air filtering

[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.

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.

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]

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.

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.

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.

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]

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.

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.

[engineheat]

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.

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.

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.

How about sealing the door/windows to minimize dirty air exchange with the outside. For fresh air intake, place a fan outside the window which forces air through HEPA filter before entering the apartment. Of course this will be well sealed to minimize leakage. An one way valve of some sort can be located elsewhere for exhaust.

In order to minimize the risk of breakdowns, two or even three such fan/filter setups can be used. It'll be extremely unlikely for multiple fans to breakdown within hours of each other.

In order to minimize risk of power outage, each fan can be backed up with uninterruptable power supply. The power supply doesn't need to be super big since it only needs to run the fan for minutes. Even 1 minute of running every couple of hours should be sufficient to properly ventilate a room for a long time.

Overall, I think the above setup shouldn't be too difficult to implement and would ensure the cleanest in-door air at all times. What do you guys think?

[Marco]

Just forget about valves and have some restrictive ventilation, as long as there is overpressure no air comes in and when fan gives out there is still some diffusion. Then again unless the home was leak tested and had all the airgaps fixed it might have enough natural ventilation with everything closed to begin with.

[Miyuki]

I have a big recuperation unit for the house
It has F7 Bag Filter to remove any particular pollutants
Just add any filter to air system

[engineheat]

Normal fans can move a relatively high air flow but they are not good at increasing air pressure -- blowing a fan against a filter with significant air resistance does not work well because normal fans can't compress the air to compensate for much air resistance.

Other designs of air impellers such as blowers like the "cage" / wheel type blower typically used in HVAC systems are efficient at moving a higher volume of air at good air speeds also against the kinds of back-pressure that typical air filters present to the air flow.  So you must use the right kind of forced air impeller / pump / blower / fan according to your needs: size, noise, energy efficiency, reliability, ability to achieve X air speed at Y duct pressure feeding into the filter given air intake from the open air environment.

Regardless of the type of fan used, it should be "ducted" or sealed around the edges for maximum air pressure right? I'm only familiar with "normal" fans. Can you provide a bit info on the cage/wheel type blower so I can look into those?

Thanks

[NiHaoMike]

Regardless of the type of fan used, it should be "ducted" or sealed around the edges for maximum air pressure right? I'm only familiar with "normal" fans. Can you provide a bit info on the cage/wheel type blower so I can look into those?

Those are most common in HVAC equipment. However, most of those use PSC motors which are less efficient than BLDC motors (and BLDC motors in HVAC equipment often use some proprietary interface that makes them more difficult to repurpose), so unless you really need to move a lot of air (which won't be the case for getting a little fresh air), 120mm BLDC fans would likely be the best value. Look for the ones with director vanes, which boost the static pressure.

[Halcyon]

I've actually been looking into air purification for my home for quite some time and had a discussion with Dave about it. Just a week ago, I settled on two Dyson units. Over here in Australia they are known as the "HEPA Cool Formaldehyde" models but I think elsewhere they are marketed as the Dyson TP09 model. https://www.dyson.com.au/for-business/air-treatment/hepa-cool-formaldehyde

At the moment in Australia, these are only available to business/commercial buyers direct from Dyson.

I was looking at the larger Blueair units but settled on the Dyson for the following reasons:

• The Dyson cost me AUD$618 per unit versus AUD$1699 for the Blueair Classic 680i (shipped).
• The Dyson TP09 has a similar air handling/filtering capability compared to the Blueair model.
• The Dyson is slightly quieter at maximum fan level.
• The Dyson filters are cheaper at AUD$99 versus AUD$175 for the cheapest Blueair filter set.
• The Dyson filters are a combined HEPA/Carbon filter. This is a AUD$275 option on the Blueair model.
• The Dyson models are compatible with Home Assistant server. Blueair isn't and you need to rely on their app.
• WiFi can be completely disabled on the Dyson and two infrared remote controls are included in the box.
• The physical footprint of the Dyson unit is much smaller.
• The Dyson TP09 destroys any Formaldehyde in the air as opposed to simply capturing it in the filter material.
• Dyson has a much bigger presence and better customer support in Australia compared to Blueair.
• The Dyson will filter particular matter down to 0.1 microns (same as the Blueair filters).
• There are 10 fan speed settings on the Dyson whereas the Blueair only has 3.

I'm fortunate enough to live in a part of the world where clean air is "normal" and even expected, but there are still times where dust, pollen and bushfire smoke can impact indoor air quality.

On a "normal" day, both my Dyson units report the following figures with windows closed:
0-1 µg/m³ of both 2.5 and 10 micron particulate matter.
Nil VOCs
Nil Nitrogen Dioxide
Nil to 0.002 mg/m³ of Formaldehyde

Even cooking something will send the VOC levels peaking quite quickly. I've found they work best with the ducted air circulating air around the house (the return air has its own filter which seems to be quite effective at filtering particulates). I've placed one unit in the master bedroom and the other in the main open plan living area. I mostly just leave them on the full-auto mode (with continuous monitoring enabled) so they ramp up when they need to.

Continuous monitoring is a feature (that is disabled by default) that runs the fan continuously at very low speed, just enough for all the sensors to sample the air. It also reports this data constantly in the app.

[engineheat]

I've actually been looking into air purification for my home for quite some time and had a discussion with Dave about it. Just a week ago, I settled on two Dyson units. Over here in Australia they are known as the "HEPA Cool Formaldehyde" models but I think elsewhere they are marketed as the Dyson TP09 model. https://www.dyson.com.au/for-business/air-treatment/hepa-cool-formaldehyde

At the moment in Australia, these are only available to business/commercial buyers direct from Dyson.

I was looking at the larger Blueair units but settled on the Dyson for the following reasons:

• The Dyson cost me AUD$618 per unit versus AUD$1699 for the Blueair Classic 680i (shipped).
• The Dyson TP09 has a similar air handling/filtering capability compared to the Blueair model.
• The Dyson is slightly quieter at maximum fan level.
• The Dyson filters are cheaper at AUD$99 versus AUD$175 for the cheapest Blueair filter set.
• The Dyson filters are a combined HEPA/Carbon filter. This is a AUD$275 option on the Blueair model.
• The Dyson models are compatible with Home Assistant server. Blueair isn't and you need to rely on their app.
• WiFi can be completely disabled on the Dyson and two infrared remote controls are included in the box.
• The physical footprint of the Dyson unit is much smaller.
• The Dyson TP09 destroys any Formaldehyde in the air as opposed to simply capturing it in the filter material.
• Dyson has a much bigger presence and better customer support in Australia compared to Blueair.
• The Dyson will filter particular matter down to 0.1 microns (same as the Blueair filters).
• There are 10 fan speed settings on the Dyson whereas the Blueair only has 3.

I'm fortunate enough to live in a part of the world where clean air is "normal" and even expected, but there are still times where dust, pollen and bushfire smoke can impact indoor air quality.

On a "normal" day, both my Dyson units report the following figures with windows closed:
0-1 µg/m³ of both 2.5 and 10 micron particulate matter.
Nil VOCs
Nil Nitrogen Dioxide
Nil to 0.002 mg/m³ of Formaldehyde

Even cooking something will send the VOC levels peaking quite quickly. I've found they work best with the ducted air circulating air around the house (the return air has its own filter which seems to be quite effective at filtering particulates). I've placed one unit in the master bedroom and the other in the main open plan living area. I mostly just leave them on the full-auto mode (with continuous monitoring enabled) so they ramp up when they need to.

Continuous monitoring is a feature (that is disabled by default) that runs the fan continuously at very low speed, just enough for all the sensors to sample the air. It also reports this data constantly in the app.

Do you know if the Dyson HEPA filter contains fiberglass? I heard those with fiberglass can leak fiberglass fibers.

[engineheat]

Regardless of the type of fan used, it should be "ducted" or sealed around the edges for maximum air pressure right? I'm only familiar with "normal" fans. Can you provide a bit info on the cage/wheel type blower so I can look into those?

Those are most common in HVAC equipment. However, most of those use PSC motors which are less efficient than BLDC motors (and BLDC motors in HVAC equipment often use some proprietary interface that makes them more difficult to repurpose), so unless you really need to move a lot of air (which won't be the case for getting a little fresh air), 120mm BLDC fans would likely be the best value. Look for the ones with director vanes, which boost the static pressure.

What does "director vanes" mean? Thanks

[Halcyon]

Do you know if the Dyson HEPA filter contains fiberglass? I heard those with fiberglass can leak fiberglass fibers.

It appears so. According to their website the PM filter material is made from borosilicate microfibres. I'd say it's not unlike a lot of filter material. I'm sure they've done their homework. I've never heard of the filters breaking down in such a way that would cause them to impart impurities into the air. Maybe some cheap filters from China of dubious quality would do that? I'd say if you're buying an expensive filtration device, you should also be buying the genuine filters. $100 per unit every 6-12 months is hardly a lot of money.

[NiHaoMike]

What does "director vanes" mean? Thanks

Those are stationary vanes that direct the air to come out exactly perpendicular to the fan itself, converting the spin imparted by the blades into more static pressure. Here's an example of a fan with them:
https://www.newegg.com/p/1YF-00B0-00JW6?Item=9SIAAESD2T6194&Description=120mm%20delta%20fan&cm_re=120mm_delta%20fan-_-9SIAAESD2T6194-_-Product&cm_sp=SP-_-518473-_-0-_-1-_-9SIAAESD2T6194-_-120mm%20delta%20fan-_-120mm|delta|fan-_-12

[engineheat]

Do you know if the Dyson HEPA filter contains fiberglass? I heard those with fiberglass can leak fiberglass fibers.

It appears so. According to their website the PM filter material is made from borosilicate microfibres. I'd say it's not unlike a lot of filter material. I'm sure they've done their homework. I've never heard of the filters breaking down in such a way that would cause them to impart impurities into the air. Maybe some cheap filters from China of dubious quality would do that? I'd say if you're buying an expensive filtration device, you should also be buying the genuine filters. $100 per unit every 6-12 months is hardly a lot of money.

I've read that even "good" HEPA filters will release a bit of fiberglass fibers, especially when new:

https://smartairfilters.com/en/blog/hepa-air-filter-fiber-harmful-health/#:~:text=Bottom%20line%3A%20HEPA%20filters%20shed,other%20dust%20in%20the%20air.

That's why I'm looking at Filtrete filters as those don't contain fiberglass.

[engineheat]

What does "director vanes" mean? Thanks

Those are stationary vanes that direct the air to come out exactly perpendicular to the fan itself, converting the spin imparted by the blades into more static pressure. Here's an example of a fan with them:
https://www.newegg.com/p/1YF-00B0-00JW6?Item=9SIAAESD2T6194&Description=120mm%20delta%20fan&cm_re=120mm_delta%20fan-_-9SIAAESD2T6194-_-Product&cm_sp=SP-_-518473-_-0-_-1-_-9SIAAESD2T6194-_-120mm%20delta%20fan-_-120mm|delta|fan-_-12

Thanks. After doing some research, it seems what I need is a centrifugal fan with forward-curved blade with single inlet, like this:

https://www.ebmpapst.com/de/en/campaigns/product-campaigns/centrifugal-fans/forward-curved-centrifugal-fan.html

I believe this can generate enough air pressure for HEPA filters.

The HEPA filters I'm interested in have larger areas than the outlet opening of the fans that I'm interested in. I can build a "funnel-like" duct to mate the fan opening to the filter with very little air leakage. Would that decrease the air pressure to the filter? I don't think so since whatever air pressure generated by the fan should be uniform in the duct, but just to make sure...

Thanks

[Halcyon]

Do you know if the Dyson HEPA filter contains fiberglass? I heard those with fiberglass can leak fiberglass fibers.

It appears so. According to their website the PM filter material is made from borosilicate microfibres. I'd say it's not unlike a lot of filter material. I'm sure they've done their homework. I've never heard of the filters breaking down in such a way that would cause them to impart impurities into the air. Maybe some cheap filters from China of dubious quality would do that? I'd say if you're buying an expensive filtration device, you should also be buying the genuine filters. $100 per unit every 6-12 months is hardly a lot of money.

I've read that even "good" HEPA filters will release a bit of fiberglass fibers, especially when new:

https://smartairfilters.com/en/blog/hepa-air-filter-fiber-harmful-health/#:~:text=Bottom%20line%3A%20HEPA%20filters%20shed,other%20dust%20in%20the%20air.

That's why I'm looking at Filtrete filters as those don't contain fiberglass.

I'm not saying it doesn't happen at all (since I haven't looked into it properly), but as Dave would say, "I smell bullshit". A lot of this so-called advice seems to come from websites and companies who sell non-fibreglass filters. Of course they are going to say the competitors product is bad. Instil fear into consumer then offer an alternate "remedy" (at a price). Surprise surprise, the website you linked sells their smallest HEPA filters at the rip-off price of USD$180 (also no mention anywhere of them being free of fibreglass either).

Fibreglass-based HEPA filters have been around for a long time yet this "problem" isn't a problem. You can bet that if it was a real problem, customers would be up in arms about it, demanding bans on these types of filters etc... etc...

As I said, you have very large companies with very large R&D budgets whose job it is to research and test this stuff. Why haven't they found this shedding of fibreglass filters to cause any problems?

After a quick Google, the only reputable source for fibreglass versus synthetic media comparison comes from a peer-reviewed paper from Clemson University (link). It talks about health concerns regarding fibreglass filters and concluded that the analyses show that both types of filter shed fibers, with some of them being respirable. Also the difference in fiber shedding between the two filter media is insignificant. The test results also show that fibers shed from the media decrease with time. As compared to the contaminate particles present in ambient air, the number of fibers shed by the filter media is negligible. The vast majority of other search results were advertisements or opinion pieces at best.

There have also been numerous studies which show no relationship between the inhaled fibreglass and lung disease (asbestos fibres are obviously quite different). After crawling around in my roof cavity for an hour, I'd inhale more fibreglass and other particulates compared to a lifetime of using fibreglass-based filter materials.

Personally, I wouldn't worry. The crap that a good air filter pulls out of the air is far more harmful to your health (by comparison).

[SteveyG]

The Xiaomi purifiers seem to be working excellently here - I ended up buying a couple, the smaller one I use in the lab and within a few seconds of doing some soldering it picks up the reduced air quality and whizzes into action despite being 5 meters away.

The filters last between 6 and 12 months, and although they have an RFID tag in to log the usage, you can carry on using them even if they're "expired". You can also use compatible filters which don't have the RFID tag. They're also very nicely built and well priced.

[engineheat]

FYI, Filtrete filters are HEPA and are not made out of fiberglass. They are made out of polypropylene material. (I specifically asked their tech support). I'd think polypropylene might be a bit "safer" than fiberglass relatively, but who knows.

[engineheat]

I found some centrifugal fans that can output up to 800 Pa of air pressure which should be enough for HEPA filters. The filter I want to use has much larger area than the fan outlet. I'm thinking about building a customized funnel shaped "duct" for this. If I remember correctly about fluids, if the air pressure at the fan outlet is say, 100 Pa, then 100 Pa will also be at the filter despite its larger area right? In another word, pressure is transmitted inside an enclosed space?

Thanks