I'm having a hard time understanding the math behind DIY air purifiers. I bought a 592mm x 592mm F7 filter (pic related, similar to MERV 13) and put it over an 80cm long box with an axial fan pulling the air through it. Producer states it does >5200m3/h + max. 125Pawhile the filter itself has an "initial pressure drop" of>110PaIn general, the thing sort-of works. I can't keep it running indefinitely because it generates 62dB and the fan gets quite hot after an hour. It pulls dust particles of any size like 30cm away from the filter but barely attracts the visible shit beyond this range.What can I do to improve it? I was thinking F7 might generate too much resistance and should probably not use anything beyond coarse G4 filters with an axial fan but I'm still not sure how to max out the range. Is it high static pressure or large volume of air per hour that I should be looking for?
Here's the fan curve. If I'm reading this right, it should pull around 3000m3/h at 110Pa but it certainly doesn't feel like it does...
I'm also starting to think the shape of the filter has something to do with the "effective range". Maybe if I bought pic related it would get better? I noticed that air doesn't travel very fast if I put my hand right in front of that pocket filter but it accelerates quite a but inside these pockets (which are covered from all sides by wooden panels)
>>2828093Any reason you don't just make a bunch of Corsi-Rosenthal boxes? You're going to have a hard time creating something based on theory. You're creating something based on experience (guess & check) or creating/using something already developed/built.
>>2828338If I use one filter instead of four (assuming the same pressure drop for all filters), what I imagine will happen is that the volume of exhausted air stays the same but the velocity at which it enters the filter is much higher
>>2828486>volume stays the sameYou said the fan gets hot, it may not be pulling air as well when it’s going through 4 filters and volume may drop. How oversized is the fan for this application?
>>2828490My current setup is one 592x592x600mm pocket filter (top) and one 450mm fan (bottom) with sides made out of plywood. I presume it gets hot because the fan's maximum static pressure is 125Pa while the initial resistance of the filter is supposedly 110Pa, so to my knowledge it's almost as if it was trying to pull the air out of an airtight enclosure.
>>2828490>>2828497To clarify, I mentioned 4 filters referring to the Corsi-Rosenthal box idea, AFAIK those have 1 fan, 4 filters and the bottom covered by anything (since it won't be pulling air through the floor anyway).
>>2828497If I’m picturing the setup correctly I’d imagine volume would decrease quite a bit then through 4 filters. I’d say you were correct in the OP and you’ll have to get a bigger fan or less dense filters. Overly thick filters is so common, I feel like the manufacturers have to be aware, but they sell better since the average person doesn’t go nearly as far as you and just thinks “thicker filter = better”. Can’t tell you how many AC systems I’ve seen get messed up bc someone used the logic “thicker filter must collect more, so thicker is always better”. Anyway, I’m no purifier engineer, but I know they measure filtering capacity in “air changes” per unit of time. Essentially, how many times it can cycle through the air in the room. So the volume is the probably the most important metric by far, since we’re making an air filter and not a vacuum cleaner. Assuming the air in the room isn’t completely stagnant at least… which would be pretty hard to do since we have a fan running, and the heat/cool cycle… but idk your setup, might be something to think about. Also, filter density is typically chosen based on what specifically you’re trying to filter out. The best dust filter will have holes juuuust large enough to catch the dust, while allowing as much airflow as possible so it can get through the max amount of air and therefore catch the most dust. So yea, I would focus on getting the most amount of air through the filter, and focusing less on how much suction is happening, obviously they’re correlated tho, so it’s just an optimization problem. The best solution being a purifier in a room with very good airflow, that cycles as much air as possible through the filter, with your filter sized to allow as much as possible through it… assuming it’s catching whatever particulate you’re worried about catching.
>>2828093>What can I do to improve it?Assuming the filter itself already catches particles as small as you want, then more airflow. It's all you can ever do with a filter.>the shape of the filter has something to do with the "effective range". Maybe if I bought pic related it would get better?You can't get more "range" out of a filter intake. Not practically, anyway. The physics of suck do not allow for it, requiring orders of magnitude more airflow for noticeable increases. If you want to filter an entire space effectively, you need to ensure the entire air volume is adequately circulated. In your case, that can probably be done with the exhaust of your filter box. Just point it somewhere around the middle-ish of the room to churn up air.>>2828497>the fan's maximum static pressure is 125Pa while the initial resistance of the filter is supposedly 110PaThat is not how that works. The filter was specified in the OP for "5200m3/h + max. 125Pa", meaning that, at a pressure of 125Pa across it, the filter will allow at LEAST 5200m^3/h through it. The fan doesn't even get up to flow rates that high, so the actual pressure drop seen will be significantly less than 125Pa. Hard to say without a matching curve for the filter, but, at a wild guess, it's probably sitting around the 4000m^3/h range.That's easily 10+ full exchanges every hour for even a large room. You don't really need to change anything here. What do you mean the fan "gets quite hot"? Constant-duty motors can get too hot to touch in normal operation. 70°C wouldn't be atypical, though a little odd for a motor cooled by its own large fan.
>>2828510>assuming it’s catching whatever particulate you’re worried about catchingI might have to do some research but right now I'm mostly worried about hair, visible dust (the one that glimmers under sunlight) and pollen. Depending on the source, I'm either looking for G4 or beyond F7, which is rather frustrating. Considering I already have the F7 one, I might just go for G4, see what happens.>>2828551>The physics of suck do not allow for it, requiring orders of magnitude more airflow for noticeable increasesDamn, I was worried this might be the answer. But it kind of makes sense, industrial dust collectors seem to be very close to the equipment generating dust.>Just point it somewhere around the middle-ish of the room to churn up airI pointed it directly towards the floor, which probably wasn't the smartest move. The air gets pulled through the filter from the top and the fan has like 20cm clearance. Doesn't seem to matter when it comes to airflow but it results in a lot of dust bundling together and gathering in small piles in every corner of my room lol>That is not how that works. The filter was specified in the OP for "5200m3/h + max. 125Pa",I should have phrased that differently. The fan curve is in this post >>2828094, so these are the official numbers but I might be reading them wrong.>What do you mean the fan "gets quite hot"?It gets too hot to touch after an hour and the max. operating temperature listed by the manufacturer is 60°C (or 40°C with RPM regulation). Aside from the heat issues, it also blows air in a very wide cone for some reason. Like if you crouch in front of it 2m away, it's going to feel like a gentle breeze, barely anything. I'm not sure if it's an indicator of a fan being overloaded but the flow of air is pretty much cylindrical without the filter
>>2828586>the max. operating temperature listed by the manufacturer is 60°C (or 40°C with RPM regulation)That's maximum *air temperature* the fan it specced to work in.>blows air in a very wide cone for some reason.Axial fans with restricted intakes tend to do that. The lower airflow results in the exhaust stream picking up significantly more sideways momentum than forward.Anyway, if you don't actually have specs for that filter, I'd go back to my initial suspicion of an axial fan not being enough. The low maximum pressure capability of axial fans inherently limits them to fairly coarse filters with low pressure drops. Anything else is going to be paired with a centrifugal blower. That's just suspicion, though, and it might be fine as-is. No real way to know unless you measure. An anemometer and manometer is cheap enough/diy-able.
>>2828591>That's maximum *air temperature* the fan it specced to work inOh shit, I'm a retard then. In that case it should be fine.I posted the filter specs earlier, it's supposed to be 110Pa when clean and it *does* work, somewhat. I see quite a bit of dust on the bottom of each pocket and the whole thing looks dirty. I guess I was just disappointed by the fact that it's infinitely better at kicking shit up with the exhaust air than pulling dust towards the filter. Still, having a fan blowing directly at the floor is probably a bad idea as it does a great job distributing floor dust all over my desk and shelves :|
>>2828586If the fan is getting too hot with the size (size as in density) filter you want, you may be able to slow it down as well. If we’re just talking about an indoor area like a room or even an apartment, it probably doesn’t need many air changes per hour to keep things clear, since you’re probably not going to be creating a ton of dust nonstop, or tracking in a ton of pollen all the time. I got a decent air purifier a few months ago, and it’ll run hard sometimes when I have the doors and windows open for an extended period, but after an hour or two it slows way down since it’s already got most everything cleared. Maintenance loads for indoor areas isn’t all that high, you many not need a very large air capacity since it’ll be running nonstop at a single speed.
>>2828604>it's supposed to be 110Pa when cleanAgain, that doesn't MEAN anything on its own. Pressure drop is velocity dependent.There won't be as much drop with lower air flows, and there will be more with higher flow.>it's infinitely better at kicking shit up with the exhaust air than pulling dust towards the filter.That's what you want. That's how it gets things into the filter. It will stabilize after a while. If it doesn't, you have some serious vacuuming/sweeping to do.Really, though, you're getting way too autismal about this without going all-in. Either call it good and let it do its thing, or get some measuring equipment, make some charts, and drag out your iteration process into a 15-minute YouTube video like normal.>>2828737>you may be able to slow it down as wellThat's liable to actually make it hotter. That's why the maximum working temperature spec is noted to be less "with RPM regulation".
Use more filters so the air can flow relatively freely. Use the fan to push air through the filters, not pull. And vacuum your shit before you try and clean your room with an air filter.
