Filter performance with non-uniformly distributed concentration of dust-evidence from experiments and models

Abstract

Cleanable fabric filters such as bag filters are widely used in industrial processes to remove large quantities of solid particles from gas streams. Intrinsically, such a filtration process is as non-uniform as it is semi-continuous; that is, filter cake build-up processes are successively followed by a short-duration cleaning step. Moreover, heterogeneous flow conditions and, thus, concentrations of particles entering the filter housing and flowing toward the filter contribute to rather non-uniform filter conditions. Since cleaning is not necessarily performed in a homogeneous manner, and patches of cake might remain on the filter fabric, the subsequent filtration is also non-uniform. Finally, the filter medium itself is largely heterogeneous certainly on micro-, but often also on a macro-scale.The influence of a heterogeneous dust concentration as a common filter non-uniformity is investigated on filter performance, i.e., pressure drop increase as a measure for filter cycle cleaning frequency being a precursor for filter life-time, dust emissions and operating costs.Laboratory tests were performed whereby the pressure increase curves are recorded at basically constant gas flow. As a baseline the entire filter area was exposed to a homogeneous dust concentration. Comparatively the filtration area was split into two zones where cake formation occurred under a high and a low particle concentration mimicking a distinctive heterogeneous dust concentration situation. The transient pressure increase profiles of both, the homogeneous and heterogeneous cake formation situation, were compared with each other.The apparent filter cake resistance was slightly higher under homogeneous dust concentration conditions, i.e., the pressure drop increased faster compared with the situation of a heterogeneous dust concentration. These results suggest that, in practical terms, a filter exposed to non-uniform dust concentrations exhibits a better overall performance. Consequently aiming for a most uniform dust concentration is to no avail, though often intended in industrial operation.