The Cuboid: A DIY air purifier that's better than a box fan | Hacker News Reader

The Cuboid: A DIY air purifier that's better than a box fan

dynomight.net

245 pointsdynm141 comments

You can also build your computer into an air filter like I did.

I'm currently running MERV 13 filters and it pulls over 100 CFM at a very livable noise level, currently equipped with Noctua NF-A12x25 fans.

The computer stays very cool but also incredibly clean.

That's freaking brilliant. You simultaneously cool the CPU, keep the CPU clean, and clean your air. I might be sslliigghllyyy worried about the filter decreasing airflow if you used a HEPA filter, but probably not much of an issue with a MERV filter.

I'd be all for a case manufacturer to clone my design in metal but I doubt that common HVAC air filter form factors are the same around the world; you'd either need an oversized design with adapter plates, or many case size variants.

Custom cut acrylic (or other plastic sheet) might be an option too. Pretty cheap if you stick to rectangles, though there are places that will cut based on your 2D vector files also.

Silverstone MM01 is such a PC case, it's been out for a while

I knew the Mammoth existed when I made mine, but it's not what I wanted.

The Mammoth is designed to withstand water spray and dust and still cool the components satisfactorily, while mine is designed to clean as much air as possible for the synergy with computer cooling.

Additionally, I wanted to use HVAC filters due to their price/availability and large area, and I wanted a vertical motherboard like the Raven so I could access all the motherboard ports on the top.

If anybody would do it, I think Silverstone would. They think out of the box.

Modern design considerations would be allowing for top-mounted radiators on the exhaust side, and having the power supply attached to the motherboard tray so you could route power cables behind.

$500+ for the case, $160+ for new filters. Ouch.

We got a stand-alone Electrolux air purifier, it takes filters like these[1]. Guessing one could find a really common variant.

[1]: https://www.amazon.com/Genuine-Electrolux-Carbon-Filters-EL0...

Those filters cost way more than normal HVAC filters though.

By comparison, I sourced 16"x20" MERV 13 filters for under $8 each from cleartheairinc.com.

meh, just buy more fans. Also, putting a filter coming in will significantly reduce the amount of dust that your fans and other parts suck in

More (or more powerful) fans will definitely work, but isn't "free" in terms of noise or electricity. Unlike a normal desktop it doesn't work to put this one in a different room. Still, with a MERV filter, I doubt it's too big a deal.

Merv is just a level, i think merv 12 is hepa quality, 14 is like hospital grade, something like that.

MERV 13 is hospital grade, apparently, but HEPA is far better (and more restrictive) than even a MERV 16.

Thanks, I forgot that was the case, I looked it up and you're right. I use merv 14 in my home and it eliminates smoke from forest fires, but not allergens, I'm going to get a hepa filter next time.

Pollen or animal allergens should be totally captured by a MERV 14 filter. Smoke is much more challenging to filter out, and that's where a HEPA would be better (but not necessary unless you're filtering incoming air directly).

When I was a teen I worked a few months in an office where people chain smoked constantly. My computer kept dying, and I started ranting about how much old these computers were. Based on how much yellow gunk was inside it. Like quarter inch layer on everythin.

I was informed they were less then six months old. Resigned by end of month.

Chain smoking inside while working would disgust the average smoker! I wouldn’t put up with that unless it was well ventilated and I grew up with a parent who smoked and did so myself for a number of yrs.

You're gonna run those fans anyway... And you've already paid a premium for quiet ones that can run 24 hours a day, and in some cases with variable speed. Why not do this?

One thing to watch out for: You want HEPA filters with a low static pressure. Less wear on the fan. Generally more surface area (folds) is a good rule of thumb. Those 3M filters for furnaces tested well (like top 3) the last time I bothered to look at reviews, and they're everywhere.

Bonus points for exhausting vertically. Convection.

You can run less of them if they're not having to push air through a filter first.

Yes, if I ignore the entire thread I can have a different outcome.

What?

omg I was looking for this! I remember seeing this build on /r/battlestations or somewhere like ages ago, but I could never find it. (Googling for "carvac hepa filter" doesn't seem to yield any relevant results)

It's not a hepa filter, which is probably why it doesn't come up when you search that.

That's awesome! I wonder if a centrifugal fan would be better than the axial fans though. Centrifugal fans, IIRC, can handle resistance better than axial fans. They're better for forcing air through a filter. They're probably louder though, so it's a tradeoff. If I was building something like this and I owned a house, I might build a centrifugal fan, like a Fantech FR100 or something, into the attic suspended by elastic like I did for my old grow room. You could get great airflow with relatively little noise.

I wanted a centrifugal fan but there was (and still is) simply no way for me to comparison shop for them, unlike with consumer computer fans.

I'm just going to sacrifice my dignity for all the other people who may be looking at that and thinking "how does that design keep the computer clean?"

I'm sure it works like you say, but I guess I'd expect the case to be on the other side of the filter if you wanted to use the filter to keep the computer clean. I assume that's not the point, and the cleanliness is coming from having a larger fan blowing over it?

I connected an SDS011 dust sensor (didn't check US prices, but I'm guessing ~20$) to a raspberry pi and then used remote controlled power sockets to switch my air purifier on and off based on some limits I set.

Of course when building the purifier yourself, it might make sense to attach the sensor directly, use something like an ESP32 and maybe control the fan speed based on pollution levels.

Here's some Grafana graphs: https://i.imgur.com/wYjERxW.png

The dust sensor isn't absolutely silent due to the tiny fan, and the purifier obviously isn't either, so I have a "silent mode" that turns off both. There's also a CO2 sensor that shows you when I opened the windows. For some reason, dust didn't spike when I opened them in the afternoon, only in the morning and last night. The purifier is a BlueAir Pure 221 which is built a lot like the one in the article.

What accounts for the CO2 cycling?

Me opening my windows for a few minutes, but leaving them closed in general.

I never measured indoor CO2 level, but yours approaching 1400 ppm seems to be alarmingly high. I thought any level above 1000 ppm requires some intervention.

It's also notable that CO2 sensors utterly suck, none of the consumer ones do absolute measurement, they assume a baseline and calibrate their output to that.

I live in a relatively small apartment. From everything I've read, levels between 1000ppm and 2000ppm seem to be quite normal indoors, e.g. in an office. Not healthy, but normal.

One of the reasons may be that houses have become more air-tight in an effort to improve insulation, while the cognitive effects of high co2 aren't quite as easy to detect as a high energy bill. That afternoon slump may be a biorhythm / digestion thing, or maybe nobody in your office bothered to open the windows since yesterday.

The concentration also rises pretty fast. I think I could make a (slightly delayed) intrusion alarm just by reading CO2 values... or a window-open-detector for automatically turning off the heating...

Cross analysis of smart-home sensors to arrive at secondary or validation checks for other sensors (like home invasion) is the feature that will actually make a smarthome useful. Individual sensors trying to detect an event get too many false positives.

I also have a CO2 meter, and it's not hard at all to get above 1500. 5000+ is where you start getting concerned, but the air is noticeably stuffy way before that with all the other organic compounds humans exhale.

I measure mine, and if I don't leave my windows cracked, it can easily get up to 2,000 in my house. Keeping them cracked usually keeps CO2 between 500-900.

I’m surprised how quickly the CO2 level rises! Any idea how high it would get if you never open a window?

I live in an old building that has a lot of cracks and gaps for air to circulate, so it rarely comes close to 2000ppm. When I leave the house for a longer period of time (hasn't happened in a while), you can see the value slowly approaching the outside baseline. And in the night, while I sleep with the windows closed, I produce much less of it, so I don't get super high values there either.

But the rise is quite interesting! It differs a lot based on some factors, such as am I asleep, awake, or working out / are any doors open, i.e. how big is the volume of air I'm in?

Curious. I have a HPMA115S0-XXX dust sensor and I can't hear any noise from its fan at all.

Is this the dust sensor the same as a particle counter? All the particle counters I could find were in thousands of dollars.

Look up the PMS7003, I think I got it for $40 bucks on aliexpress and it's used in many air filters to monitor PM2.5 and PM10 etc sized particles. Easily run by a raspi and i output to a regular segmented display for my particle counts averaged every 5 minutes to see the california fires and how quickly my air filter could clear the room. For reference the winix 545 ($110 from costco iirc) on full blast got me down to about 0 particles in 15 minutes.

edit: oh and regarding fan noise, the PMS7003 has a tiny centrifugal fan in it but I can barely hear it with my ear on top of it. The winix air filter on full blast though... crazy loud compared to the other low/med/high settings it's kind of hilarious how big of a jump it is in fan speed

This Old House made a great video on the subject of using box fans with filters:

https://youtu.be/aw7fUMhNov8

I've been using the design they showed in the video, but I will definitely try this duct-fan method, saving electricity is always a bonus.

Great site! I love the "What happens if you drink acetone?" article: https://dynomight.net/2020/09/14/what-happens-if-you-drink-a...

I accidentally drank acetone once. It tasted like icy death. I learned after that hastily, worried i was going to die, you can consume up to a pop can's worth and not suffer ill effects as your body can actually break it down.

It's not a very pleasant experience though.

It's difficult to imagine how one could accidentally consume more than a swallow.

You think you are shotgunning a beer, but it's an acetone. Happens all the time.

Personal Note: Stop storing acetone in beer steins on the dining room table.

So my problem with acetone is how quickly it evaporates. I figured that cooling it down would slow the rate, so I kept my beer steins of acetone in the refrigerator.

Still didn't prevent accidental chugging. Any other suggestions?

I suppose you know what you are doing, but high risk of explosion/fire with the fumes, that is why the original container usually have a correct seal, the beer bottles don’t. Twice treacherous:

- For most gases, the stoechiometric proportion is very low (6-9% for methane — won’t ignite well above that) so it’s not like you have to keep a large quantity to be at risk,

- Artist houses and painting schools often burn because rags are kept in a corner and self-ignite.

Maybe not valid for you, maybe I’ve just heard horror stories ;)

The other classic "rags in a corner" ignition source is that drying oils, ie vegetable oils like you'd use for paint or wood finish, generate heat as they "dry" (actually polymerize).

In my case...it was a gatorade bottle full of acetone my coworker placed next to my gatorade bottle full of water...but yeah pretty much.

I guess the lesson is, gatorade bottles make for poor acetone storage.

I've had a family member take a swig of gasoline from an old beer bottle while working on a car.

The obvious fix here is not to store your molotov cocktails next to your regular cocktails.

Kind of like the etiquette for small arms when cleaning them at the dinner table.

Hmm might have worded that poorly. Was just a swallow, after in my readings i learned up to a soda can will not affect you too badly.

Would the experience be similar to a hangover? Or is my understanding biology that bad?

I would have liked a little more detail in the fan/filter trade off section.

It's not entirely true that you need a loud fan to move more air through a filter. That looks like a duct booster fan in the picture. Those have very weak (and usually inefficient) motors and impeller design. You could use a PSC motor running a squirrel cage fan. PSC motors are efficient and can handle higher torque. The torque can be used with an appropriate fan/impeller design (like a squirrel cage) to handle the pressure drop through the filters.

That said, the duct boosters are cheap and readily available. Here's a homemade hybrid flow/glove box for inoculating mushroom substrate.

https://postimg.cc/NKrGGRFH

The article does address the type of fan and tradeoffs:

  > An 8inch (20.32cm) diameter inline duct booster fan ($30).
  > Inline duct booster fans are fairly weak fans often
  > used to help with grow rooms or range hoods. An inline
  >  duct fan would probably perform even better (see the
  >  discussion) but at a higher cost. The one I bought 
  > is rated to push around 12 m³ of air per minute.

Yeah, but they didn't really go into fan efficiency with pressure drops or motor efficiency.

One other thing is longevity and even safety. Maybe their use of multiple filters reduces the pressure enough. But if it doesn't, then the motor will wear out pretty quickly and could potentially start a fire if it's generating enough heat. The motor in my setup gets pretty hot when running at full power.

The barrel-shaped channel can be completely within the cube. The air will just go around.

The fan should be chosen for static pressure. Typically static pressure fans feature less angle of attack and more blades.

Sealing is very important. Make sure to use tape anywhere between the filters and make sure that the fan fits well its barrel.

What kind of fans are you recommending given the ~30$ constraint? I'm not exactly sure what static-pressure fans are, care to explain.

Static pressure in fluid dynamics is just pressure, so the parent poster is suggesting a fan that can produce more pressure (in other words, uses more force to move air).

For example, if you're just setting up a simple "fan blows air through filter" setup, more static pressure is needed to force more air through the filter; a fan that can move 30 units of air per minute might not be able to move 30 units of air per minute when an obstruction, like a fine-particulate filter, is placed in front of it.

If you look at the fan used in the article, you can see that the fan blades don't reach all the way to the outside of the cowling, meaning that some air could actually make its way through that gap, and that the blades are large but sparse, meaning there's more room for unwanted airflow; in other words, the air flow you're getting might not all be coming through the filters, but rather flowing around the fan and getting blown back out.

The article is recommending a larger fan to move more air. However, the opposite may be true, as smaller fans tend to have a higher static pressure.

Given that the airflow is only 1/4 of the rated airflow for the fan, it seems that the resistance of the filter is causing a pressure drop across the fan that is fairly close to its static pressure. Adding another fan in parallel to try to move more air likely won't make an iota of difference, as the pressure drop will be about the same. However, giving it a fan with a higher static pressure likely will increase the airflow, even if the rated airflow is lower. Or, add a second fan in series with the existing fan (with appropriate distance between the two to smooth out the turbulence), which could double the static pressure and therefore the airflow. Yes, adding more filters would also help.

There's a reason that most fans in air filters are centrifugal (radial) fans rather than axial fans - they have a higher static pressure.

Smaller fan will require much more RPM for the same airflow and pressure and this will cause more (and more annoying) noise.

What you want is a fan that is designed for particular circumstance.

https://images.app.goo.gl/ULuRqRvEA6m3qYnQ8

Since HEPA filter is significant obstruction you want a fan that is designed for static pressure (which is ability to push air against a force/obstruction).

Static pressure is not about fluid dynamics. The name comes from how it is measured. You block the outflow of the fan (for example with a blocked pipe) and measure the pressure difference between inside and outside of the pipe, while the air is static (ie. it does not effectively flow through the fan).

The pressure difference caused by the fan will be different from the same fan but running in free air (not obstructed).

The "I built a better DIY air purifier" trend really reminds me of swamp coolers for Burning Man, except it doesn't yet have its FIGJAM.

Both have people doing interesting experiments, suggesting alternatives, pointing out inefficiencies, etc.

Most important thing is to buy these HEPA filters now. They will all sell out next fire season.

they should be sold out now. They reduce covid in the room. Granted they probably reduce your chances of getting covid by only 5% compared to not having them, but unless you are one of the lucky ones who got a vaccine a few weeks ago that is one of the few things you can do.

Fans are rated by CFM (cubic feet/min) but one interesting aspect for builds like this is static pressure because of the need to push air through filters. I'm not sure you would be able to find a static pressure number for commercial fans.

To be precise, you need the intersection between the pressure-vs-flow graphs of the filter and the fan.

Finding those for fans is _somewhat_ doable if you're buying fans from a components store like Mouser, but that doesn't help that much if you don't also have at least some values for the filter.

Actually, for fans, you'd ideally want a 3d graph of pressure-vs-flow-vs-noise.

Use 4" thick furnace filters. They cost less than specialized air purifier filters and flow a lot more air.

They don't filter the same things, which may or may not be an issue for you.

Do they not? They have the same ratings scale, based on filtering certain particulate sizes.

That's BS. Look at the MERV rating. MERV 14 and above filters out increasingly large quantities of particles at the sizes of COVID virus particles and pollen. 4 inch thick furnace filters can easily be rated above MERV 16 (I run a MERV 16 filter in my home). For most peoples purposes, they are almost as good (and far cheaper) than HEPA.

> For most peoples purposes, they are almost as good

That's what "may or may not be an issue for you" meant.

Bit off topic, but seeing a lot of talk about air purifiers here recently. I've never even considered purchasing one, but now my interest is a little piqued. I've never really felt my air to be unclean, and I don't really have allergies. Still worth looking into? Anyone w experience tell me any benefits? cheers

Just because you don't have allergies now it doesn't mean you'll never have one. Until a few years ago, I've never heard of ragweed alleries. Guess who's allergic to it now?!

I bought one last summer when the air quality outside was terrible (Pacific northwest, fire season yay). It was actually necessary, the air quality in my home had gotten really bad, like to 200 AQI. I don't have asthma, allergies, etc. but my esophagus was irritated and I felt generally awful.

I got the Coway purifier that's currently the Wirecutter pick. It was about $150, has a HEPA filter and an ionizer. The replacement filters are pretty cheap. I leave it on 'auto' in my bedroom 24/7.

Am I getting great benefit from it right now, when the AQI outside is 10? Probably not. Opening your windows for 5 minutes every morning is totally sufficient to keep your indoor air in the green. But it will be great to have around when the inevitable fires start later this summer. If you're in a similar region, pick one up while they're cheap and in stock!

Look up the air quality in your area. If it's bad, an air purifier is probably a good idea for your long-term lung health.

If you leave your windows open for longer periods of time, then an air purifier might be ineffective.

We live in British Columbia, and now that we have "wildfire season" every year, it's very easy to tell the difference between good air and bad air. I suspect a lot of people are jumping on this as well for the same reason - Silicon Valley is in California, and they're harder hit than anyone else with this.

I bought one cause my wife had bad allergies in our bedroom even after cleaning, most likely because of our dog.

I get itchy eyes with allergies, and I don’t think the purifier helped at all. Just my anecdote. An allergy pill helped me more haha.

Looks like the same concept as the BlueAir 211+ but way cheaper! Unfortunately, for me, it needs to look somewhat nice.

Seems like that issue would be fairly simple to solve.

I think a good solution would be some kind of box that you could just put over the whole thing while maintaining airflow. Ideally you'd also be able to sit on it or use it as a kind of side table.

It wouldn't be too hard to build something, e.g. out of wood, to do the job, but that would surely increase the total build time and tools needed by at least 10x.

There's got to be some cheap and widely available product out there that you can just buy and use. (Something from IKEA?) I looked around but haven't found any good options so far.

Must be a good design - I have that one because it has proven to be very effective in various tests. I think the DIY one could copy another aspect of it - the washable, "coarse" fabric filter you put around the entire device that catches bigger dust particles.

This is super well written and explained and shows the research and methods used.

A couple of days ago, I had deliveries of 3 box fans and a case of 6 MERV 13 filters. I just put them away in the attic. It'd be nice not to use them but here in Portland we've had bad wild fires nearby in 2018 and 2020, bad enough that people I know were required to evacuate.

Total cost was about $18 * 3 (from Wal-Mart, free delivery) and $42 (from Amazon, hopefully not counterfeit, free delivery). So less than $100 total. I hope not to use them at all but to get 1-2 weeks per filter.

In 2020, we had one box fan and I went to Home Depot, Target, Lowe's, etc. and the furnace filter shelves were absolutely empty. So I resolved to buy them at a time when they're not in peak demand.

If you want good filters at a fair price, and don’t want to risk counterfeits open Amazon, I’ve had good luck ordering directly from Nordic Pure. Not a shill, just someone who is leery of ordering anything that’s high margin and easy to counterfeit on Amazon.

All articles I've read on DIY box fan purifiers suggest sealing the filter to the fan with tape; the box fan filter they used for this definitely does not do so.

There's experiments on this in a different post!

https://dynomight.net/2020/12/15/some-real-data-on-a-DIY-box...

The one error in that post is he worries about wearing out the fan. If air flow is restricted the fan is doing less work, and will last longer while consuming less electricity. An easy and fun experiment.

But it may overheat due to a lack of cooling airflow.

It isn't doing less work, the same amount of power is going to it but it won't be able to spin as fast.

It is doing less work. There is less air moving, and it is in a partial vacuum/low pressure environment - as physics dictate less work is being done. Simply connect an electricity monitor and you will see it.

If you're going to build something like this, look to see how much cartridge filters cost locally. Nice big cylinders with a great many pleats. It'll simplify construction, may improve airflow (depending on the model, pleats, etc, and will make it look more like a missile launcher than an IED.

Availability is king. People buy one purifier, getting filters becomes harder and harder, they give up and buy another.

If you can find a filter that has a particular use, like a furnace or dust collection system, then you will be set for a long time.

Exactly. We don't really gave furnace filters in the UK. I'm sure they exist somewhere but they're not omnipresent like they are in the US.

Automotive and industrial air filtration is what's easiest for me to get my paws on.

Are there any of these DIY filters that can handle VOCs? I'm renting a place where the land lord used some really fumie paint on the floors just before I got the keys, and it's was basically unusable for a month. Sansiberia isn't doing the job.

Potentially, sure. You'd need a filter with an activated charcoal/carbon layer.

I see a lot of DIY air purifiers — you don’t see many household appliances with a big DIY scene since the Walmart price/quality point is hard to beat. Any reason why the market hasn’t filled this niche yet? What is the advantage of DIY here?

Air purifier manufacturers use odd shaped filters to force lock-in. Those proprietary filters are also expensive. And of course they're constantly going out of business, since it's consumer electronics, so there's no guarantee at all you can even buy that filter a year from now. (And it's not like the purifier itself is cheap either! The Blue Air purifiers I see people on here talk about start at two hundred bucks. Ten times more than a box fan.)

Furnace filters, on the other hand, are standardized and have many vendors. I wouldn't be surprised to see 20x20 filters still being made a century from now.

So why doesn’t someone sell air purifiers that take furnace filters?

That could work as some kind of open source hardware project, like OP, but I don't know if you'd make a lot of money with something whose competition is a box fan and some duct tape.

But surely the Shenzhen guys selling $20 box fans at Walmart could get $25 for them if they had 25 cent clips for filters?

Regarding power consumption, the calculation seems to assume you run the thing constantly. Is it a mistake to run these things constantly? Presumably if it does that good a job pulling out incense, and if you're not constantly smoking or burning stuff, and you're in a reasonably sealed building (and if those assumptions don't hold, why would a filter help?), you should be able to get most of the benefit from only turning it on when something has added a bunch of particulate matter (like, you're cooking or cleaning, or you opened the window and there's pollen out)?

"Advice" section has an unfinished sentence ('you')

One thing that people never talk about the box fan air purifier design is how it pushes air through the filter, rather than pulling it. If you look at any commercial air purifier, you'll see that they pull the air through.

Pushing the air through the filter has the major downside that some of that air will bounce back out, and may help disperse the dust and bad particles that have been accumulating on the back side of the filter.

put the filter on the other side of the fan?

Most designs I’ve seen have it pulling. Pressure attaches it, rather than tape.

Yeah, you can just as easily make it a puller design. I made mine not long ago, simply taped a MERV13 4" filter to the back of a 20x20 box fan. Works like a charm, no dust getting pushed off the filter.

IQAir GC MultiGas models do both. There's the 'prefilter' on the bottom (inlet), a big fan in the middle, and a bunch of gas filter canisters on top.

The internal components also don't get dusty if it's sucking instead of blowing air through the filter

I bought sqair from https://smartairfilters.com/en/ and it's same principle. One of them keeps my 700sq ft village flat clean. First year my daughter doesn't need doses of siflex at night.

Can with a HEPA filter is good either as proven by an article shared on HN about 6 months back.

Despite the low cost construction, it actually looks decent. Swap in some black bungee cords and it would look even better.

That's brilliant. I'd like to make a mini-box variant sometime using a 1x3 grid of 80mm 120mm fans.

Just one note regarding the comparison chart with the crude strap on boxfan filter. It would be nice to see one baseline chart showing the PM 2.5 curve with no fan running (just watching the PM 2.5 decrease by diffusion and settling).

The same filters were used in the second trial. If they perform worse over time, that seems like an unfair comparison.

Typically they filter better over time, but at the expense of less airflow. Thus it is really hard to draw any conclusions.

Upon searching for videos, there is a similar design "Comparetto":

https://www.youtube.com/results?search_query=cuboid+air+puri...

Looks interesting, but where do I find the 5cm thick rigid hepa filters? The ones I see aren't strong enough to hold that shape.

Also, any good way to protect the fan blades from kids/pets or drunk roommates?

That's pretty awesome, really great improvements. I am curious if it could be improved further by using a radial fan, in either price, efficiency, or both.

Can any of these DIY filters eliminate the odours from cooking fish ? I love fish but I cant stand the smell of it cooking -- it literally gets into everything.

Pretty much your only and best option for odors is going to be active charcoal filters. Which you can by and attach under the filters in any regular air filter or diy filter like this one. I have one behind the filter in my hvac air intake.

From testing I have seen, baking soda does nothing.

A bit off topic, but have you tried cooking fish using sous-vide method?

Why is the comparison 3 small filters on a box fan? One large one fits perfectly over the box fan.

I found this strange, too. They don't actually cover the fan as well as the intake to the cuboid. I think comparing to the single large filter would be more honest.

The fan will burn out if you use a single filter. See my other comment in the thread.

Can't argue with that, I've got two burnt out box fans ;)

I saw that pic and immediately wondered why they didn't seal off the areas of the fan that the filters didn't cover. I'm willing to bet that had a negative effect on the box fan setup's performance.

I'll take that bet! https://dynomight.net/2020/12/15/some-real-data-on-a-DIY-box...

I couldn’t find a way to contact you—could you specify the filters you used for your cuboid design?

If a single filter is used, the fan might fail. Using additional filters decreases the load on the fan, because there are more ways for the air to come in, so the torque required for a certain CFM value (which stays fixed for a certain fan speed) is lower.

This might seem like a silly question, but why point the fan out of the box, as opposed to in?

"Resembles IED", a most interesting metric

if the fan is kept inside with it pulling and pushing through filters, the noise levels could be considerably lower.

A 3 filter Blueair 680i delivers 4x as much volume (400 cfm) of clean air (PM1, PM2.5, PM10) with an output 0 ppm (undetectably low). I can get my place (500 m^3) down to 1 ppm in 4-5 hours. No pollen, no dust, just damn clean air. Lets suppose 10 of these contraptions could accomplish the same thing owing to their less output and less efficiency: this would use several times more energy, be louder, be more maintenance, look awful, take up more outlets and space, and cost more.

I use Plantower PMS5003's, the same ones used in PurpleAir sensors.

This sew shop in L.A. https://suayla.com/pages/our-mission was claiming you could use a double layer of common blue shop towels as an N95 filter. I can't find that information on their site anymore.

If it is N95 effective, to my understanding it's pretty equivalent to HEPA air filter standard. It's on my to do list to make an indoor filter this way. If nothing else I hope to reduce the volume of indoor dust that settles in my office. Cheaper than HEPA filters.

It was probably removed for being inaccurate. If using blue shop towels worked as well, then manufacturing N95s would be a lot simpler.

Shop towels are not going to offer anywhere near the filtering performance of melt-blown polypropylene for N95 masks.

It takes a lot of specialized aerosol science equipment and expertise to measure the percentage filtration, so it's questionable that they could claim to replace N95 filter with towels.

HEPA is more like N100 than N95.

HEPA is 99.95% of particles > 0.3 micrometers

N95 is 95% of particles.

They aren't equivalent.

> I wanted a new design that preserves the best aspects of using a box-fan: ... No proprietary parts.

Does HEPA filter qualifies as a proprietary part? Can it be created at home?

It's a commodity part that you can pick up at many hardware stores, and if the filters you get are different size from the article, it's easy to modify the design to match. Contrast to, say, a Dyson air purifier: you need to buy Dyson-brand filters (or a knock-off).

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