[lmilcin]

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.

[0-_-0]

> 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:

https://www.frozencpu.com/products/8147/fan-500/Delta_Mega_F...

[qpiox]

> 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.

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.

[maxerickson]

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).

[avian]

> 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.

[labawi]

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.

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

[avian]

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.

[labawi]

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.

[wlesieutre]

Lemme go send my air intake through a time machine and as long as the weather keeps changing we can use that to drive air through a filter!

[avian]

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.

[thatguy0900]

If you had a time machine you might as well just skip the filter and pull in air from before the industrial revolution directly

[eeZah7Ux]

https://noctua.at/en/nf-f12-pwm/specification

Static pressure 2,61 mm H₂O <-- according to the vendor

[0-_-0]

I got it from here:

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

[ddeck]

>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.

[lmilcin]

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.

[cosmolev]

+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.

[OJFord]

> Lungs of a person act as a perfect purifier.

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?

[cosmolev]

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.

[lm28469]

The whole point of filtering air for PMs is to avoid having them in your lungs so I'm pretty sure that's not "good".

[OJFord]

Yes, that is what I said.

[DoctorOetker]

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...