Gas flow through the cake formed over the surface of a filter generates an aerodynamic drag that increases mechanical compression on the layers closer to the filter surface, reducing their thickness and permeability. A procedure was developed to calculate the pressure drop during cake build-up, taking into account the adjustment of the cake properties to the compressive stress through a constitutive equation. The rise of pressure drop with time in a cycle was determined assuming that the gas velocity was constant and that the layers were formed in equal time intervals. Darcy's law, the Happel cell model and the Carman-Kozeny equation were tested in calculating the pressure drop through each layer of filter cake. Part of the particles from the incoming gas stream may settle down before reaching the filter, and not participate on cake formation. Prior knowledge of the settling process was shown to be necessary in order to predict the filter cleaning period, more so when the gas face velocity was lower. Three laboratory filtration experiments, under coal gasification conditions, were used to exemplify the procedure. (C) 2002 Elsevier Science B.V. All rights reserved.426495501