10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Evaluation of Mobile Air Purifiers under Realistic Conditions
STEFAN SCHUMACHER, Daniel Spiegelhoff, Miriam Küpper, Ute Schneiderwind, Hartmut Finger, Christof Asbach, Institut für Energie- und Umwelttechnik e.V. (IUTA)
Abstract Number: 502 Working Group: Indoor Aerosols
Abstract Over the last years, air pollution has more and more moved into the focus of public attention. This not only concerns the atmospheric pollution, but also the indoor air quality, which can be deteriorated either by infiltration from outside or by indoor sources. Since people in industrial nations spend most of their time inside buildings, the latter is of special importance. A way to improve the indoor air quality are mobile air purifiers, which draw the polluted air through a filter (here a fibrous electret filter) and release the purified air back to the room so that the pollutant concentration decreases exponentially with time.
Different national standards classify the cleaning performance of mobile air purifiers by their clean air delivery rate (CADR), which describes the flow rate of clean air delivered by the air purifier. It is determined by measuring the temporal decay rate of an initial pollution concentration in a closed room while the air purifier is running. Most standards only consider the number concentration of particles larger than 0.3 µm, although relevant indoor aerosols mainly consist of much smaller particles. To address the question whether such CADR measurements yield a praxis-oriented measure, the size-dependent CADR for particles down to 0.03 µm was investigated. It was found that the CADR significantly drops with decreasing particle size, which means that recent standards overestimate the cleaning efficiency with respect to many relevant indoor aerosols.
It is known that the efficiency of electret filters can drastically degrade with time due to exposure to particles, gases, or humidity. Therefore, we developed a method to reproducibly age the filters by burning cigarettes in steps of 12 at the same time in a 25 m3-room (Finger 2015). Also the aging behaviour strongly depends on the particle size, which can be explained by the size dependence of the electrostatic contribution to the total filtration efficiency (Schumacher 2018). Furthermore, the results were compared to aging with 48 cigarettes at the same time in a 3 m3-cube as proposed by a recent Chinese standard, which yields a very distinct aging behaviour. This shows that for a praxis-oriented rating of the long-term stability the considered particle size range and the conditions of aging are of great importance.
Standardized tests are always conducted in test chambers without furnishings and with good air mixing. In contrast, in occupied rooms the flow and hence the pollutant dispersion can be much more complex. To investigate possible deviations, an air purifier was operated in an office room with the pristine outdoor aerosol as target pollutant. To measure the local CADR, portable particle monitors based on diffusion charging were located at different positions in the room. At all tested positions they showed similar decay curves, indicating that the cleaned air was homogenously distributed within the room. Also the location of the air purifier did not have a significant influence. However, the CADR determined in the office room was generally slightly smaller than in the standardized test chamber, which might be attributed to the different particle size distribution of the test aerosols.
The IGF project 19145 N was funded via the German Federation of Industrial Research Associations (AiF) within the program to support industrial collective research (IGF) by the German Federal Ministry for Economic Affairs and Energy (BMWi) based on a decision of the German Bundestag.
References
H. Finger et al. Gefahrstoffe - Reinhalt. Luft 75 (2015) 497-502.
S. Schumacher et al. Chem. Eng. Technol. 41 (2018) 27-34.