Modeling the surface filtration pressure drop of PTFE HEPA filter media for low load applications

Recently, PTFE filters have been widely used in low load applications like industrial cleanrooms because of their low initial pressure drops, while little attention to their pressure drop growth has been paid in previous studies. In this study, surface filtration pressure drop models for PTFE media were established by evaluating the particle drag force in dust cakes, where the shielding effect between neighboring particles was quantitatively calculated using the kinetic theory. Dust loading experiments for PTFE HEPA membranes were conducted using monodisperse SiO₂ with different diameters and polydisperse SiO₂ with different geometric standard deviations, the pressure drops for PTFE membranes were real-time monitored. A novel on-site laser scanning porosity measuring system was built to accurately measure the changes in thickness and porosity of dust cakes. Experimental results showed that the thickness growth rate of a dust cake was gradually decreased with an increase in loaded dust mass and the porosity was linearly decreased consequently. The pressure drops for dust cakes formed by monodisperse SiO₂ aerosols agreed well with those predicted by the monodisperse model after the diffuse reflection rather than specular reflection hypothesis was applied. Lower experimental results than the prediction by the polydisperse model could be explained by the aggregation effect inside the dust cake which led to a decline in the windward area of particles. The comparison between the models developed in this study and previous studies indicated the better applicability of the new-established models for surface filtration pressure drops of PTFE media for low load applications.