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A small DIY air purifier

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Re: A small DIY air purifier

#151

Post by dredmorbius »

chewz wrote: post #23

This seems like useless toy. 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.

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.)

Re: A small DIY air purifier

#152

Post by dredmorbius »

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 no, don't do this.

Re: A small DIY air purifier

#153

Post by dredmorbius »

pureliquidhw wrote: post #61
adrianN wrote: post #3

You could probably buy a box fan and tape a HEPA filter in front.

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.

Re: A small DIY air purifier

#154

Post by dredmorbius »

cwkoss wrote: post #140

I've always wondered if you could essentially hook a blower up to a bong to filter particulates out of the air using water. Would be much cheaper to replace the 'filter', but I suspect it wouldn't filter as well. Seems like it could be an interesting tradeoff though.

There is a set of "Luftwascher" ("Air washer") designs which effectively do this.

These do filter out particulates, but also add enough humidity to the air that this is billed as a feature of the system. In high-humidity conditions this is less than ideal.

https://venta-usa.com/product-category/airwashers/

Re: A small DIY air purifier

#155

Post by dredmorbius »

amluto wrote: post #127

My general theory is that HEPA makes sense for filtration where a single pass is all you get. So it makes sense for vacuum cleaners, air intakes, and such. But, for room air cleaning, what matters is the rate at which particles are removed, not the cleanness of the air coming out. So, for recirculating, something like a MERV 13 filter seems like a better choice: it lets more particles through on each pass, but it als…

PM2.5 particles are small enough that they pass through many materials.

You can use lower-efficiency (higher effective particle-size) filters ... but you'll just be blowing the bad stuff through them rather than removing it.

The net metric is how fast the room particulate level drops, which is the treatment goal. Flow rate is a red herring.

Re: A small DIY air purifier

#156

Post by dredmorbius »

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.

Re: A small DIY air purifier

#157

Post by dredmorbius »

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.

Re: A small DIY air purifier

#159

Post by 0xffff2 »

sl1ck731 wrote: post #98
majos wrote: post #94

May as well ask here: do air purifiers have meaningful health benefits for generally healthy people? I get that you change the filter and it's filled with grey muck and that's gross, and that the filters change particulate content in air, but is there good evidence that doing those things improves health if you don't already have some sensitivity to air pollution or live in a highly polluted area? I don't know if "Lu…

I would assume the majority of people (in the US, anyway) are already filtering their air enough through their central air to make any additional "room-sized" filtering negligible. Similarly they make some very dense HVAC filters if standard ones aren't catching the level of particles you need, so I'm not sure what the purpose of these filter-fan things are for most people.

Do a large majority of people in the US even have central air? It's virtually unheard of to have an HVAC system in older buildings in California.

Re: A small DIY air purifier

#160

Post by DoctorOetker »

lmilcin wrote: post #13

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 o…

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...

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