Dust cake detachment from gas filters

Most forms of barrier filter for particulate removal from gases are cleaned periodically by administering a short pulse of pressurized gas to the downstream side. This is intended to remove the dust cake which deposits during filtration. The mechanisms of dust cake detachment from both rigid and flexible filter media are considered, with particular reference to the effect of the 'cake loading', the cake mass per unit area of filtration medium. Small-scale experimental methods for the quantitative determination of the conditions necessary for cake detachment are reviewed, and experimental data on cake detachment at ambient temperature are summarized. At low cake loadings (particularly below 300 g/m(2)) experiments carried out under conditions of reverse gas flow and acceleration show different trends: in general, the 'cake detachment stress' measured in an acceleration test increases with increase in cake loading while the opposite is true for cake detachment by reverse Bow. At higher cake loadings (about 1000 g/m(2)) the results of the two methods converge. Cakes are removed much more easily from rigid media than from flexible ones under corresponding conditions. Experiments on rigid media show that if the cake detachment stress under reverse flow conditions is taken at the first point of significant cake removal, (the 'burst pressure'), the resulting values are in good agreement with median detachment stresses obtained by acceleration. It is likely that cake removal by reverse flow is influenced by 'hinging' of cake patches, which remain loosely attached to the surface after their apparent detachment stress has been overcome.