Operation stability of non-uniformly covered bag filters

Fabric filters are frequently used for the separation of fine dust particles from gas streams. Very high dust removal efficiencies can be obtained when the dust laden gas stream pervades the filter material and fine particles are withheld forming a filter cake. But pressure drop across the filter augments as the filter cake grows. Therefore the filter cake is to be removed periodically which is often achieved by jet pulses i.e. a short high pressure air pulses are directed against the usual gas flow direction. There the filter cake is removed from the filter cloth and it drops at least partially to the bottom of the filter housing where it is removed. In practice most filters operate at a partial cleaning mode which can be due to an imperfect removal of the filter cake from the fabric (referred to as "patchy cleaning") or which is the consequence of cleaning only a part of the total filter area. Irrespective of its origin, the filter is unevenly covered with filter cake which results in significantly different filtration velocities and consequently in a different filter cake build-up. Such a different cake formation procedure can in turn have consequences on the cleaning behaviour of the filter cake. An instable operation mode is expressed by an increasingly reduced time span of the dust filtration between cleaning pulses at otherwise unchanged operation conditions. In the worst case such an instable operation of a fabric filter results in a severe failure of the filter which is most undesirable. The operation of existing jet-pulsed filters has already been evaluated with special emphasis on "patchy cleaning" based primarily on global operational data from pilot and large scale filter units. This will be continued and improved. Moreover the instability of filters will be investigated experimentally and by means of mathematical simulations. During a cooperation with the Norwegian University of Technology and Science, Trondheim, Norway, the applicability of the simulation model will be extended towards panel bed filter systems, where a granular solid particle bed is used instead of a filter cloth for filtration. The simulations will allow to also predict the operational features at varied operational parameters. Finally the goal will be to outline the stable versus the instable range of filter operation. The applicability of the model will be tested by detailed measurements of the spatially resolved cake formation growth at a small scale filter unit which comprises only three filter bags of commercial size, and local gas flow measurements.

Fabric filters are frequently used for the separation of fine dust particles from gas streams. Very high dust removal efficiencies can be obtained when the dust laden gas stream pervades the filter material and fine particles are withheld forming a filter cake. But pressure drop across the filter augments as the filter cake grows. Therefore the filter cake is to be removed periodically which is often achieved by jet pulses i.e. a short high pressure air pulses are directed against the usual gas flow direction. There the filter cake is removed from the filter cloth and it drops at least partially to the bottom of the filter housing where it is removed. In practice most filters operate at a partial cleaning mode which can be due to an imperfect removal of the filter cake from the fabric (referred to as "patchy cleaning") or which is the consequence of cleaning only a part of the total filter area. Irrespective of its origin, the filter is unevenly covered with filter cake which results in significantly different filtration velocities and consequently in a different filter cake build-up. Such a different cake formation procedure can in turn have consequences on the cleaning behaviour of the filter cake. An instable operation mode is expressed by an increasingly reduced time span of the dust filtration between cleaning pulses at otherwise unchanged operation conditions. In the worst case such an instable operation of a fabric filter results in a severe failure of the filter which is most undesirable. The operation of existing jet-pulsed filters has already been evaluated with special emphasis on "patchy cleaning" based primarily on global operational data from pilot and large scale filter units. This will be continued and improved. Moreover the instability of filters will be investigated experimentally and by means of mathematical simulations. During a cooperation with the Norwegian University of Technology and Science, Trondheim, Norway, the applicability of the simulation model will be extended towards panel bed filter systems, where a granular solid particle bed is used instead of a filter cloth for filtration. The simulations will allow to also predict the operational features at varied operational parameters. Finally the goal will be to outline the stable versus the instable range of filter operation. The applicability of the model will be tested by detailed measurements of the spatially resolved cake formation growth at a small scale filter unit which comprises only three filter bags of commercial size, and local gas flow measurements.