: Regeneration-related particle emissions in surface filters. In: Hazardous substances - cleanliness. Air . 58, No. 1/2, 1998, ISSN 0949-8036 , pp. 35-40.
- ↑ Eberhard Schmidt: On the compression of dust layers deposited on filter media. In: Dust - cleanliness. Air . 53, No. 10, 1993, ISSN 0949-8036 , pp. 369-376.
- ↑ a b VDI 3891: 2015-07 emission reduction; Systems for human cremation (emission control; human cremation facilities). Beuth Verlag, Berlin, p. 32.