Ceiling Fan Air Filter
post by jefftk (jkaufman) · 2022-05-10T14:20:02.037Z · LW · GW · 9 commentsContents
9 comments
Filter cubes are a great way to cheaply filter a lot of air, but they're bulky and noisy. Elevating them can get them out of the way if you have a high enough ceiling, but it's still not ideal. What if we built something around a device that is intended to be up there: a ceiling fan?
Let's say your blades are a foot from the ceiling, and sweep a diameter of 52". Shroud the fan with a regular octagon of 12"x24" furnace filters, and the air will flow in through the filters and down:
Relative to box fans, ceiling fans move a lot more air for a given level of noise, because they are so much bigger. Since noise often makes people turn air filters down or off, a quiet high-volume filter that doesn't get in the way could be very useful.
You could make a simple one with duct tape, but it should be possible to make one that is reasonably attractive if you used a metal grille for the outside. This could also make changing the filters much easier, if the framework held the filters and allowed you to slide them into place. Putting lights on the inside and using the filter as a diffuser could also have a nice effect, like a paper lantern.
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comment by jefftk (jkaufman) · 2022-05-10T20:55:24.262Z · LW(p) · GW(p)
I think I want to try this out and see how well it works. Looks like covid is probably primarily transmitted in particles of 9μm or larger, even though it itself is tiny. I can measure p10 levels (and p2.5), but I'm not sure the best way to generate the particles. The Wirecutter tests by burning matches but not only isn't that all that repeatable it generates particles that are mostly much finer than we're talking about -- ~0.1-1 μm. I wonder what would be a repeatable way to generate a lot of ~10μm particles?
comment by Eggrenade · 2022-05-10T17:50:25.664Z · LW(p) · GW(p)
Though I love the idea of a filtered ceiling fan that's out of my way and can therefore be larger and quieter, I don't think a jury-rigged solution would work at all. Fans tradeoff flow for static pressure (https://blog.orientalmotor.com/fan-basics-air-flow-static-pressure-impedance), and I'd bet ceiling fans are optimized for flow. The filters will up the impedance considerably compared to basically open air, and the flow would drop dramatically.
Also, some (most? all?) central air systems can run fan-only (no heat or AC), making additional filters unnecessary.
Replies from: jkaufman↑ comment by jefftk (jkaufman) · 2022-05-10T19:23:45.540Z · LW(p) · GW(p)
Box fans are also optimized for flow, but do well when used as filter cubes. Part of this is that these filters do not actually offer very much resistance to flow.
A central air system is generally not spec'd to move a large amount of air: they usually work by moving a smaller amount of more conditioned air.
Replies from: Eggrenade↑ comment by Eggrenade · 2022-05-10T22:55:01.708Z · LW(p) · GW(p)
I'm not so sure a central air system wouldn't be up to the task. I calculate this purifier at 150 CFM from the product page and assuming 8 ft. ceilings, and this implies 1,500 CFM would be a fairly typical residential HVAC system, so that seems roughly adequate to me.
Also, that product page suggests you should cycle air 5 times per hour, but that seems excessive to me. I use that unit in a much larger room on a low setting and it does just fine getting rid of any smoke smell from wildfires.
Replies from: jkaufman↑ comment by jefftk (jkaufman) · 2022-05-11T00:34:12.283Z · LW(p) · GW(p)
microcovid.org thinks outside is 1/20 the risk of inside, and 5 ACH gets you to 1/4.
The purifier you link to has a CADR of 141 CFM, so roughly equivalent to bringing in outside air at a rate of 141 CFM.
If your residential system can do 1,500 CFM on a 2,000 sqft house with 8ft ceilings, I get 5.6 ACH. That's quite a bit better than I expected; not sure how typical that is?
comment by jefftk (jkaufman) · 2022-05-18T01:06:05.919Z · LW(p) · GW(p)
Another advantage of a ceiling fan filter is an essentially ideal airflow pattern. If you have people sitting around the outside of a room,with the fan in the center, it blows fresh air down toward the people, and then sucks up the dirty air around the perimeter of the room.
comment by [deleted] · 2022-05-10T16:42:32.025Z · LW(p) · GW(p)
Jeff, this is not a situation where you can do better than a prepackaged properly engineered solution. You need scale, incremental refinement, and a metric to optimize for. (In this case a mix of reliability, filter life and cost, and particle filtering effectiveness).
The simple issue with filter cubes and other improvised solutions is they leak air.
Anyways if you need cleaner air buy one of these: https://www.google.com/amp/s/www.nytimes.com/wirecutter/reviews/best-air-purifier/amp/
(I use a coway airmega bought on sale, it is always running)
Replies from: jkaufman, pktechgirl↑ comment by jefftk (jkaufman) · 2022-05-10T17:22:55.666Z · LW(p) · GW(p)
Your link's top pick, the Coway AP-1512HH Mighty, has a pollen CADR of 240 CFM [1], while a filter cube has a pollen CADR of more like 464 CFM while being much cheaper. Leaks are not a critical problem for this usage, the way they would be on an intake filter, because you are repeatedly processing the same room's air. What matters is the overall effectiveness, which is what CADR ("clean air delivery rate") measures.
[1] Per Lee 2020, covid is probably primarily transmitted in particles of 9μm or larger, and when reporting "pollen" effectiveness people use 5-11μm.
↑ comment by Elizabeth (pktechgirl) · 2022-05-10T17:35:23.299Z · LW(p) · GW(p)
This is not a situation where you can do better than a prepackaged properly engineered solution.
Why not?