The efficiency and pressure drop of filters made from synthetic fibres carrying electrostatic charges and filters made from uncharged glass fibres were measured repeatedly for more than 19 weeks in operating HVAC systems. Results showed efficiency reductions as large as 40% for the filters made from charged fibres whereas the efficiency of filters made from uncharged fibres changed little. Clean filters were sent to a testing laboratory for efficiency measurements according to ASHRAE Standard 52.2-1999. When the filters experienced accelerated dust loading according to the ASHRAE procedure, the efficiency of both the glass and synthetic fibre filters increased. Efficiency reductions like those observed for the charged filters during actual use were not replicated by this standard test.
Bibliographical noteFunding Information:
This research was supported by Johns Manville Corp and the authors' primary contact with Johns Manville was Charles Kern. Don Thornburg of Camfil Farr was instrumental in obtaining the filters. Andrew Streifel of the University of Minnesota Department of Environmgntal Health and Safety assisted the authors in setting up this project. The authors wish to acknowledge the essential support provided by Bill Allen, Ladang Hak, John Goettl and Dale Runge of University of Minnesota Facilities Management for their help in data management, filter handling and other support. •