THE EFFECT OF SOLID PARTICLE MASS LOADING ON THE PRESSURE-DROP OF HEPA FILTERS

The increase in pressure drop across a HEPA filter has been measured as a function of solid particle mass loading using three materials with different particle morphologies. Sodium chloride, ammonium chloride and aluminum oxide particles in several different size distributions were generated as challenge aerosols. For each particle size distribution, the mass of material loaded per unit area of filter per unit pressure decrease for a given filtration velocity (1/specific resistance) was measured. In all cases, specific resistance of the filter cake increased as the mass median particle diameter decreased. The experimental data for the specific resistance were found to be well correlated with the increase of the mass median particle diameter and independent of the particle density. The advantages of this approach over other methods are the use of a more convenient and easily measured particle diameter and the described approach does not require the measurement or prior knowledge of the particle cake porosity. This new correlation can be used to predict either the final pressure difference across a loaded HEPA filter or the maximum mass that can be loaded onto a filter for a specified pressure difference.