The constructive features of the horizontally or vertically installed flat-bag filter elements particularly in combination with the different - off line - cleaning systems included in the range of products allow
at low space requirement and low maintenance
Flat-bag filters are used for gas volumes of 1,000 m³/h up to more than 1,000.000 m³/h.
The filter housing is divided into crude gas and clean gas chamber by means of perforated plates. The flat-bag filter elements, each consisting of flat-bag and support cage, are precisely fixed in the housing. They are fitted in the holes of the perforated plate, secured without the use of screws and provide a perfect seal against dust leaks. The crude gas flow through the textile filter material is from outside to inside and the particles are retained on the outside.
The filter housing is divided into crude gas and clean gas chamber by means of perforated plates. The flat-bag filter elements, each consisting of flat-bag and support cage, are mounted form the clean gas side. The flat-bag elements are precisely fitted in the housing. They are fixed in the holes of the perforated plate, secured without the use of screws and provide a perfect seal against dust leaks. The crude gas flow through the textile filter material is from outside to inside and the particles are retained on the outside.
The advantages of a flat-bag filter with horizontally installed filter elements can only partially be used in case of vertical installation of filter elements.
The clean gas chamber is provided with vertically installed, easy to remove compressed air injectors, arranged in front of each bag row. Compressed air and clean gas as secondary gas are injected sequentially into two adjacent bag rows by means of injectors, designed in accordance with aerodynamic principles. The bag rows are cleaned in pairs by a brief pulse of jet air, injected into the bags contrary to the filtration flow, thus removing the particle cake from the fabric filter.
The fundamental design of this cleaning system is demonstrated in the illustration, exemplary shown by means of a flat-bag filter with vertically installed flat-bags.
The clean gas chamber is divided into several chambers. Each chamber is connected with the clean gas collective channel and can be isolated. At the beginning of the cleaning process in one chamber the corresponding isolation damper on the clean gas side will be closed. The flat-bags are cleaned during interrupted filtration process by means of compressed air.
On request, the complete crude gas area of the filter housing can also be divided into chambers, thus offering the possibility to isolate separate chambers for inspection and/or repair whilst the other chambers are still available for the filtration process. In principle this will also be possible in case of use of a compressed air – on line – cleaning system.
The cleaning of the flat-bag rows takes place sequentially in steps by means of a cleaning device, travelling within the clean gas chamber and provided with compressed air feeding and injector tubes. The cleaning device covers three vertical filter bag rows, the middle of them being cleaned with a brief, about 0.5 sec., pulse of compressed air and clean gas as secondary gas. A lip seal along the carriage travel serves as reliable sealing, proved in continuous operation.
The cleaning of the flat-bag rows takes place sequentially in steps by means of a cleaning device, travelling within the clean gas chamber. The cleaning air is injected into one flat-bag row by means of a medium pressure fan, whilst the two adjacent flat-bag rows to the row being cleaned are not charged by the crude gas during the cleaning process. A lip seal along the carriage travel serves as reliable sealing, proved in continuous operation.
This cleaning system does not require any compressed air.
Flat-bag filters are designed to achieve low residual concentration of particulate in the clean gas and maintain long filter fabric service life. The long fabric service life, compared to more conventional round vertical bag-filters, is due to low mechanical stress to which the fabric is subjected. Some corresponding aspects are mentioned below.
Support cage construction
The support cages have a small wire mesh (approx. 25 x 25 mm) thus achieving, across the entire fabric surface, an even distribution of the differential pressure forces acting on the filter bags. The smaller the mesh, the lower the load is on the textile filter fabric near the wires.
Length of flat-bag filter elements
The filter element length is limited to 2.5 m for the horizontal installation, and to 3.0 for the vertical installation. In comparison to other types of construction, this length limitation requires the installation of proportionally more filter elements for a filter surface of the same size; however, it offers considerable advantages:
In comparison to – on line – cleaning procedures, - off line – working procedures offer some advantages:
The medium pressure and compressed air – off line – cleaning systems with a travelling cleaning device in the clean gas chamber, are favourably priced flat-bag filter types. They benefit from the advantages of the – off line – cleaning systems, without having to accept the less favourable complex construction of the compartmentalised – off line – cleaning system.
Filter inlet to the filter elements
An important condition for achieving long cleaning cycles and low stress of the filter fabric is the preferably undisturbed descent of the separated particles within the filter housing. When using one compartment filters, a crude gas inlet from above is preferred. With a crude gas inlet from below, or diagonally from below, the particle agglomerate separated from the filter fabric can only descend if its descent velocity is higher than the gas inlet in counter flow. Since the descent velocity of particle agglomerates of about 200 µm totals only about 1 m/sec., it is evident that one compartment filters should be exclusively provided with a crude gas inlet from above. This will avoid an undesired and uncontrolled redeposit of even the finest of particles. This advantage holds true only in flat-bag filters with horizontally installed flat-bag filter elements.