Filtration Focus: Charged-Fiber Media
In January we attended an ASHRAE Forum [at the ASHRAE Winter Meeting] in Atlantic City where the discussion centered on "High Performance Buildings," with the goal of defining and quantifying such a facility. After considerable discussion, it was evident that so many opinions exist on how and who should define this type of building that it would be easier to solve the "chicken or egg dilemma."
A New Type of FilterWith this in mind, we would like to resurrect the idea that a clean building will always be the one that ultimately best serves its occupants. This process can best begin by cleaning the one element found in the largest quantity in every building - air.
There have been several improvements in air filtration media over the last few years. One such improvement centers on synthetic air filtration media called "electret" media because it is manufactured in such a manner as to impart a charge to the fibers during manufacture. This charged-fiber media is able to attract aerosol particulates much the same way a magnet attracts iron filings.
Most air filters start off at an efficiency lower than rated and, with mechanical dust loading, become more efficient. For example, an 85% average, ASHRAE, atmospheric, dust spot efficiency-rated filter will have an initial efficiency around 70%, and then increase in efficiency as the effect of dust loading occurs, ending up as a percentage in the low 90s and having an "average" efficiency of 80% to 85%.
Higher Initial EfficiencyElectret medias by contrast have a high initial efficiency, usually meeting the rated efficiency, then drop off as the dust accumulates on the fibers, insulating the charge, and then increase again as dust loading takes place (Figure 1). For example, an 85% average ASHRAE-rated efficient filter would begin at 90%, drop to 86% and then increase again to a final efficiency over 90%, giving an average of 85%.
Because of this charge, electret media can be manufactured having three advantages over standard filter media:
- Lower pressure drop - reduced pressure drop through the filter because of the fiber configuration that results in increased airflow and decreased electrical consumption. Correspondingly, the hvac unit operates less to satisfy the heating and cooling requirements of the space.
- Higher efficiency - even with the drop in efficiency due to dust insulating the charge, electrets tend to maintain higher efficiencies throughout the life of the filter.
- Higher dust holding - because electrets are able to collect dust around the entire fiber and can hold more dust with the buildup of particles on top of other particles, electret media may have a longer service life than other types of media.
Work most recently completed by Jim Hanley at Research Triangle Institute and others have shown that electret media can lose a portion of its charge when subjected to mass amounts of smaller particles like those found in outdoor air. In addition, depending on the manufacturer, moisture or hydrocarbons may also deteriorate the charge. ASHRAE has approved a research project that will look at electret media and its ability to hold its charge under various types of circumstances. With this research, we will be better equipped to apply the benefits electret hold for improving the quality of the indoor air. ES