EFFECT OF HUMIDITY AND PARTICLE HYGROSCOPICITY ON THE MASS LOADING CAPACITY OF HIGH-EFFICIENCY PARTICULATE AIR (HEPA) FILTERS

The effect of humidity, particle hygroscopicity, and size on the mass loading capacity of glass fiber high efficiency particulate air filter s was studied. Above the deliquescent point, the pressure drop across the filter increased nonlinearly with areal loading density (mass coll ected / filtration area) or a NaCl aerosol, thus significantly reducin g the mass loading capacity of the filter compared to dry hygroscopic or nonhygroscopic particle mass loadings. The specific cake resistance K2 was computed for different test conditions and used as a measure o f the mass loading capacity. K2 was found to decrease with increasing humidity for nonhygroscopic aluminum oxide particles and for hygroscop ic NaCl particles (at humidities below the deliquescent point). It is postulated that an increase in humidity leads to the formation of a mo re open particulate cake which lowers the pressure drop for a given ma ss loading. A formula for predicting K2 for lognormally distributed ae rosols (parameters obtained from impactor data) was derived. The resis tance factor, R, calculated using this formula was compared to the the oretical R calculated using the Rudnick-Happel expression. For the non hygroscopic aluminum oxide, the agreement was good but for the hygrosc opic sodium chloride, due to large variation in the cake porosity esti mates, the agreement was poor.