You'll need to fill in the gaps around the fan blades. I did this with pink foam insulation. After that modification,place the filter on the back of the fan. If the filter suctions onto the fan that's a good sign you're pulling air through the filter and not from the front of the fan.
There's going to be some reverse flow around the corners of the exhaust-side, but net net, so long as you've got sufficient flow and a sufficiently-rated filter, you'll be moving air through the filter and trapping particles within it.
A circular exit mask might help slightly, but you're talking a few percent efficiency improvements, not a quantum leap in efficacy.
Simple, cheap, available, and effective are the goals here. Diminishing returns set in early.
I have an "air purifier" from Amazon that's basically just that (but designed for it obviously). I'm not sure that the author is really saving much money by not just buying one.
This is a really cool project but frankly you can do it a lot cheaper. Find the cheapest box fan you can find, and find the cheapest HEPA filter you can, and strap the HEPA filter to the front. To improve air purification even futher, slap an activated carbon filter on the front too, and your air purifier now filters VoCs! For cheap Activated Carbon Filters, look into the kind used for Aquarium filters. They'll work just as well for air as far as I know.
For extra flow, set up a triangle with the fan on one side and 2 of these filters on the intake end as the other two sides, and duct tape some cardboard to seal the top and bottom.
This may or may not work, cheap box fans aren't designed to push/pull air through obstructions (ie: low static pressure) and HEPA filters are giant obstructions. So instead of the air going through the filter it'll go through gaps, around the edges of the blades, etc.
I used a cheap box fan for some DIY ventilation and the loss in airflow is massive after filter and ducting.
You could, but I think the goal would be to make sure the design will work before going through the effort of making it. If you already have all the parts though definitely worth a try.
I have a box fan on low 24x7 with a 12 on the input and a 7 on the output. That's not a HEPA rated filter set, but it's fantastic. Allergy season just went away (while in my room) when I started using it a few years ago. I also tried a 3 on the output, and that worked as well, but the flow was noticeably lower. The fan is cheap, $20. I change the filters every few months, and it always looks like I waited too long.
I used a cheap box fan and a furnace filter held simply by pressure differential.
Flow drops a lot, but it still works. Filter was dirty after some time. It was hilariously undersized for my purpose (serious belt-sander action leveling sub-floors), but air does move.
Would something like this this work sufficiently well to capture airborne fur/dander from a tabby cat? Or will there be a vastly different outcome from an expensive one?
Yes! We have two box fans with HEPA filters in our bedroom to combat the cat fur/dander. It's made a world of difference. You can see it working as the filter captures everything. Have to change them monthly... but so worth it. #YMMV #YouCatsMayVary
Do you strap them to the front or the rear of the fan? Doesn't having them on the front (i.e. where the air blows out) mean that the fan will get dirty since it's sucking the air in?
You may wish to use a cheap thin washable pre-filter either way. It will catch a lot of the hair before it hits the fine filtration, which you can then just wash off the pre-filter, making it easier and cheaper to maintain.
If you want this to actually work, you take a box fan, and 2 filters. You make a triangle and then some cardboard on the top and bottom to fill the gap. This increases surface area and reduces the stress on the fan due to pressure drop. Slapping a filter directly on to the box fan is going to kill the fan very quickly. https://i.imgur.com/SX1RloH.jpg
I ran a few cheap box fans with filters directly over them for several years and none of them failed, I have heard this claim a lot but it seems overstated. The triangle setup will significantly increase the airflow and thus the effectiveness of filtration though.
Doesn't really matter as long as the filters are in the correct orientation. However it would be beneficial if the air is clean before it goes through the fan.
Actually it does. You'll get higher CFM through the system and it will take 5x longer to saturate the filters with dust. The cost per month will actually be similar, granted you'll have a higher startup cost with 5 filters instead of 1.
The time-to-saturate is a wash when you factor in per-filter rates. You're achieving a 4x longer life per filter, but multiplying that by the 4x filters you are using.
The net filtration rate per filter will fall as that's a function of flow rate * filtration size.
Yes, the four- or five-filter version will probably have a slightly better overall filtration rate ... but that's really not the principle constraint here. A single-filter design will drop particle density by ~90% in about 30 minutes. That's sufficient. It's doubtful the final particle density measurement will be much reduced (though if someone's got the data I'd be interested in seeing it), or that this reduction is clinically significant from a health perspective. Remember that you're likely going outside the filtered area, so achieving perfection inside but spending a substantial fraction of your time in unfiltered or far-more-poorly-filtered air, isn't much of a win.
Meantime, you're tying up 3-4 filters that you don't really need, and which others could make effective use of, for a very low marginal imrovement in your own experience.
Where the filters themselves are in low supply -- typical for a region where a wildfire has errupted, and particularly true where filters aren't a high-volume item as with Australia presently -- designs which economise on filter use are a net social benefit.
they probably work around a small area where they are placed, but particulates outside of that will be unaffected and ultimately fall to the ground ready to be disturbed and recirculated as dust. I'm under the impression that for something like this to work, you would need to feel the air current at every point in the room, like a light breeze
I decided to buy one when the smoke from the Sydney bushfires blanketed my town.
In my bedroom I could see and taste the smoke. The air purifier struggled, but it cleared the air around my computer quite well. I have dividing curtains in my room that I was able to close, which made it easier on the purifier.
All in all I think it was definitely worth the buy. FWIW I picked up a secondhand Kogan SmarterHome air purifier for AUD$60.
Yes, they definitely do, although to a various degree of effectiveness. If you get a Xiaomi Mi Air 2, for example, it will be very effective. If you get a Dyson fanless thing, not so much.
In general, there is no escaping physics: you need to move a large quantity of air through something that traps dust particles. The low-tech solution of a fan and a HEPA filter is a very good one, but I would suggest getting a filter that traps PM2.5 particles as well.
Any sources on that? I have one and I swear by it. They have a glass HEPA filter and a carbon filter. I'm not quite sure how much air it moves but it's enough to scrub the air of my two floors apartment when someone smokes indoor. It's not really fanless per-se, it's a turbine in the base. You are simply moving the blades down into the base and away from the output area.
Yes they do. But you need a sufficiently powerful one to be able to circulate the air in the room enough times per hour, more that the circulation that occurs through the doors, windows and even walls.
Eco-rated houses have a circulation of about 2-3 times per hour. Older houses that are well insulated are about 5-6 times per hour. The purifier needs to be powerfull enough to spin the entire air volume couple of times faster than the natural circulation.
If your windows and doors leak too much, or if you open them too much, or if you burn oil / light up cigars too much and produce smoke then it might be pointless.
I live in one of the cities where the pollution is regularly at the top 10 in the world, and the unit I have is able to reduce the pollution inside to a level that is 5-6 times lower than outside if it runs constantly in a mode that is like 30% of the power. It could do better, but it would be too loud for me.
Not sure where you live, but if air in my room circulated once in an hour, with doors/windows closed, I'd consider it way too much. We have winters so people tend to plug holes.
You're not talking about HVAC, are you?
Nevertheless, I do know people living in leaky homes/building, and they do get >1 exchange/hour.
A few years back I had to temporarily move back to my parents while getting a new place. It had been years since I'd used my room, and it had been neglected a bit.
First days I started coughing a lot when staying the room for a bit, and I woke up quite stuffy in the morning. Got me an air purifier with HEPA filter and let it run at max setting while at work. After running it a few days like this, I didn't have any more issues.
I felt similarly, but I have a family member who is bed-bound, there’s some asbestos removal going on, so I decided to take the plunge.
I can vouch for their ability to detect impure air. If I start cooking anything, it switches on within seconds. It’ll turn on within about three minutes of sweeping the floor. And if the detectors can turn on when the air is dirty, it stands to reason that they can tell when the air is clean and they turn off as well.
Shortly after getting this, some intriguing news surfaced regarding the installation of air filters in a California school (due to a gas scare) which resulted in a jump in test scores vs other schools in the same district.
I am not a scientist, but I’m feeling pretty convinced about the efficacy of these things.
(A few minutes after posting this, morning rooftop construction started. Sounds of saws and drills buzzing, large objects and beams being dropped directly overhead. Air purifier just kicked into high gear. Yes, I’m definitely into this thing.)
I’m a big fan of thewirecutter, and I reference them frequently on most things I’m buying. But for HEPA air filters, I don’t feel like they test the models that I feel are best for someone with asthma, or other similar breathing problems.
We have several IQAir units, of at least a couple different models. None of them have sensors to automatically ramp up the filtering when they detect pollution, that would be nice. But according to the air quality sensors I have in the house, they do an amazingly good job of filtering the air, at least if you turn them up to high enough levels.
I have yet to see demonstrated clear benefit of the units with activated charcoal filters for removing VOCs from the air, but my gut feeling is they do help in those areas where VOCs are a concern. Not all parts of our house needs that, however.
Having four furballs does greatly increase the dander and other allergens we have to deal with, but we wouldn’t give them up for anything, so the IQAir units at least make our lives a bit more livable. Especially during “Cedar Fever” season.
If you're referring to "air quality" in the form of particulates… it does turn off after a while. 15-30 minutes after cooking, one or two after sweeping, etc. Presumably it's triggered by no longer detecting particulates in the air, one would have to be fairly conspiracy-minded to suggest that it's working on some kind of timer (and can tell the difference between dust-timeouts and gas range waste timeouts)… I figured this was too obvious to point out, even given the sort of pedantry we get around here.
If you're referring to some quality of the air beyond particulates… well that's not what a HEPA is even for, is it? I could be mistaken.
How does it compare to a control? If the pollution stops (eg you stop cooking) the particulates will settle on their own. You'd expect the count to go back down to normal over time. How much faster does the air purifier cause the particulate count to decrease?
I would suggest worrying much more about PM2.5 and getting a filter that does trap those particles. Many health problems are caused specifically by small particulate matter.
His statement "I'm not concerned with the ability of the device to remove PM2.5" comes across as dismissive - he's not concerned because he's using a HEPA filter health officials are recommending and therefore he trusts it. HEPA starts at 0.3μm ("PM0.3") particle size, which doesn't cover finer forms of elemental carbon (soot). "H13" is a minimum retention efficiency though, not a specific particle size or application.
Air quality test before and after or it doesn't work.
I am very suspicious with regards to air purifiers. Have you ever seen real HEPA filter? It is thick and it is quite difficult to push air through it. I can't imagine small PC fan that is designed to push air through UNOBSTRUCTED space will make any significant flow of air. I have spent some time with the guys who build PCs and who know great deal about fans and one thing I learned even an object that the air has to pass around or any kink in the path of air can severly impact performance of PC fan. Because they are designed to be quiet and still provide good flow of air they produce almost no pressure differential to speak of and the flow can be halved by a wire mesh that is a bit too dense.
You need to cycle the air in your room through the filter couple of times to clean it, assuming your filter is any good.
The cheap fan in the picture would barely be able to cycle the air in a small room maybe once an hour, if it was completely unobstructed. It is very likely that with any kind of HEPA filter the flow will drop by an order of magnitude.
Go with suggestion of other readers. If you want good, cheap filter, use a box fan and especially one that was designed to give good pressure difference. Take large cardboard or plastic box and mount large HEPA filter in it so that the air has to pass through. The larger the surface of the filter the easier time the fan will have to push the air through it and more efficient it will be.
>The larger the surface of the filter the easier time the fan will have to push the air through it and more efficient it will be.
This is quite true. Assuming the media in your particle filter is of sufficient quality, the most important metric to look at is the aggregate surface area. The larger the surface area, the more air you will be able to move through it and the more particulate matter it will hold before requiring replacement.
Particle filters use pleated media for this reason. It's not uncommon to see a filter with 1 square foot face having 50 square feet or more of media. Without specs, the thickness of the filter and the density of the pleating will give you a decent estimate of the filter area. To improve performance and longevity further, it's also typical to have a coarser, pre-filter.
The machine itself is just a fan to push air through the filter and as noted, any reasonably powerful fan will do a sufficiently good job. High quality machines are only really required where you're filtering air before it enters a given area (e.g. LCD/semiconductor plants), and thus cannot afford to have any unfiltered air leak past the filter.
The quality of the filter is important if you want to remove very fine particles.
Even good filters do not typically remove all particles, only particles above certain size. If you have cheap filter it may not even capture a portion of the particles you are interested to remove, it may remove none no matter how many times you cycle the air.
> Because they are designed to be quiet and still provide good flow of air they produce almost no pressure differential to speak of and the flow can be halved by a wire mesh that is a bit too dense
There are high static pressure fans which address this problem, specifically designed to work with pressure differential, usually used to blow air through radiators for water cooling. The fan linked in the github page (Noctua NF-F12, the "cheap fan" in the picture) is such a fan, it provides a static pressure of 7.63 mmH2O, which is very high.
Edit: you can get regular-sized 12cm pc fans that need 48 watts and rotate at 7160rpm, move 7.160 m^3/min air at 35.877 mmH2O static pressure:
No need for a time machine (or being snarky - I did say it's just for comparison, didn't I? I think it does put the "very high pressure" into perspective)
That said, your time machine idea could be simply implemented with a large, airtight box. Simply put the filter over it and weather changes will push and pull air through the filter with two orders of magnitude more pressure than this little fan. Volume of filtered air will depend on the volume of the box though, so better get a large one.
That balloon figure doesn’t make sense to me in relation to atmospheric pressure changes. It would mean that a tied-off balloon would randomly inflate and deflate itself depending on weather.
It does, slightly. Pressure change 20 mBar = 2% of absolute atmospheric pressure -> about 2% balloon volume change, with corresponding 2% internal absolute pressure change, while keeping the same low differential inside-outside pressure.
> it provides a static pressure of 7.63 mmH2O, which is very high.
Actually, it depends on the specific HEPA filter if that is enough or not. If it's not enough, the air will find the path of less resistance and circulate in and out thru the fan blades, but not thru the filter.
Even if they don't recirculate, I sort of doubt that these small fans are going to achieve sufficient air flow through the filter (a radiator is much less of an obstruction than a HEPA filter).
+1. Together with a test in the same environment without a purifier at all. PM count drops down to almost 0 in a room with closed windows with a breathing person inside. Lungs of a person act as a perfect purifier.
I mean, of course it's good that PM doesn't just pass straight into our bloodstream, but isn't our lungs doing the purifying exactly the thing that people try to avoid with mechanical air purifiers?
It is bad of course! PM does pass straight into our bloodstream.
I was just talking about the test setup. A test of any purifier should be compared to a test with a person in a room without purifier. Otherwise those results are not significant.
from a physics perspective there is nearly no lower limit on the pressure differential for a given flux (liters per second) of air (ignoring the mixing entropy).
just make the analogy pressure ~ voltage, flux ~ current, filter ~ resistance.
A doubling in flux can be achieved either by doubling the voltage across the resistor ( ~ doubling pressure differential across the filter), OR by halving the resistance of the filter ( ~ placing 2 filters in parallel, without halving the filter thickness) so technically a PC fan could blow at a typical PC fan pressure differential and flow rate purified air across an arbitrary number of layers of HEPA, as long as enough of those [N layers of HEPA] in parallel...
if you don't mind a noise you could buy a 2nd hand vacuum cleaner with a built-in hepa or water filter (ideally also with adjustable speed to make it less noisy) and just let it run without a hose, sucking the air and pushing it through the filter...
That kW will move you more air in 30 minutes than PC ventilator can in a whole day, especially if you're trying to push it through hepa filter which is fairly thick and adds quite a resistance.
Most vacs are adjustable. Also, I think they're trying to maintain a roughly constant speed of the fan, not a constant power. Which would mean that they reach peak power when airflow is obstructed, and idle at lower power when air is passing freely and motor is spinning with little resistance.
Also, ozone purifiers not need to keep working all the time. A usual usage it's have a burst of ozone to sanitize a place, and wait a few minutes to decompose to O2. The result is that any smell is removed, and the ambient is sanitized. I saw this a lot on hotels where they use "ozone cannons" where they are cleaning rooms.
The problem for areas presently affected by high PM2.5 concentrations, most notably Australia, is that materials which are commonplace in the US, box fans, but especially box-fan-sized square HEPA furnace filters, are rare as hen's teeth.
So alternate designs using available materials are required.
(Or someone might drop ship a few containers worth of box fans and HEPA furnace filters down under.)
Same problem here. I've just spent close to an hour trying to locate a box fan for sale in Europe, couldn't find any resembling those I can easily find on amazon.com!
Speaking of air purifiers, does anyone have recommendations for one that is quiet and still effective?
I have a pair of Electrolux EAP300s, in small rooms.
Unfortunately on the lowest manual setting ("Quiet") they hardly do anything for air quality. And they're still louder than I like. I think they really optimized for cost; these devices feel very light and plasticy, the fan sounds cheap and has an uneven tone to it, and its speed keeps oscillating back and forth. The purifier itself is also not stable and it can make an annoying noise if placed on a surface that ends up vibrating along.
I also thought about DIYing one with noctua fans (which I know are quiet, and I have plenty of them in my PCs), but I suspect they're not going to be very effective.
It depends on what noise level is acceptable for you and the room size.
Out of 30 or so air purifiers available in shops here, of all the ones that are in the $100-500 category, the Winix P/U450 are able to push the largest amount of air under the constraint of <40db.
HEPA filters are rated at which air speed they are most effective and how much pressure is needed. The ammount of pressure will only rise as the filter is saturated with dirt. You need a sufficiently powerful fan, and fans that are both powerful enough and quiet enough are very expensive.
I have two Winix HR1000. They have extremely powerful blowers and are quiet when not in "turbo" mode. The turbo mode is actually pretty good for white noise when sleeping. I've been running them 24/7 for years now and they are still working great.
Levoit makes ones that are a lot quieter than the ones you see for around $100, but you will pay a lot more for them. I still wouldn't call them "Quiet" though.
One thing to keep in mind is that an air purifier with a squirrel-cage fan (the kind of fan needed to pull enough air through a HEPA filter) makes a very pleasant sound that is good for masking, e.g., banging noises from neighboring apartments. I regularly run mine just to mask noise. So being loud can be considered a feature.
Pollution sensors for indoor use that can be plug via USB to the mobile phone are really cheap, you can find laser ones like Nova SDS for $17 and some that are not so precise come even cheaper than that.
So unless the author buys a real PM2.5 sensor and does a test in a true room and not a box of the size of a Desktop PC, with the sensor on the opposite side of the room, and proves that the pollution drops to below 8micrograms/m3, in a reasonable time frame, than we can say that it is working.
Until than we can only say that the purifier is only able to purify a desktop PC.
It does not "wash" the air. It has a HEPA-filter on the intake side before the humidifying cycle. So it is basically the same thing as all other examples on this comment page.
You can actually run it without water (which we do), then it does only filtration.
Don't buy this particular product. It is overprized and has dramatic production quality issues (we sent two back, amazon reviews indicate larger issues).
How about a 'filter' that can 'wash' the air with clean water. To me it appears to me the best possible solution to clean air in a confined space. To put it simply, air can be bubbled through the clean water, to scrub it of both particulate and chemical contaminants. No doubt the apparatus will lend itself to several technical challenges like noise and the need for periodic changes of the water.
The main problem of air washers is that it is not enough to change the water.
Fungus and spiders love the wet environment. Algae start growing if there is sunlight using just the dust particles on air as food. Also very bad bacteria like salmonella and others could grow if the water gets warm.
Pretty cool project.... Except for the fact that I cannot buy any of the parts from pccasegear. They deny certain geographical areas access to their website. In fact, this seems to be the new normal. Last week, I was denied access to a recipe from a .au website. Flipping on my VPN restores access..
I don't really think airfilters in that size so much. They need to push a serious amount of air through the filters to be able to accommodate for natural air circulation. Some pc fans don't do the trick I'd suspect
Many (most?) proper ones use a combination of a coarse washable pre-filter, charcoal, HEPA, and ionizer.
Which makes sense, since they all basically do different things.
I don't know how effective they are compared to these devices, but I much prefer to keep plants with air purifying qualities. NASA did a study on a variety of air purifying plants in 1989: https://ntrs.nasa.gov/search.jsp?R=19930073077
You would need to live in an indoor rainforest (approximately 680 plants for a typical house) to see an effect on air quality, a spider plant in the corner isn't doing anything: https://www.youtube.com/watch?v=AmeiXikh0v8
That's an interesting study, but it appears to only study the plants' ability to filter out a few VOCs. HEPA filters won't help with those (you need an activated charcoal filter for that... or plants, I guess?), but they will help with particulates, and I'm not sure plants will do anything for those.
there's still a bunch you can buy. apparently many "toxic" plants are only mildly toxic for cats/dogs.
i don't think they make much difference for air quality, but i bought some "air purifying" plants and cross-checked them against the aspca list for safety:
If you own a Shop-Vac (a wet-dry vacuum, generically), could you just turn it on without the hose? It would draw much more power, but presumably would work much quicker, and so may end up being more efficient.
177 comments
[ 4.3 ms ] story [ 218 ms ] threadFor some strange reasons it immediately reminded me of the scene in Apollo 13:
https://www.imdb.com/title/tt0112384/quotes/qt0476826
A circular exit mask might help slightly, but you're talking a few percent efficiency improvements, not a quantum leap in efficacy.
Simple, cheap, available, and effective are the goals here. Diminishing returns set in early.
Seriously? 600mA? Air purifiers are rated like 30W for a reason.
In China there is a whole company founded on doing just this, although they've since pivoted into selling specific air purification hardware at a significant discount. See: https://smartairfilters.com/cn/en/product/diy-1-1-air-purifi...
https://www.homedepot.com/p/Honeywell-20-in-x-20-in-x-1-in-A...
For extra flow, set up a triangle with the fan on one side and 2 of these filters on the intake end as the other two sides, and duct tape some cardboard to seal the top and bottom.
I used a cheap box fan for some DIY ventilation and the loss in airflow is massive after filter and ducting.
Flow drops a lot, but it still works. Filter was dirty after some time. It was hilariously undersized for my purpose (serious belt-sander action leveling sub-floors), but air does move.
Cardboard is the vastly more cost-effective option. And taping a HEPA filter directly to the back of the fan effective and proven.
The net filtration rate per filter will fall as that's a function of flow rate * filtration size.
Yes, the four- or five-filter version will probably have a slightly better overall filtration rate ... but that's really not the principle constraint here. A single-filter design will drop particle density by ~90% in about 30 minutes. That's sufficient. It's doubtful the final particle density measurement will be much reduced (though if someone's got the data I'd be interested in seeing it), or that this reduction is clinically significant from a health perspective. Remember that you're likely going outside the filtered area, so achieving perfection inside but spending a substantial fraction of your time in unfiltered or far-more-poorly-filtered air, isn't much of a win.
Meantime, you're tying up 3-4 filters that you don't really need, and which others could make effective use of, for a very low marginal imrovement in your own experience.
Where the filters themselves are in low supply -- typical for a region where a wildfire has errupted, and particularly true where filters aren't a high-volume item as with Australia presently -- designs which economise on filter use are a net social benefit.
I went for ventilation systems that take air from outside, filter it and then bring it inside the room. Namely Xiaomi MJXFJ-300-G1: https://www.xiaomitoday.com/xiaomi-mijia-air-purifier-mjxfj-... I've installed one in every bedroom.
My "measurement" was my lungs when working from home: coughing before, no coughing after. Filters turned black.
In my bedroom I could see and taste the smoke. The air purifier struggled, but it cleared the air around my computer quite well. I have dividing curtains in my room that I was able to close, which made it easier on the purifier.
All in all I think it was definitely worth the buy. FWIW I picked up a secondhand Kogan SmarterHome air purifier for AUD$60.
In general, there is no escaping physics: you need to move a large quantity of air through something that traps dust particles. The low-tech solution of a fan and a HEPA filter is a very good one, but I would suggest getting a filter that traps PM2.5 particles as well.
Or read the specs and compare with norm
https://en.wikipedia.org/wiki/HEPA#Specifications
Any sources on that? I have one and I swear by it. They have a glass HEPA filter and a carbon filter. I'm not quite sure how much air it moves but it's enough to scrub the air of my two floors apartment when someone smokes indoor. It's not really fanless per-se, it's a turbine in the base. You are simply moving the blades down into the base and away from the output area.
Eco-rated houses have a circulation of about 2-3 times per hour. Older houses that are well insulated are about 5-6 times per hour. The purifier needs to be powerfull enough to spin the entire air volume couple of times faster than the natural circulation.
If your windows and doors leak too much, or if you open them too much, or if you burn oil / light up cigars too much and produce smoke then it might be pointless.
I live in one of the cities where the pollution is regularly at the top 10 in the world, and the unit I have is able to reduce the pollution inside to a level that is 5-6 times lower than outside if it runs constantly in a mode that is like 30% of the power. It could do better, but it would be too loud for me.
You're not talking about HVAC, are you?
Nevertheless, I do know people living in leaky homes/building, and they do get >1 exchange/hour.
First days I started coughing a lot when staying the room for a bit, and I woke up quite stuffy in the morning. Got me an air purifier with HEPA filter and let it run at max setting while at work. After running it a few days like this, I didn't have any more issues.
I can vouch for their ability to detect impure air. If I start cooking anything, it switches on within seconds. It’ll turn on within about three minutes of sweeping the floor. And if the detectors can turn on when the air is dirty, it stands to reason that they can tell when the air is clean and they turn off as well.
Shortly after getting this, some intriguing news surfaced regarding the installation of air filters in a California school (due to a gas scare) which resulted in a jump in test scores vs other schools in the same district.
I am not a scientist, but I’m feeling pretty convinced about the efficacy of these things.
https://www.vox.com/2020/1/8/21051869/indoor-air-pollution-s... https://news.ycombinator.com/item?id=18219391
https://thewirecutter.com/reviews/best-air-purifier/
We have several IQAir units, of at least a couple different models. None of them have sensors to automatically ramp up the filtering when they detect pollution, that would be nice. But according to the air quality sensors I have in the house, they do an amazingly good job of filtering the air, at least if you turn them up to high enough levels.
I have yet to see demonstrated clear benefit of the units with activated charcoal filters for removing VOCs from the air, but my gut feeling is they do help in those areas where VOCs are a concern. Not all parts of our house needs that, however.
Having four furballs does greatly increase the dander and other allergens we have to deal with, but we wouldn’t give them up for anything, so the IQAir units at least make our lives a bit more livable. Especially during “Cedar Fever” season.
If you're referring to some quality of the air beyond particulates… well that's not what a HEPA is even for, is it? I could be mistaken.
https://invidio.us/watch?v=kH5APw_SLUU
I am very suspicious with regards to air purifiers. Have you ever seen real HEPA filter? It is thick and it is quite difficult to push air through it. I can't imagine small PC fan that is designed to push air through UNOBSTRUCTED space will make any significant flow of air. I have spent some time with the guys who build PCs and who know great deal about fans and one thing I learned even an object that the air has to pass around or any kink in the path of air can severly impact performance of PC fan. Because they are designed to be quiet and still provide good flow of air they produce almost no pressure differential to speak of and the flow can be halved by a wire mesh that is a bit too dense.
You need to cycle the air in your room through the filter couple of times to clean it, assuming your filter is any good.
The cheap fan in the picture would barely be able to cycle the air in a small room maybe once an hour, if it was completely unobstructed. It is very likely that with any kind of HEPA filter the flow will drop by an order of magnitude.
Go with suggestion of other readers. If you want good, cheap filter, use a box fan and especially one that was designed to give good pressure difference. Take large cardboard or plastic box and mount large HEPA filter in it so that the air has to pass through. The larger the surface of the filter the easier time the fan will have to push the air through it and more efficient it will be.
This is quite true. Assuming the media in your particle filter is of sufficient quality, the most important metric to look at is the aggregate surface area. The larger the surface area, the more air you will be able to move through it and the more particulate matter it will hold before requiring replacement.
Particle filters use pleated media for this reason. It's not uncommon to see a filter with 1 square foot face having 50 square feet or more of media. Without specs, the thickness of the filter and the density of the pleating will give you a decent estimate of the filter area. To improve performance and longevity further, it's also typical to have a coarser, pre-filter.
The machine itself is just a fan to push air through the filter and as noted, any reasonably powerful fan will do a sufficiently good job. High quality machines are only really required where you're filtering air before it enters a given area (e.g. LCD/semiconductor plants), and thus cannot afford to have any unfiltered air leak past the filter.
Even good filters do not typically remove all particles, only particles above certain size. If you have cheap filter it may not even capture a portion of the particles you are interested to remove, it may remove none no matter how many times you cycle the air.
There are high static pressure fans which address this problem, specifically designed to work with pressure differential, usually used to blow air through radiators for water cooling. The fan linked in the github page (Noctua NF-F12, the "cheap fan" in the picture) is such a fan, it provides a static pressure of 7.63 mmH2O, which is very high.
Edit: you can get regular-sized 12cm pc fans that need 48 watts and rotate at 7160rpm, move 7.160 m^3/min air at 35.877 mmH2O static pressure:
https://www.frozencpu.com/products/8147/fan-500/Delta_Mega_F...
If anyone else is wondering, that's about 0.8 mbar (roughly 8 ten-thousands of an atmosphere), or 80 Pa of over-pressure.
To put this into perspective, my local atmospheric pressure varied about 2000 Pa in the last 48h due to weather changes.
That said, your time machine idea could be simply implemented with a large, airtight box. Simply put the filter over it and weather changes will push and pull air through the filter with two orders of magnitude more pressure than this little fan. Volume of filtered air will depend on the volume of the box though, so better get a large one.
Car tire pressure is about 1.8-2.5 Bar.
Your 2000 Pa atmospheric pressure change is about 20 mBar.
Pressure in an inflated balloon is about 2 mBar (0.002 Bar).
Noctua's static pressure is 0.8 mBar. Let's call it 0.5 mBar dynamic working pressure.
Not as low as I expected. Not sure if it will have a semi-reasonable speed, but that will definitely push some air.
Another comparison, 0.5 mBar is 50 Pa = 50 N/m². That gives about 50 * 0.3 * 0.3 / 10 ~ 0.5 kg force on a 30x30 cm² surface.
Finally, as the article video and data shows, it does actually work.
[1] http://scipp.ucsc.edu/outreach/balloon/labs/InflationExp.htm
Static pressure 2,61 mm H₂O <-- according to the vendor
https://noctua.at/en/nf-f12-industrialppc-3000-pwm/specifica...
Or if you just follow the link on the linked page:
https://www.pccasegear.com/products/27866
Actually, it depends on the specific HEPA filter if that is enough or not. If it's not enough, the air will find the path of less resistance and circulate in and out thru the fan blades, but not thru the filter.
For some HEPA filters you can find specs. These (https://www.airclean.co.uk/download/4724) are spec to 250 Pa which would be about 25 mm H2O if I am correct.
Is that good?
I mean, of course it's good that PM doesn't just pass straight into our bloodstream, but isn't our lungs doing the purifying exactly the thing that people try to avoid with mechanical air purifiers?
I was just talking about the test setup. A test of any purifier should be compared to a test with a person in a room without purifier. Otherwise those results are not significant.
just make the analogy pressure ~ voltage, flux ~ current, filter ~ resistance.
A doubling in flux can be achieved either by doubling the voltage across the resistor ( ~ doubling pressure differential across the filter), OR by halving the resistance of the filter ( ~ placing 2 filters in parallel, without halving the filter thickness) so technically a PC fan could blow at a typical PC fan pressure differential and flow rate purified air across an arbitrary number of layers of HEPA, as long as enough of those [N layers of HEPA] in parallel...
If not, then cycling the vacuum on and off would actually Dave energy compared to the small fan setup.
Also, ozone air purifier make wonders to keep at bay smells and fats (wonderful to have on the kitchen), and kills bacterias, spores and fungus.
Also, ozone purifiers not need to keep working all the time. A usual usage it's have a burst of ozone to sanitize a place, and wait a few minutes to decompose to O2. The result is that any smell is removed, and the ambient is sanitized. I saw this a lot on hotels where they use "ozone cannons" where they are cleaning rooms.
You should be aiming for something larger that would work for an apartment.
[1] https://youtu.be/kH5APw_SLUU
[2] https://youtu.be/8Hkdpx-59kk
There is thousand vidoes on Youtube on how to DYI one.
You can also make DYI air-quality instrument on a cheap using SDS011 sensor and Raspberry Pi. [2]
[1] https://aqicn.org/sensor/sds011/
[2] https://www.raspberrypi.org/blog/monitor-air-quality-with-a-...
So alternate designs using available materials are required.
(Or someone might drop ship a few containers worth of box fans and HEPA furnace filters down under.)
I have a pair of Electrolux EAP300s, in small rooms.
Unfortunately on the lowest manual setting ("Quiet") they hardly do anything for air quality. And they're still louder than I like. I think they really optimized for cost; these devices feel very light and plasticy, the fan sounds cheap and has an uneven tone to it, and its speed keeps oscillating back and forth. The purifier itself is also not stable and it can make an annoying noise if placed on a surface that ends up vibrating along.
I also thought about DIYing one with noctua fans (which I know are quiet, and I have plenty of them in my PCs), but I suspect they're not going to be very effective.
Out of 30 or so air purifiers available in shops here, of all the ones that are in the $100-500 category, the Winix P/U450 are able to push the largest amount of air under the constraint of <40db.
HEPA filters are rated at which air speed they are most effective and how much pressure is needed. The ammount of pressure will only rise as the filter is saturated with dirt. You need a sufficiently powerful fan, and fans that are both powerful enough and quiet enough are very expensive.
I don't mind expensive, I'm tired of buying poor quality products.
Just did this recently actually here's a pic https://mobile.twitter.com/jonathanfly/status/12155378275654...
So unless the author buys a real PM2.5 sensor and does a test in a true room and not a box of the size of a Desktop PC, with the sensor on the opposite side of the room, and proves that the pollution drops to below 8micrograms/m3, in a reasonable time frame, than we can say that it is working.
Until than we can only say that the purifier is only able to purify a desktop PC.
https://www.engineersedge.com/filtration/hepa_filter_pressur...
Here is one from Phillips: https://www.youtube.com/watch?v=n3xC3qxVphw
It does not "wash" the air. It has a HEPA-filter on the intake side before the humidifying cycle. So it is basically the same thing as all other examples on this comment page.
You can actually run it without water (which we do), then it does only filtration.
Don't buy this particular product. It is overprized and has dramatic production quality issues (we sent two back, amazon reviews indicate larger issues).
I believe such air purifiers already exist.
The main problem of air washers is that it is not enough to change the water.
Fungus and spiders love the wet environment. Algae start growing if there is sunlight using just the dust particles on air as food. Also very bad bacteria like salmonella and others could grow if the water gets warm.
They do an excellent job of humidifying the treated space, which may or may not be ideal.
I don't really think airfilters in that size so much. They need to push a serious amount of air through the filters to be able to accommodate for natural air circulation. Some pc fans don't do the trick I'd suspect
With something as simple and effective as HEPA-filters, it seems unreasonable to try other technologies.
i don't think they make much difference for air quality, but i bought some "air purifying" plants and cross-checked them against the aspca list for safety:
https://www.aspca.org/pet-care/animal-poison-control
(my cat likes to munch on my spider plants, even with cat grass available)
If you own a Shop-Vac (a wet-dry vacuum, generically), could you just turn it on without the hose? It would draw much more power, but presumably would work much quicker, and so may end up being more efficient.
It's just airflow + filter, which is why DIYing them is so popular.