Comment by LTL_FTC
15 hours ago
Those fans will not have sufficient static pressure to be useful. These makeshift purifiers may be great in a pinch (wildfire season and stores are all out), but far from the best.
15 hours ago
Those fans will not have sufficient static pressure to be useful. These makeshift purifiers may be great in a pinch (wildfire season and stores are all out), but far from the best.
This is indeed the "conventional wisdom," but if you do the math it turns out this is incorrect.
The trick is to use high-airflow MERV 13 filters (ideally 3M 1900, which has high pleat count and low resistance) instead of high-resistance HEPA filters, so axial PC fans can generate enough pressure.
At that point, you’re not doing any actual air filtering. Just use them as fans instead.
Merv-13 means it filters at least 75% of the particles passing through it.
For a standalone air filter which is pushing the filtered air into the same room its drawing from, we can assume reasonably good mixing of the intake and exhaust air. Thus to be equivalent to a HEPA-filter it only needs 1/3rd more airflow... Which really isn't much.
This doesn't follow for a a fancier setup where you're taking dirty air and blowing it into a cleanroom or something... but that's not how we're typically using household air filters.
2 replies →
The linked PC-fan purifier has been tested and shown to purify air faster than the conventional alternatives, and are not makeshift, so I'm not sure what you are referring to.
From the specs it looks like they are probably closer to higher-static-pressure server fans, the CADR is a few hundred CFM and it pulls 30+ watts. And as with many of these designs, you can compensate for low pressure performance by increasing filter surface area... the greater the surface area the lower the pressure drop. Of course, the problem with this is you are just spending more money on filters, which is the expensive part.
The filter cost for the AirFanta 3 doesn't seem particularly high? In fact it seems cheaper than competitors.
All the purifier companies realized they make more money selling overpriced filters, so $60+ for the OEM filters is common nowadays. Despite it just being a standard HEPA filter cut to a specific size.
Airfanta 3 Pro filters (four) - $57
Coway Mighty Filter - $60
Coway Mighty2 Filter - $70-$80
I'd be concerned about the availability of filters after a few years. Some of these companies have been changing their models and filter type every year to add some nebulous feature. Others just go out of business.
Yeah, the retail prices of filters is all over the place because of the popularity of razor/blade model of pricing. I'm not talking about that, I'm talking about the BOM itself -- this device is using 4x the filter media of comparable filters, which would typically have one filter that size.
HEPA media itself is a commodity and aftermarket filters (and the BOM cost to the OEM) tend to level out at whatever rate is minimally profitable per area of filter media.
The upfront cost of this device is high for what you get, because you're mostly buying filter media. The device itself is not much of a polished product. I'm sure it works fine but I wouldn't want one in my living room.
1 reply →
There's an Open Source tool for building your own that does all the calculations for you: https://filterboxbuilder.com
For up-front cost, this is true.
For cost over time, twice the filters means twice the replacement interval, so the additional filter cost cancels out.
But since you bought mostly media, you didn't get a housing that hides the media, and you get to look at any collected dirt for that longer interval as well...
I'm sure it works but like any engineering decision it has pros and cons.
3 replies →