Air filters produced with charged, or electret, media for HVAC filtration applications have gained significant market share and acceptance over the past few years. While these filters provide the advantage of high initial efficiency and low pressure drop, there are concerns about their ability to maintain efficiency in service. Furthermore, there is a tendency to classify all electret media in the same general category without any consideration of media structure, fiber size, and charging technique. Current research suggests that a variety of factors influence the loss of efficiency in use including humidity, exposure to certain chemicals, aging, temperature, etc. While this is true of some charged media, the effect of environmental factors on filtration performance is highly dependent upon the media technology itself. This paper provides an overview of current electret media types'on the market detailing media structure and charging techniques. Fundamental impact of environmental factors on filtration performance is presented along with field studies detailing in-use performance of pleat filters.
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