Air molecules are a lot smaller than the particles getting trapped, so presumably a filter with a low enough density can minimize heat transfer except to things getting stuck in it.
Also, many filters actually can stop smaller particles than you might expect based on the rating. Most people think of a filter as working the way a net does. There is a gap size between the fibers, and anything smaller than that gets through easily, and anything larger does not.
It's more complicated because there are actually 4 mechanisms that might stop a particle from getting through.
1. For particles bigger than the gaps, it does work like a net as most people expect.
2. For particular smaller than that but massive enough that they cannot change direction fast enough to follow the air stream they can miss turns of the air flow and get embedded in the fibers.
3. For even smaller particles they can get jostled around significantly by hits from gas molecules (like with Brownian motion). This can knock them into fibers where they can become embedded.
4. Some filters materials have an electrostatic effect that can attract and hold passing particles.
One of the reasons filters are commonly rated by their efficiency at 0.3 micron is that this is in the region where the filter is least efficient. For bigger particles, the gains from #2 and later #1 more than make up for #3 being irrelevant at those sizes. For smaller particular, the gains from #3 increase efficiency.
So you have the filter roughly getting worse down to the 0.3 micron ballpark, and then getting better as particles become still smaller, and at some point as you continue getting smaller it is going to get worse again. (I don't know where that final turn around is).
I've never thought about air filters, but the explanation on why they also filter smaller particles is very similar to size exclusion chromatography, a very common method used in a biolab. This is also a method that might appear counter-intuitive at first.
The idea there is to separate molecules according to their size. So you press them through a column of porous beads. Small molecules can enter these pores, which delays them and they travel through the column slower than large molecules that cannot enter them. This is pretty counter-intuitive, especially as other similar methods work as you'd expect with smaller molecules being faster to move through the material because they don't bump into it as much as larger molecules.
I think that is an idea behind many air filters. You will get build up in areas of the "maze", but ideally the lighter portions of the air will be able to basically bounce out and through. Some bigger particles will get lucky and find an exit.
HEPA filters block particles smaller than it's rated size, and they block those particles more easily than larger particles because of Brownian motion and the physics of electrostatically capturing particles.
Presumably smaller particles are also more difficult to filter (and therefore more likely to make their way indoors without dedicated filtration), and also expected to remain airborne for a greater distance and duration (because of square cube things).
No, the filter gets heated to 250 °C to destroy trapped viral particles. The air itself won't get heated up much, its contact time with the filter is too short.
>Presumably smaller particles are also more difficult to filter
So, that may or may not be true for just passive filtering (like door and window frames), but it's actually not true for actual hepa-style filters, because the physics of particulate filtering is totally unintuitive from the macro level -- past a certain point, finer particulates actually become easier to filter, because the physics is dominated by two different effects at different sizes.
>>> By making the filter electrically heated, rather than heating it from an external source, the researchers said they minimized the amount of heat that escaped from the filter, allowing air conditioning to function with minimal strain.
Dense HVAC filters make your HVAC systems run much less efficiently. There is less airflow and less heat transfer (either cooling or heating). Anyone that has come to service my air conditioner or furnace has always checked the filter and recommended never getting a denser filter.
The filters in the HVAC system are more there to prevent your furnace from getting clogged up with larger particles than to help the indoor air quality.
These filters look "not enough" to filter very small particles. There should be more than one layer in them (judged by the pictures/imho). Also, a channel in the middle is needed (the "Dyson principle") for the particles being centrifuged in the center and not being just blown into the air again (just my humble engineer opinion).
Also, it needs to be taken into account that the virus filled droplets dry faster, the more dry the air is. But the more dry the air, the more the particles stay in the air. So maybe a good idea would be to prolong the way the air stays within the box and apply some heat or use a UVC lamp inside or something.
Otherwise, there is a possibility that the very fine droplets won't stick to the filter but being blown into the air again.
- still better than having a droplet-cloud waving through the room -
> Even thought the pores are larger than a virus the masks are statically charged causes smaller things to get trapped.
Even filters that aren't statically charged stop particles that are smaller than the gabs between the fibers. There are actually several mechanisms by which a filter can stop a particle.
1. Big particles don't fit between the fibers of the filter. Think fish in a fish net. This is called sieving.
2. Particles too small for sieving but heavier than the surrounding flow don't make the turns as well as the surrounding flow when the flow goes around the fibers. The particles can get embedded in the fibers. This mechanism is called inertial impaction.
3. The smallest might be too small to actually be affected much by the flow of the surrounding fluid through the filter. The move by diffusion, and many will randomly hit the fibers and get stuck.
4. Particles too big for diffusion but too light for inertial impaction still can run into fibers and get stuck. This is called interception.
5. As you mentioned, some filters have an electrostatic charge which can help trap particles.
The effectiveness of sieving, inertial impaction, and interception all follow S shaped curves that start out low for small particles, then at some point start rising, and then level out. The sieving curve's rise is almost vertical. The rise for inertial impaction is steep but not nearly as steep as it is for sieving. The curve for interception's rise is much more relaxed.
The effectiveness for diffusion goes the other way. Much more effective for very small particles, then above some size drops down and is low from then one.
When you put all these together, you get a curve that is effective at the small end, and at some point as size goes up effectiveness drops, reaching a minimum, and then rises again to reach high effectiveness for particles above some certain size.
The reason 0.3 microns is used for many HEPA filter ratings is that is in the middle of the low part of that U shaped curve, so when you get a filter that removes, say, 99.97% of 3 micro particles, it should actually do better for both larger and smaller particles.
Placing tight filtration on the HVAC system can substantially reduce airflow over the coils and/or heat tubes, decreasing efficiency and possibly causing the system to malfunction.
The HVAC system just needs enough filtering to avoid impacting its operation. Any additional air particulate removal should be done separately.
Read the rest of the article. The cheap furnace filter filters nanoparticles too, just not as well. Both larger and smaller particles are pretty easy to filter. It's the ones around 0.3um that are the toughest to filter.
When you're dealing with a machine that is exchanging air in a room a certain number of times per hour and a continuous influx of particulates, with less efficient filters you need more air exchanges which means more filters, bigger fans, etc.
The takeaway is that less particle efficient filters won't ever get you down to the same steady state as better filters in real world applications.
And from personal experience with filters and devices to measure airborne particles, it is hard enough getting good numbers with the best filters I can find.
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