Study Of Filtration Characteristics Of Electret Filter Media Against PM_2.5

Recently, China was frequently hit by severe smog. The PM2.5 pollution has become a big issue for China. To protect human, it’s effective to capture particles from outdoor to indoor by applying HVAC(Heating, Ventilation and Air Conditioning) air filter. Filter media is essential for air filtration. With significant higher filtration efficiency due to fiber charge, electret filter media are widely used in HVAC filter. However, the filtration efficiency of electret media decreases a lot during loading. Efficiency reduction makes it easier for particulate pollutants to infiltrate into indoor environment and threaten human health. It’s necessary to study the mechanism of electret filtration and improve the reliability of electret media.Currently, there is no international standard for filtration testing against fine particles in the PM2.5 domain. The working group 6 of ISO CEN/TC 195 is drafting the standards. This study involved in this project. The filtration testing system was designed and built. There were routine communication and data comparison with renowned universities and institutes, which made the testing system more reliable. The initial efficiency against monodisperse particles was tested. The polydisperse particles, with similar size distribution compared to typical ambient PM2.5, were used for loading test. During the loading test, the curves of filtration efficiency evolution and pressure drop increment were obtained.Based on the aforementioned testing system, this paper studied the initial and loading filtration properties of commercial HVAC electret filter media. The filtration properties against PM2.5 was analyzed, and the theoretical model of initial and loading efficiency was established. This paper also proposed the strategies of particle charging and ultrafine fiber lamination to improve the reduction of efficiency in the loading. This study showed:(1) For electret media with high charge level, the MPPS(Most Penetration Particle Size) was around 15 nm-30 nm under different face velocities. For electret media with medium charge level, the efficiency curves were bimodal under different face velocities.(2) Based on classical single fiber efficiency model, the computation model for initial efficiency of electret media was deduced. The squared correlation coefficient R2 was close to 1, which meant the modelling results were close to experimental data.(3) The mechanism of electret filtration was the combination of image force, polarization force and Coulombic force. With particle size dependency, the image force and Coulombic force were effective for initial efficiency against finer particles, while polarization force was effective for larger particles.(4) The loading of electret media was conducted against polydisperse particles with similar size distribution of typical ambient PM2.5. During the loading, the efficiency decreased to the minimum value and then gradually increased. For minimum loading efficiency against particles with diameter of 50 nm, 80 nm, 100 nm, 200 nm, 300 nm and 500 nm, the charged particle loading can be increased 31.4%, 27.1%, 26.5%, 22.2%, 18.2% and 11.9% respectively compared to neutralized particle loading. The filtration properties were improved.(5) The minimum loading efficiency of electret media against particles with diameter of 50 nm, 80 nm, 100 nm, 200 nm, 300 nm and 500 nm decreased 46.0%, 57.4%, 58.4%, 58.4%, 54.8% and 51.0% compared to initial efficiency, while the minimum loading efficiency of composite media decreased merely 5.1%, 11.0%, 14.4%, 11.6%, 5.7% and 5.3% compared to initial efficiency, which was much lower than electret media loading. The loading performance was improved compared to electret media.