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air filtering

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engineheat:
Hi,
This is kind of technical as it deals with fluid movement. I'd like hear what experts has to say. The problem is this:

Suppose one lived in a city with poor air quality, in a high rise condo that one can modify or install anything, and cost is not a factor. The goal is to ensure the cleanest air possible in the unit at all times.

One can place multiple air purifiers in the unit. Those air purifiers sucks in air via a fan and force the air through a filter (say, HEPA), and eject the filtered air out. If the unit is large, I would assume one air filter might not be enough since it might only be able to clean the air in one area, and diffusion would be needed for all the air in the unit to eventually get cleaned. But this assumes the condo is a closed system with no air exchange with the outside. We need oxygen and air exchange with the outside.

One solution would be to completely seal the unit so that it is a closed system except for an air intake and exhaust that can be controlled. All the intake air would go through a filter. But this is a bit risky if the system breaks down there'd be no air/oxygen exchange with the outside. And if this happened while one is asleep, it is risky.

The simpler solution is to place multiple powerful air purifiers in different areas of the unit, while not closing the unit from the outside completely (say, the door gaps, or only closing the windows 99% of the way). This way, natural wind and diffusion wouldn't introduce much dirty air into the unit (the rate will be slow) so the unit would be semi-closed off. But the small openings would still allow outside air to be drawn in via the indoor air purifiers (they have fans after all). And as long as there are multiple air purifiers working all the time, this should still result in indoor air orders of magnitude cleaner than the outside right? Because the rate at which the air gets "cleaned" by the air purifiers will be higher than the rate at which dirty air gets introduced via those small gaps. But I would assume there would still be an equilibrium level of pollutants in the indoor air.

The goal is to achieve at least 95% reduction in air pollutants.

Someone who is more well versed in this area please provide some suggestions or comments on whether that's achievable.
Thanks

thm_w:
Close the doors, windows, etc whatever you want. Then use the CO2 reading of an air monitor to determine when you need to pull in a lot of fresh air, say when CO2 level exceeds 900ppm or so. Use an intake fan with a filter to do this.
Use the PM2.5 reading to determine how well smoke/dust has been filtered out to verify your system is working. With a good electrostatic/hepa unit like blueair I wouldn't expect much more than 1hr or so before the level drops to almost nothing.


--- Quote from: engineheat on August 03, 2021, 04:29:11 pm ---One solution would be to completely seal the unit so that it is a closed system except for an air intake and exhaust that can be controlled. All the intake air would go through a filter. But this is a bit risky if the system breaks down there'd be no air/oxygen exchange with the outside. And if this happened while one is asleep, it is risky.
--- End quote ---

Its not risky overnight, do a simple calculation of how long that oxygen will last, many many days. But it still seems like a poor idea, unless the outdoor air is incredibly toxic.

engineheat:

--- Quote from: thm_w on August 03, 2021, 09:23:14 pm ---Close the doors, windows, etc whatever you want. Then use the CO2 reading of an air monitor to determine when you need to pull in a lot of fresh air, say when CO2 level exceeds 900ppm or so. Use an intake fan with a filter to do this.
Use the PM2.5 reading to determine how well smoke/dust has been filtered out to verify your system is working. With a good electrostatic/hepa unit like blueair I wouldn't expect much more than 1hr or so before the level drops to almost nothing.


--- Quote from: engineheat on August 03, 2021, 04:29:11 pm ---One solution would be to completely seal the unit so that it is a closed system except for an air intake and exhaust that can be controlled. All the intake air would go through a filter. But this is a bit risky if the system breaks down there'd be no air/oxygen exchange with the outside. And if this happened while one is asleep, it is risky.
--- End quote ---

Its not risky overnight, do a simple calculation of how long that oxygen will last, many many days. But it still seems like a poor idea, unless the outdoor air is incredibly toxic.

--- End quote ---

If we take this route, then not only is an intake fan needed, but there also need to be an exhaust mechanism. I don't know if there are off the shelf components one can buy to build something like this, or will this be a DIY project? I mean, most HVAC systems that draws in outside air are commercial roof-top units right?


Thanks

engineheat:

--- Quote from: evb149 on August 03, 2021, 06:06:33 pm ---
If you consider the building rooms to be filtered you can calculate the volume of the air by the volume of the rooms and you end up with some volume figure measurable in cubic meters of air space or equivalently cubic feet in those units.

Considering you have X cubic meters volume of air and you would like to filter it all therefore your filter will be passing X cubic meters of air through it somehow.
If you would like 100% of that air volume to be filtered totally within some time T seconds then you must have air movement as well as filtration that passes X cubic meters / T seconds through the filter.  This is a key specification because it defines the needed air flow and amount of air passing in, out, and through the filter over time.


--- End quote ---

I like the big filter idea in terms of decreasing noise. I believe someone built a quiet air purifier by using a huge filter. I'm talking HEPA filters, those usually have high air resistance so like you said, it'll probably require ducted fans even if the area is large right?

I guess if the room is not large (say X cubic meters), and there are multiple air purifiers working such that X cubic meters of air gets filters in T seconds where T is small (like 5 min?) Then even if one doesn't close the window 100%, the indoor air will have way less contamination than outside right? I think there will be an equilibrium point. Perhaps if one wants to open the window a little bit (for oxygen exchange), maybe it would be beneficial to place intake of one of the air purifiers at the window such that the fan will suck in outside air and filter it before the pollutants have a chance to diffuse itself throughout the room. This might be largely true if the window opening is small relative to the purifier intake, and the intake is placed near the window.

NiHaoMike:
For the air inlet, you could use a high powered fan outside blowing through a filter to the inside. Basically, the filter doubles as a muffler. Can also add a HRV if it makes sense for the weather, otherwise just rely on leakage to exhaust stale air.

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