Fundamentals of the theory of industrial dust protection with bag filters

The oldest cleaning method for bag filters (still used in mobile filters these days) consists of motorised or manual jolting equipment. With automatic regeneration, a motor is started, either at certain intervals or when reaching a maximum filter resistance. This oscillates the filter element. During the resulting movement, the deposited dust cake detaches from the filter surface and drops down into a dust collector, which is usually cleaned via dust dischargers. Mechanical cleaning is performed after interruption of the filtration. The filter bags are stressed mechanically through shaking during the cleaning operation and therefore have relatively short service lives. The periodic reversal of the flow direction (backwashing filter) is a much gentler cleaning method. Here, the filter system features several separate chambers which are cleaned individually. A combination of both cleaning methods was also often realised. For the last decades, however, the jet pulse method has become the standard cleaning method. The filter media are regenerated through cyclic, intense blasts of compressed air. This briefly causes overpressure in the filter bag during cleaning. The filter bags are briefly inflated, the flow direction is reversed and the filter cake detached (fig. 1). A supporting cage gives the bag the required stability during the filtration phase. Among other things, the cleaning cycles depend on the filter load (volume flow per filter surface and unit of time), the gas density, the raw gas charge and the particle attributes. The regeneration can be controlled in time intervals or via defined differential filter pressures.