Portland State University. Department of Mechanical and Materials Engineering
Term of Graduation
Date of Publication
Master of Science (M.S.) in Mechanical Engineering
Mechanical and Materials Engineering
air quality, clean air delivery rate, low-cost air cleaning, particulate matter, wildfires
1 online resource (viii, 50 pages)
Air cleaning reduces indoor exposure to fine particulate matter (PM2.5) during wildfire events. However, resource and cost restraints may limit access to air cleaning during such an event, as both commercial devices and the high-rated MERV filters that homemade assemblies typically rely upon tend to be expensive and in short supply. With these barriers in mind, we sought to develop and evaluate the potential for air cleaners that use common household fabrics as filtration media. Evaluated designs use a box fan to move air across fabric filters; box fans are inexpensive and readily available to many households. Ultimately, this research aims to advance both fundamental understanding and practical considerations for recommending emergency-use air cleaning technologies.
Using mass balance principles to model a hypothetical indoor space during a wildfire, a target PM2.5 clean air delivery rate (CADR)--or volumetric flowrate of clean air delivered by an air cleaner--for an effective do-it-yourself (DIY) device was predicted to be 127 m3/h. Three distinct experimental methods were employed to determine if various configurations of the air cleaner met or exceeded this target. First, particle decay tests in two residential homes were conducted using incense combustion emissions as the challenge aerosol. A prototype air cleaner--which consisted of a box fan equipped with a cotton batting filter--yielded a PM2.5 CADR of 177 m3/h, 39% greater than the target. This CADR resulted in a net PM2.5 removal effectiveness of >80% within 30 minutes of operation.
We then conducted laboratory testing of the device--again using incense combustion emissions as the challenge aerosol--independently characterizing air flowrates and single-pass removal efficiencies to determine size-resolved, predicted CADRs. Five fabric filters (cotton batting, polyester, felt, flannel, and chiffon) were tested, as well as two popular, homemade air cleaning configurations with and without flowrate-increasing shrouds. Of the five fabrics tested, cotton batting yielded the highest predicted CADRs: at the highest fan speed setting with a single layer of fabric, average CADRs of 98, 80, and 192 m3/h were realized at particle size ranges of 0.02-0.3, 0.3-1, and 1-2.5 µm, respectively.
Finally, particle decay testing in a large-scale chamber was conducted on the device with a cotton batting filter attached, including alternative configurations that featured a second filter and flowrate-increasing shroud. Across triplicate experiments with combustion emissions from pine needles local to Portland, Oregon as the challenge aerosol, the device with two layers of fabric and shroud yielded the highest average CADRs: 190, 158, and 243 m3/h at particle size ranges of 0.02-0.3, 0.3-1, and 1-2.5 µm, respectively.
In an effort to directly compare results across laboratory and large-scale chamber experiments, an additional round of chamber testing was performed with a single cotton batting filter using the same box fan and challenge aerosol as the laboratory experiment (incense combustion emissions). The device yielded average CADRs that were 49%, 1%, and 6% higher than the corresponding laboratory experiment predicted CADRs, at particle size ranges of 0.02-0.3, 0.3-1, and 1-2.5 μm, respectively. While there was agreement between these experimental approaches in the two larger size bins, laboratory testing underpredicted CADRs in the 0.02-0.3 μm size range, a discrepancy that could be explained by relative humidity and peak PM2.5 injection concentration inconsistencies across experiments.
The three distinct approaches used here to determine CADRs yielded generally consistent results, demonstrating the fundamental scientific principles that govern active indoor air cleaning. While single-pass fabric removal efficiencies are generally low, large surface areas and high air flowrates make for an effective, low-cost air cleaning device, constructed of materials readily available to most during a wildfire event. There is limited data in the literature regarding the performance of DIY air cleaners, especially for the novel fabric-based designs developed as part of this work. These designs are simple, effective, and inexpensive, such that they represent a viable option for improving indoor air quality during a wildfire event.
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Stinson, Brett William, "Developing and Testing Low-Cost Air Cleaners for Safer Spaces During Wildfires" (2023). Dissertations and Theses. Paper 6444.