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A small DIY air purifier

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Re: A small DIY air purifier

#161

Post by spookthesunset »

dredmorbius wrote: post #152
spookthesunset wrote: post #122

In addition to the old "strap an HVAC sized air filter on the front of a box fan" tip, I'd add that it is important to maintain a positive air pressure in your living space. The only path air should enter your dwelling from the outside is via your air filter. You can do this by putting the box fan in a window and sealing up any gaps with something like cardboard or thick plastic sheeting. On the outdoor end of the fa…

Bad advice. A decent filter's throughput is going to be multiples of your structure's air-exchange rate. What you want to do is to filter the interior air , by passing it, multiple times, through the filter. If you put the fan and filter in a window, you're hugely multiplying the air-exchange rate (to no net benefit), but are filtering that air only once. The net result is a much higher indoor particulates level. So…

Interesting perspective. Wouldn’t it be best to do some combo of both? A low volume fan/filter that maintains positive pressure and an internal filter/fan combo?

I mean in labs and manufacturing facilities that need ultra pure air, I’m pretty sure they maintain a positive pressure inside the sensitive areas. Otherwise you’ll be sucking in bad crap from every nook and cranny...

Re: A small DIY air purifier

#162

Post by littlejohnny »

dredmorbius wrote: post #156
littlejohnny wrote: post #45

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. I believe such air pu…

See: https://venta-usa.com/product-category/airwashers/ They do an excellent job of humidifying the treated space, which may or may not be ideal.

dredmorbius, do you own one? At what frequency do you normally change the water?

Re: A small DIY air purifier

#163

Post by sliken »

dredmorbius wrote: post #149
logfromblammo wrote: post #106

Earlier quoted context omitted.

Or ditch the cardboard, and use 5 square HVAC filters to make a true "box fan".

The HEPA filters are the expensive component. Adding more filters doesn't improve net effectiveness. Cardboard is the vastly more cost-effective option. And taping a HEPA filter directly to the back of the fan effective and proven.

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.

Re: A small DIY air purifier

#164

Post by dredmorbius »

littlejohnny wrote: post #162
dredmorbius wrote: post #156

Earlier quoted context omitted.

See: https://venta-usa.com/product-category/airwashers/ They do an excellent job of humidifying the treated space, which may or may not be ideal.

dredmorbius, do you own one? At what frequency do you normally change the water?

No, I don't. Ran across them discussing filtering options recently.

Re: A small DIY air purifier

#165

Post by dredmorbius »

spookthesunset wrote: post #161
dredmorbius wrote: post #152

Earlier quoted context omitted.

Bad advice. A decent filter's throughput is going to be multiples of your structure's air-exchange rate. What you want to do is to filter the interior air , by passing it, multiple times, through the filter. If you put the fan and filter in a window, you're hugely multiplying the air-exchange rate (to no net benefit), but are filtering that air only once. The net result is a much higher indoor particulates level. So…

Interesting perspective. Wouldn’t it be best to do some combo of both? A low volume fan/filter that maintains positive pressure and an internal filter/fan combo? I mean in labs and manufacturing facilities that need ultra pure air, I’m pretty sure they maintain a positive pressure inside the sensitive areas. Otherwise you’ll be sucking in bad crap from every nook and cranny...

Various lab setups have different requirements.

For biocontainment you generally want negative pressure, such that all exhaust is filtered.

For clean rooms, positive pressure, to ensure filtration and minimum particle reduction levels within the conditioned space. These also have staged zones, much like a cascade refrigerator, with finer levels of purity as you approach the core. Humans themselves are excluded from the higher-purity zones (we're leaky bags of flakey skin, hair, dander, sweat, and mucus). Most especially in chip-fab, but also I suspect various nanofab facilities.

All of which is pretty much entirely outside my paygrade, so I can't really comment intelligently.

Re: A small DIY air purifier

#166

Post by dredmorbius »

sliken wrote: post #163
dredmorbius wrote: post #149

Earlier quoted context omitted.

The HEPA filters are the expensive component. Adding more filters doesn't improve net effectiveness. Cardboard is the vastly more cost-effective option. And taping a HEPA filter directly to the back of the fan effective and proven.

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.

Re: A small DIY air purifier

#167

Post by verdverm »

dredmorbius wrote: post #157
verdverm wrote: post #91

I use a box fan with filters taped to each side. You want to do both sides or the air gets pushed/pulled around the edges of the blades, rather than going through the filters.

That's doable, though you're increasing initial cost by about 1/3, and doubling consumables, by using two filters. The net effective increased filtration rate is marginal. If you can afford this without concern, it's fine, but it's not necessary.

It works with two and not with one, so pretty much required. The effect is far from marginal in my setup. Also far cheaper then the post link, less than half.

Re: A small DIY air purifier

#168

Post by dredmorbius »

verdverm wrote: post #167
dredmorbius wrote: post #157

Earlier quoted context omitted.

That's doable, though you're increasing initial cost by about 1/3, and doubling consumables, by using two filters. The net effective increased filtration rate is marginal. If you can afford this without concern, it's fine, but it's not necessary.

It works with two and not with one, so pretty much required. The effect is far from marginal in my setup. Also far cheaper then the post link, less than half.

Numerous documented and tested builds with only one.

https://invidio.us/watch?v=kH5APw_SLUU

YMMV.

Re: A small DIY air purifier

#170

Post by avian »

labawi wrote: post #146
avian wrote: post #32

Earlier quoted context omitted.

> it provides a static pressure of 7.63 mmH2O 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.

IMO, absolute pressure doesn't give much of a perspective. 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…

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.
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