I also worked on a very large extremely quiet air filtration appliance, using an extremely quiet quite big fan (500 mm blade diameter, 540 mm square frame) I have designed, and continue to improve. This is the google folder for cad files etc, related things such as the SDP810-125A based micromanometer code are also here:
https://github.com/adouglas89/Big-quiet-air-purifier and there is more info here, I stopped paying for the domain name for that project.
www.bigquietfan.com <--- This is the big one for now, which I spend most of my time on.
UVC Nukit torch mod for external power/constant operation:
https://docs.google.com/document/d/1BQh_6jLaWd9nHVWsMZghqf-FgCTMaG_XzB_kwg550y4/edit?tab=t.0
The under chin personal hepa is also now nearly complete, I just want to reinforce some areas of plastic to make them a little stronger:
https://github.com/adouglas89/Personal-Under-chin-HEPA-purifier
I made a zine on improvised air purifiers, which I used as the basis of an air purifier workshop. They are 5 pc fan air purifiers, with 20x25x1 merv-13 filters, dafco brand ($16 each after tax and shipping 12.5 before, CAD). They use high efficiency power supplies, and 2 of the three used generic fans which had daisy chainable connectors, the frames were primarily cardboard. The finger grille was so called hardware cloth, which works well and is extremely cheap. The fans are attached with 30 mm long M4 bolts, and washers. Total cost was $79 CAD, about 53 USD, including filtres. Using tuck tape is important, there is transparent tuck tape available. As with other 5 fan pc air purifiers, total noise was about 40 dBa, with a cadr of about 220-250 (the generic fans are actually a bit better than the p14 ones, however reliably sourcing the same ones is the challenge). You can find a copy here, it discusses many of the fundamentals of design for these types of air purifiers.
I held a workshop twice for making highly cost effective air purifiers with PC fans, cardboard, hardware cloth (plastic type, so basically plastic mesh/grille) tuck tape, some bolts and the filters. They were $79 CAD each, and got 220-250 CFM CADR, depending on the fans (measured by measuring airflow with an anemometer and assuming merv-13 efficiency). We did not take pics of the people as we do not want to support surviellance, however here are the air purifiers, so you can see the type of construction, the cardboard actually works pretty well. To replace the filters just cut the tape with a box cutter and put a new layer on there. The filters should last a year or something till they suffer 30% cadr hit (efficiency tends to go down, based on CR foundation research):
