The Long-Term Performance of Electrically Charged Filters in a Ventilation System

Abstract

The efficiency and pressure drop of filters made from polyolefin fibers carrying electrical charges were compared with efficiency and pressure drop for filters made from uncharged glass fibers to determine if the efficiency of the charged filters changed with use. Thirty glass fiber filters and 30 polyolefin fiber filters were placed in different, but nearly identical, air-handling units that supplied outside air to a large building. Using two kinds of real-time aerosol counting and sizing instruments, the efficiency of both sets of filters was measured repeatedly for more than 19 weeks while the air-handling units operated almost continuously. Pressure drop was recorded by the ventilation system's computer control. Measurements showed that the efficiency of the glass fiber filters remained almost constant with time. However, the charged polyolefin fiber filters exhibited large efficiency reductions with time before the efficiency began to increase again toward the end of the test. For particles 0.6 μm in diameter, the efficiency of the polyolefin fiber filters declined from 85% to 45% after 11 weeks before recovering to 65% at the end of the test. The pressure drops of the glass fiber filters increased by about 0.40 in. H ₂ O, whereas the pressure drop of the polyolefin fiber filters increased by only 0.28 in. H ₂ O. The results indicate that dust loading reduces the effectiveness of electrical charges on filter fibers.

Keywords:

• air filtration
• dust loading
• efficiency
• electrostatics
• filters
• ventilation

ACKNOWLEDGMENTS

This research was supported by Johns Manville Corporation and the authors' primary contact with Johns Manville, Charles Kern. Don Thornburg of Camfil Farr was instrumental in obtaining proper filters; Andrew Streifel of the University of Minnesota Department of Environmental Health and Safety assisted the authors in setting up this project. The authors acknowledge the essential support provided by Ladang Hak, John Goettl, Dale Runge, and, especially, Bill Allen, of the University of Minnesota Facilities Management for their help in data management, filter handling, and other support.