Bag filter
A bag filter is in the fluid technology , a filter ( surface filters ) in which the filter medium to a tube is formed. Bag filters have established themselves as filtering separators in filter technology, gas cleaning and dedusting in numerous industrial processes.
Wilhelm Beth from Lübeck is considered to be its inventor .
In earlier years the filters were cleaned by knocking, shaking or in combination with low-pressure flushing, today this is done with compressed air .
execution
Bag filters are made of fiber material that is woven into tubes . While natural fibers were often used in the past , synthetic fibers have now largely established themselves in industrial applications . In the hot gas filtration find metallic wire mesh application.
The length of individual filter bags is usually between 1.5 m and 8 m.
Filtering process
The raw gas is ideally fed to the filter bags in a cross- flow manner in order to avoid an upflow against the sedimentation direction of the particles. The raw gas is directed over a distributor plate, where a pre-separation takes place and the raw gas flow in the filter housing is evened out. The actual separation of the particles takes place on the surface of the filter medium or the filter cake deposited on it. The cleaned gas flows upwards out of the hose.
The corresponding flow resistances result from the pressure losses in the filter cake and in the filter medium directly after the jet pulse cleaning ( residual pressure loss ). Especially with long hoses (e.g. a hose length of 8 m with a hose diameter of 160 mm) and high filter surface loads , the pressure loss across the hose base, i.e. H. when exiting the hose through the inlet nozzle into the clean gas area, also important. This and all other flow resistances of the filter housing (raw gas inlet to the filter cake surface, clean gas flow from the hose outlet to the clean gas duct outlet) are combined in the housing pressure loss.
Cleaning
Vibration method
In the oldest cleaning method, using manually operated or motorized shaking devices ( vibration ), a motor is started depending on time or when a maximum filter resistance is reached, which causes the filter element to vibrate. During the resulting movement, the accumulated filter cake becomes detached from the filter surface and falls down into a dust collecting container, which is usually emptied by dust discharge devices. Mechanical cleaning is carried out after the filtration operation has been interrupted. The disadvantage of cleaning by shaking is that the filter bags are mechanically stressed and therefore only have a relatively short service life .
Backwash method
A more gentle cleaning process consists in the periodic reversal of the direction of flow using scavenging air (backwashing). The filter system is designed in several separate chambers that are cleaned individually. In many cases, the combination of the cleaning methods vibration and backwashing has also been implemented.
Pressure surge method
The most modern form of cleaning in filter technology, now largely used as a standard, is the pressure surge process (jet pulse cleaning). With this method, the filter bags, unlike the two above. Methods that flow through from the outside to the inside in filter operation and therefore require support cages that give them the necessary stability. The cleaning is carried out by a cyclical, intensive burst of compressed air into the interior of the hose, which briefly puts the filter hose in excess pressure . The filter bags are briefly inflated, the direction of flow is reversed ( now from inside to outside ) and the filter cake is detached from the outside of the hose. After the filter bags have been cleaned, the dust particles settle into the dust collection chamber, and from there the material is mostly transported away via screw conveyors and rotary valves .
The cleaning has to be done in such a way that the filter cake is completely detached over the entire length of the hose. At the same time, the impact of the medium on the support cage must be minimized by modulating the pressure curve accordingly.
The cleaning cycles depend u. a. from
- the filter surface load ( volume flow per filter surface and time unit )
- the gas tight
- the raw gas load
- the particle properties.
The transition from purge air cleaning to jet pulse cleaning has made a significant contribution to increasing energy efficiency through the effective removal of the periodically deposited filter cakes.
Modes of operation
In the online mode of operation, the particles in the raw gas chamber are continuously filtered. Immediately after the jet pulse cleaning, the particle concentration near the filter hose is very high. In this state, especially in the case of finely dispersed dusts with a low tendency to agglomerate , the particles that have been cleaned off are filtered again. This “internal” dust circulation can cause a considerable proportion of the filter cake mass and thus contributes to the pressure loss.
In order to increase energy efficiency, filter modules are therefore put into a flowless state by means of shut-off devices on the raw and / or clean gas side during cleaning. In this offline mode, the immediate repositioning of dust on neighboring filter bags is prevented. This can also be achieved by cleaning the filter with a compressed air pulse of much lower intensity.
control
The cleaning is now controlled using microprocessor technology and fieldbus systems . In addition to controlling the membrane valves , the pneumatically or electrically operated raw and clean gas flaps are activated and signals from field sensors, such as " broken bag monitors ", are processed.
When pulsing the pressure surges, a distinction is made between
- a fixed time control and
- differential pressure control with variable cycle times.
Another control parameter is the continuous regulation of the tank pressure of the compressed air reservoir. The compressed air requirement is adapted to the respective operating conditions by continuously adjusting the cleaning pressure. The filter differential pressure is used as a control variable for the inlet pressure-controlled cleaning (see fig.). Advantages of this method: The operating data of the dedusting system are kept at the desired operating point with minimal compressed air requirement , the dust accumulation is more even, the capacity of the dust discharge elements is better utilized, and the filter bags are less mechanically stressed.
Injector technologies
The two-stage injector system, which introduces the compressed air during periodic regeneration, is decisive for the energy-efficient operation of bag filters.
Injector systems can, for example, consist of a blowpipe with simple bores from which the compressed air flows out (left part of the figure). Secondary air is sucked in through the downstream Venturi inlet nozzle and an increase in the static pressure in the filter hose is achieved. The inlet nozzle represents an optimization with reduced flow losses.
By necking the inlet bore into an “ideal nozzle”, a further increase in efficiency is achieved when converting the compressed air energy into a cleaning impulse (middle part of the figure).
The Coanda injector is a very efficient cleaning technology (right part of the figure). This cleaning system uses the Coanda effect , in which the compressed air emerges from an annular gap and is guided over a curved surface. The primary air follows the boundary layer , which is not detached due to the geometry of the Coanda injector . This creates an extremely high negative pressure within the first injector stage, which sucks in further secondary air and forms a propulsion jet that has a significantly increased amount of air compared to the variants described above. This propulsion jet enters the inlet nozzle as a second injector stage, in which further secondary air is sucked in.
literature
- Friedrich Löffler: Dust separation. G. Thieme Verlag, Stuttgart-New York (1988), ISBN 3-13-712201-5
- Intensiv Filter GmbH & Co. KG (Ed.): Dedusting technology, filter media. Intensiv-Filter paperback. 3rd edition, 1989.
- H. Meyer zu Riemsloh, F. Kordas: Dedusting large exhaust gas volume flows with filtering separators. Processing technology 33 (1992) No. 12, ISSN 1434-9302 , pp. 673-683.
- Gunnar-Marcel Klein, Tim Neuhaus, Peng Bai, Theo Schrooten, Tobias Daniel: Reduction of the pressure losses caused by particle deposition in industrial bag filters . F & S Filtrieren und Separieren 23 (2009) No. 3, ISSN 1432-3559 , pp. 134-139.
- Tim Neuhaus, Peng Bai, Theo Schrooten, Gunnar-Marcel Klein: Increasing energy efficiency in industrial gas cleaning through optimized surface filtration. In: Hazardous substances - cleanliness. Air . 70 (2010), No. 6, ISSN 0949-8036 , pp. 231-236.
- VDI guideline VDI 3677 sheet 1: 2010-11 Filtering separators - surface filters . Beuth Verlag, Berlin.
Individual evidence
- ↑ a b VDI 3677 sheet 1: 2010-11 Filtering separators; Surface filters (Filtering separators; Surface filters) . Beuth Verlag, Berlin. P. 28.
- ↑ 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 1: 2010-11 Filtering separators; Surface filters (Filtering separators; Surface filters) . Beuth Verlag, Berlin. P. 13.
- ↑ VDI 3677 sheet 3: 2012-11 Filtering separators; Hot gas filtration (filtering separators; high-temperature gas filtration). Beuth Verlag, Berlin, p. 23.