Effects of relative humidity and particle hygroscopicity on the initial efficiency and aging characteristics of electret HVAC filter media

Because of the low pressure drop, electret filter media are widely used in indoor environments to remove particles for air quality control. The effects of water soaking, air relative humidity (RH), and particle hygroscopicity on the filtration performances of fresh and aged media, i.e., size-fractioned efficiency, however remains unclear. Electret media from a commercial high-efficiency particulate air (HEPA) filter, a commercial Heating, Ventilating, and Air Conditioning (HVAC) filter with minimum efficiency reporting value of 13 (MERV13) and a homemade filter (CM) were investigated in this work. In the initial test, the performance of above three media were evaluated at 10, 60 and 90% RHs. In the aged test, the media performances were characterized at 30, 60 and 80%, and using three different sample particles, i.e., hygroscopic NaCl, non-hygroscopic SiC, and NaCl + SiC mixture). In addition to clean filters, the collection efficiency and pressure drop of used MERV13 electret media, which had been utilized in a house for few months, were evaluated. SEM analysis was conducted to examine particle loading characteristics. The major finding of this study is that operating the filtration systems having electret media at ~60% RH can greatly enhance the particle holding capacity and minimize the efficiency decline. The result of this study provides the insights of the RH effects on the performance of electret media.