10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Combined Effect of Aerosol Concentration and Humidity on Laboratory Filter Loading Experiments
QISHENG OU, David Y. H. Pui, University of Minnesota
Abstract Number: 1411 Working Group: Control and Mitigation
Abstract Current air filter test standards and laboratory evaluations employ artificial testing aerosols to assess filter performance. Solid particle size distribution was found to significantly affect filter lifetime, so it is often reported specifically in reports and literature, together with other operational parameters, such as filtration velocity, temperature, and pressure if applicable. Test aerosol concentration and humidity are another two factors that may affect filter performance, however, in a long time, enough attention has hardly been paid. In order to achieve reasonable test durations, laboratory filter loading tests are often accelerated with aerosol concentration much higher than real application in the field, which is particularly true for filters facing ambient air, such as those in air intake, HVAC, or air cleaning applications. Although suspicious, it is often accepted that the orders of magnitude difference in concentration (µg/m3 in field vs. mg/m3 or even g/m3 in laboratory) has negligible impact on filter lifetime. Relative humidity (RH) was found to affect filter holding capacity, especially for those loaded by hygroscopic aerosols. Unfortunately, humidity is often not controlled, monitored, or reported in literature even when hygroscopic salt particles present.
In this study, the effect of aerosol concentration and test humidity is experimentally evaluated by loading air intake filter media with hygroscopic salts at varied concentrations and RHs. Besides potassium chloride as a typical laboratory salt, ammonium sulfate and ammonium nitrite, which are more abundant in atmosphere, are employed to better represent ambient particle speciation. At same concentration level, filter holding capacity increases with RH increasing as long as particles kept “dry” below their deliquescent RH, which is mainly attributed to the morphological change on particle dendrite and cake due to moisture-salt interaction by capillary condensation. This effect is more pronounced at lower concentration as more time is allowed to develop towards a steady state, which is very difficult to reach from observation in this study. The rate of morphological change competes with the loading rate, so that the observed pressure drop evolution during a loading test represents a combined effect of aerosol concentration and testing RH. At high RHs, where rapid interaction between salt and moisture occurs, the effect of concentration becomes significant. Particles with different hydration-dehydration cycles response differently to RH and concentration level. Detail discussion will be given in the presentation.