Filtration efficiency and loading characteristics of PM_2.5 through commercial electret filter media

Atmospheric PM_2.5 pollution has been attracting much public attention for decades because of its adverse health effects. China and other Asian countries are frequently exposed to high PM_2.5 concentrations, and filtration techniques through HVAC systems and indoor air cleaners are often used to mitigate PM_2.5 in indoor environments, e.g. commercial and residential buildings, schools, and hospitals. Electret filter media provide lower pressure resistance and are frequently used in air purification systems. However, existing test standards, e.g. ASHRAE 52.2-2012, only challenge filters with particles larger than 300 nm which makes it impossible to assess filter performance at the most penetrating particle size (MPPS) since the inherent MPPS of electret media is smaller than 100 nm. In this study for investigating the performance of the electret filter media against PM_2.5, the filter media were first challenged with monodisperse particles of 3–500 nm (the majority of PM_2.5 number concentration resides in this range) at face velocities of 0.05, 0.25 and 0.5 m s⁻¹ to obtain initial efficiency curves. Then the time-dependent efficiencies during the loading process were investigated using polydisperse NaCl particles which mimicked the size distribution of typical atmospheric PM_2.5. Additionally, the effects of fiber charge and particle charge on initial efficiency and loading characteristics were evaluated by intentionally removing the fiber charges and charging the particles bipolarly (neutralized) or unipolarly. Results showed that the effects of fiber and particle charges on initial efficiency enhancement depended on particle sizes. The polarization force enhanced the collection of particles larger than ∼20 nm particles, the image force enhanced the collection of particles smaller than ∼100 nm, and the Columbic force enhanced the collection of particles smaller than ∼300 nm. Charging the particles unipolarly can slow down the reduction of electrical efficiency during loading and enhance the minimum efficiency by 10–30% for 50–500 nm particles, compared to that of bipolarly charged particles. A model using an electric degradation parameter and mechanical collection efficiency raising factor was applied to explain the performance of electret media in loading processes.