By: Nancy Zimmerman, Senior Research Scientist, Kimberly-Clark Professional |
In the United States, most people select filters based on their Minimum Efficiency Reporting Value (MERV) rating under ASHRAE 52.2. In Europe, EN 779 is the standard and test procedure that classifies filter performance. Unfortunately, the EN 779 test suffers from some of the same weaknesses as does the “MERV-A” option of ASHRAE 52.2.
As we reported earlier this month, when evaluating filter performance, it’s always best to look beyond a filter’s simple MERV rating and evaluate its E1, E2 and E3 particle capture efficiencies as well. We also cautioned against accepting “MERV-A” results in a vacuum, without considering the full ASHRAE 52.2 test results.
Similar cautions are in place for those evaluating filter performance under EN 779. This European Standard and test procedure also classifies filter performance. To compare results for different filters, the test requires filters to be tested against two synthetic aerosols:
- A fine aerosol for measurement of filtration efficiency as a function of particle size within a particle size range of 0.2 to 3.0 microns.
- A coarse aerosol for obtaining information about dust-holding capacity and, in the case of coarse filters, filtration efficiency with respect to coarse loading dust (arrestance).
This European Standard applies to air filters having an initial efficiency of less than 98 percent with 0.4 micron particles. Filters should be tested at an air flow rate between 0.24 m3/s (850 m3/h) and 1.5 m3/s (5400 m3/h). The performance results obtained cannot by themselves be quantitatively applied to predict performance in service with regard to efficiency and lifetime. Other factors influencing performance to be taken into account are described in Annex A (Normative) and Annex B (Informative).
Like with the MERV-A option, EN 779 requires electrostatically charged filter to be “discharged” through immersion in a solvent. Thus, the procedure has a similar problem that it does not represent real-world conditions.
Remember: electret treatments are an enhancement of an underlying mechanical structure of the media. It is impossible to isolate the structural and physical properties of an electret-charged filter media from the charge distribution without impacting other filtration mechanisms and/or other filtration properties.