WHAT DID IT TAKE to provide sufficient purified air in our Chapel earlier today, in order to allow it to be used again safely for an adult group event?
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| Air Purifier #1 |
Executive Summary
We assembled a do-it-yourself (DIY) air purifying unit that produces about 90% of the amount of purified air per hour as a typical commercially available HEPA air purifier.
We made 3 of them, which together are estimated — based on recently published engineering performance tests — to have provided a sufficient quantity of purified air for the Chapel during today’s Kiddush Lunch (namely, purifying the room’s air every 10 minutes on average, when they are run on their quietest, i.e., low setting).
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| Air Purifier #2 |
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| Air Purifier #3 (needs extension cord) |
By attaching two types of filters (off-the-shelf at Home Depot) to 3 box fans that we already had on hand, the total cost of our DIY air purification system was $90 (and you can add another $90 if we had needed to purchase the box fans). In contrast, 3 commercial HEPA units would have cost more than $1500.
Recommendations
I recommend that these DIY units now be regularly used at Mishkon whenever people work, or meet, or talk, or eat, or sing without masks. Two such units would together supply 6–7 air changes per hour in rooms that are the size of the large office or the library. (The units should be placed so that together they circulate air around the periphery of the room, rather than blowing directly on someone.)
[These results probably don’t apply to our largest indoor spaces, namely the sanctuary and the Social Hall. Different and more complex considerations apply there. As for the sanctuary’s bimah, it requires a quieter and more visually appealing solution—which I hope to discuss in another post.]
Placement in the Chapel
This plan view shows the suggested placement (and direction) of the 3 air purifiers in the Chapel, assuming that the serving table is near the kitchenette, and the eating tables are near the windows. (Fan #3, near the door, needs a 6' extension cord.)
Performance Tests and Engineering Calculations
The author addressed the question of what standard of performance to aim for. The answer is not straightforward. As he lamented, “to date there is no large-scale, real-world data to validate a minimum threshold of ACH to control SARS-Cov-2 transmission in a room.” He laid out the fundamental principle: “the rate of aerosol (particulate) removal by air filters… must exceed the rate of introduction of these aerosols by a significant margin in order to avoid accumulation of aerosols and achieve rapid clearance (e.g. when introduced by an infected person).” He noted that the California Department of Public Health recommended in 2021 that the air in rooms with marginal ventilation be filtered 4 to 6 times per hour (i.e. 4–6 air changes per hour, or ACH).
Srikrishna tested a basic 20" Lasko-brand box fan on its low speed. (A Lasko representative told me that their box fans have a rating of 1500 cubic feet per minute, or cfm, at that speed.) He attached and tested the fan with various filters. The MERV-13 filter rating is considered the minimum for protection against airborne viral or bacterial infection. The tests included a 1"-thick MERV-14 filter (two different brands), and a 2"-thick MERV-13 filter, among others.
Srikrishna measured the filtration efficiency at the particle size equivalent to the size of aerosols or airborne droplets from a typical covid-19 infected person’s exhalation (0.3 microns or larger). With the 3 types of filters mentioned above, the unit’s efficiency was in the 60–70% range—and better for larger particle sizes. He then calculated an estimated Clean Air Delivery Rate (CADR) for each DIY purifier system, as the measured airflow (in cfm) times the filtration efficiency (%).
The overall DIY purifier performance, expressed as CADR, was 244 and 298 cfm using the two competing 1" MERV-14 filters, and 293 cfm with the 2" MERV-13 filter.
Significant Findings (and Notes)
1. Srikrishna compared his homemade units with several well-regarded commercially available HEPA air purifiers. The results were instructive. He found that although the filtration efficiency of those units was much higher (95%), their overall CADR is essentially the same as the DIY units, because the density of a HEPA filter dramatically restricts the commercial units’ airflow rate!
2. The author’s conclusions:
As we discovered in the test results, lower-efficiency air filtration by combining off-the-shelf components (box fans with heating, ventilation and cooling or HVAC filters) in tested DIY configurations compares favorably in performance (clean air delivery rate, noise) to the tested HEPA air purifiers but at approximately five to ten times lower cost, and can be an affordable, complementary option for rapid aerosol removal indoors in homes, clinics, schools, offices, and other public venues.
Our results suggest a sweet spot between speed of setup, simplicity, size, and cost [namely] a single 20" box fan with 2" MERV-13 ($35) [or] 4" MERV-14 filter ($58) of 20" length and width.” [However, those thicker filters are not sold at Home Depot.]
3. He found that the results were similar regardless of whether the filters were attached to the fan in a manner that sealed up gaps between the filter and the fan intake; i.e., leaks created by a loose fit did not seem to make much difference.
4. Srikrishna also measured the noise (in decibels) generated by the various DIY purifiers. The low-speed fan setting was the only speed at which the noise was found to be unobtrusive — and even quieter than the commercially available HEPA air purifiers. [That’s why I considered only the low-speed setting.]
5. Long-term performance was not evaluated.
My Calculations
I am using the target of 6 ACH in my calculations, which is equivalent to clearing out the air on average every 10 minutes. This target rate does not guarantee that nobody will get infected at Mishkon, but it does make a superspreader event highly unlikely.
The filter type that I was able to locate at Home Depot was somewhat similar to the three types mentioned above: 1"-thick MERV-13.
(Other things being equal, a higher MERV rating only slightly improves the amount of purified air delivered. Meanwhile, other things being equal, thicker filters perform significantly better because they don’t restrict the air flow as much. They also last longer, so they may be a better value in the long run. But for this initial pass, I went with what was readily available.)
To estimate the system performance with the purchased filter, I conservatively use the CADR figure 200 cubic feet per minute (cfm), which is >20% lower than Srikrishna’s estimated performance with any of the similar tested filters. (That adjustment seems reasonable because I put a $6 washable, polyester dust filter on the outside of the main filter, to keep that inner filter clean of dust — and thus prolong its useful life.)
Chapel volume: 30.5’ x 19.75’ x 9.25’ = 5572 cu. ft. (not including the unoccupied kitchenette)
Performance of each purifying unit in that space:
CADR divided by ROOM VOLUME = air change rate
200 cu ft/min * 1 AC / 5572 cu ft. * 60 min/hr = 2.1 AC/hr
3 units => More than 6 air changes per hour
(The Chapel’s windows were also open today, but I’m not convinced that they needed to be.)
Materials
I used straps or packing tape to attach a MERV-13 filter (and a dust filter) to the upstream side of an ordinary box fan.
1. Typical 20-inch box fan:
Lasko #3733 Save-Smart Energy Efficient 20-in. 3-Speed White Box Fan
2. MERV-13 HEPA filter (rated as appropriate for droplets that carry viruses):
Honeywell 20" x 20" x 1" Elite Allergen Pleated MERV 13 (FPR 10) Air Filter
Less expensive when buying 4 or more
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