Electret Filters—From the Influence of Discharging Methods to Optimization Potential

Abstract

Electret filters are electrostatically charged nonwovens which are commonly used in aerosol filtration to remove fine particles from gases. It is known that the charge and thus also the filtration efficiency can degrade over time. Thus, many testing standards require to remove the charge by treatment with liquid isopropanol (IPA) or IPA-saturated air. However, the parameters influencing this discharge have not been completely clarified yet. The aim of this work was, on the one hand, to experimentally investigate the influence of the IPA treatment on different electret filters and, on the other hand, to show the optimization potential of electret filters with respect to efficiency and long-term stability by numerical simulations. The experiments revealed that the air permeability is a central influencing parameter. Small pores lead to a reduced discharge efficiency using liquid IPA, while both treatment methods are suitable for larger pores. The simulations showed that a homogeneous charge distribution within the filter depth is advantageous for the initial performance. In contrast, charge penetrating deeper in the filter medium delays the charge decay and thus increases the operating time, with the trade-off of a lower initial performance.

Testing Filter-Based Air Cleaners with Surrogate Particles for Viruses and Exhaled Droplets

“…However, uncharged particles are also polarized by the spatially inhomogeneous electric field surrounding the fibers and thus are increasingly filtered without increasing the pressure drop [21]. Although electret filters typically reach a high initial filtration efficiency, it is known that they can drastically degrade with time due to the exposure to particles [23,24], organic solvents [25,26], or air humidity [27,28]. This effect also concerns mobile air cleaners, as shown in previous studies [29,30].…”

High‐Pressure Reverse Osmosis for Industrial Water Recycling: Permeate‐Sided Pressure Drop as Performance‐Limiting Factor

“…To gain information about how and especially where exactly intrusion may mostly affect the permeate sided fluid dynamics, the flow through a permeate spacer was resolved using Direct Numerical Simulation (DNS) and was analyzed by means of the simulation software DNSlab , which is specialized on the analysis of porous structures for technical applications [18][19][20][21][22]. Due to the rather complex geometry of the woven fabric as well as the strong impact of the specific structure on the intrusion effects, the permeate spacer was scanned via nano-computer tomography (nano-CT).…”

“…To gain information about how and especially where exactly intrusion may mostly affect the permeate sided fluid dynamics, the flow through a permeate spacer was resolved using Direct Numerical Simulation (DNS) and was analyzed by means of the simulation software DNSlab , which is specialized on the analysis of porous structures for technical applications [18][19][20][21][22].…”

Fabrication and characteristics of electret ultrasonic‐sensor using laser patterning

“…Conventional methods to manufacture electrets by charging solids or liquids (below referred to as “electretization”) use corona discharge or soft x‐ray irradiation and contact electrification 3–5 ; however, the most common is electretization of PTFE and other fluororesins with corona discharge. There are studies on electret applications such as non‐woven fabrics used as aerosol filters to remove fine particles from the air and energy harvesting using porous polymer electrets to convert small vibrations into electric power 6,7 . We have done research on performance improvement of an electromechanical transducer made of an electret sandwich between two electrodes and called electret capacitor sensor (ECS) 8–10 .…”

“…There are studies on electret applications such as non-woven fabrics used as aerosol filters to remove fine particles from the air and energy harvesting using porous polymer electrets to convert small vibrations into electric power. 6,7 We have done research on performance improvement of an electromechanical transducer made of an electret sandwich between two electrodes and called electret capacitor sensor (ECS). [8][9][10] We also developed on an ECS with electret of colloidal silica formed by spraying or silica aggregate formed by spin-coating of fine powder; in that ECS, electret is also used as a spacer to form microgaps.…”