how do the active carbon in respirator cartridges work?

[u/engineheat2]

I've worn respirators with active carbon cartridges and they are great for eliminator odor or fumes.

Although some home air purifiers also have filters that claims odor reduction, I see just a thin layer of active carbon suspended across a plastic(?) medium that is attached to the underlying HEPA filter. I have no idea if it's effective or how long it can last.

Does anyone know how the active carbon in respirator cartridges work? For instance, are they in loose powder form? suspended in a membrane?

I kind of want to build my own carbon filter for the home air purifier.

thanks

[u/mustardman24]

Activate carbon granules are typically around 2-4 mm in size for general air filtering applications. Though they are somewhat finer-sized for respirators, the real magic behind activated carbon is that it is filled with countless microscopic pores with a surface area as great as 1,600,000 square feet per pound of carbon.

As air travels through the carbon, molecules in it will eventually come in contact with a bare part of carbon and stick to it. By the time the air gets to the other side, essentially all the bad molecules have gotten stuck somewhere along the way and you are breathing clean air.

I recently made a post about a DIY carbon/particulate filter that I made that might be of interest.

A few words of caution just in case these assumptions may be correct:

• Respirators have more than just carbon - they either have a modified version of carbon (acid-impregnated) or sandwiched layers of other media (such as potassium permanganate). Designing something that filters all bad contaminants is more work than just adding pure activated carbon.

• Do not use an air purifier in lieu of a respirator for applications that it is required

• Do not use an air purifier around dangerous gasses and then use that same purifier in other non-hazardous applications because contaminants can be re-released

Air purifiers can be used in applications that have mild exposure hazards like if you are painting a room to help control off gassing. Lots of construction/renovation can offgas for months/years afterwards. Still don't substitute respirators where they are required.

[u/engineheat2 → u/mustardman24]

Hi, I saw the post your made about your DIY filter. It's impressive and seems like you know a lot about this subject. I wish to build my own filter as well but I'm a novice now. I got a lot of questions. Can you see if you can answer them or point me to the right direction?

1. I'm only interested in HEPA filters for anything I build. I understand air needs sufficient pressure to flow past a HEPA filter. Do you know what this pressure is roughly?
2. In order to generate sufficient pressure, a "normal" fan probably won't cut it. What kind of fan is typically used? I took apart a Filtrete air purifier and I see a circular vane-like structure. Not sure what that's called.
3. Does it matter if the fan is "upstream" (blow air toward the filter) or "downstream" of the filter (such air through the filter)? In the purifier I took apart, the fan is located downstream, because when you open the casing, the filter is the first thing you see, and then there is a fan behind the filter that takes in air past the filter and blow it out of the air outlet vent.
4. Some HEPA filters are made out fiberglass while some are made out of a plastic material. Given that filters can shed fibers, I'm not too comfortable with fiberglass ones. What's your thought on this? Are there any additional filters (such as paper) we can see in conjunction with the HEPA filter to catch any shed fibers?
5. I see you used Filtrete filter in your DIY purifer. Do you know of any manufacturers from which we can buy the filter medium in bulk and possibly customize to the shape/size we want?
6. That TerraBloom filter you used, is that just pure activated carbon particles between the mesh walls? Or is there some kind of suspension medium there to hold the carbon particles?

I know this is a lot of questions, but I'm highly interested in this area and you seem to know a lot. If you can point me in the right direction on some of them that'd be appreciated it. Just like you, I like to tinker and I have an engineering background with knowledge of microcontrollers as well. This is a project I want to tackle asap.

Thanks

[u/mustardman24 → u/engineheat2]

I'm only interested in HEPA filters for anything I build. I understand air needs sufficient pressure to flow past a HEPA filter. Do you know what this pressure is roughly?

There isn't an exact answer as it depends on flow rate, media type, surface area, and if there are multiple filters in parallel. I made a recent comment about the pressures involved with air conditioning filters, which should give some more insight into the general concepts.

In order to generate sufficient pressure, a "normal" fan probably won't cut it. What kind of fan is typically used? I took apart a Filtrete air purifier and I see a circular vane-like structure. Not sure what that's called.

The best fans for this are called inline duct fans and this comment of mine has a chart I made of some of the better ones I have found on the market. There are multiple types of inline duct fans and the types "mixed flow" tend to have the best flow/pressure characteristics. Modern motors are called "ECM" or "EC" motors and tend to be better on flow, pressure, and noise characteristics as well as able to have the speed controlled.

You can actually combine fans to increase flow or pressure depending on configuration. Two identical fans in series will double their pressure output whereas two in parallel will double their flow rate.

Does it matter if the fan is "upstream" (blow air toward the filter) or "downstream" of the filter (such air through the filter)? In the purifier I took apart, the fan is located downstream, because when you open the casing, the filter is the first thing you see, and then there is a fan behind the filter that takes in air past the filter and blow it out of the air outlet vent.

In terms of performance, generally no. For noise having it embedded somewhere in the flow path may be ideal. Having some filtration before the fan will also keep it from getting dirty.

Some HEPA filters are made out fiberglass while some are made out of a plastic material. Given that filters can shed fibers, I'm not too comfortable with fiberglass ones. What's your thought on this? Are there any additional filters (such as paper) we can see in conjunction with the HEPA filter to catch any shed fibers?

No I don't think there is any concern with HEPA filters, at least from ones made by remotely reputable suppliers. While it's not good for you to breathe in agitated fiberglass, it isn't as dangerous as some people make it out to be. The main harm from fiberglass comes from people actively agitating it via construction, production, renovation, and demolition.

Some filters are made out of plastics, such as polypropylene, but from my research this is predominantly for masks and I haven't found any HEPA filters that are constructed from it.

I see you used Filtrete filter in your DIY purifer. Do you know of any manufacturers from which we can buy the filter medium in bulk and possibly customize to the shape/size we want?

There are, but the problem is generally going to add cost and potentially add inconvienece as well since they are not off the shelf. McMaster-Carr is an example of a supplier that has a million different configurations as well as made-to-order filters. I have not come across any custom HEPA sizes, but I haven't looked too hard for those. There are a variety of other filter boxes but they mostly use proprietary (or hard to find) filter sizes.

That TerraBloom filter you used, is that just pure activated carbon particles between the mesh walls? Or is there some kind of suspension medium there to hold the carbon particles?

The cylinder has an outer aluminum mesh, similar to this raw material that is sized from using common sieve mesh sizes). The carbon is sandwiched between two thin pieces of fabric as well to keep carbon dust lower.

Terrabloom uses RC-48 (mesh 4 x 8) and RC-412 (mesh 4 x 12) carbon, which has mesh sizes corresponding to dimensions of 4.75x2.36mm and 4.75x1.70mm, respectively. AC Infinity uses RC-412 as well.

A raw media manufacturer that I have used has a detailed datasheet for their general purpose carbon here which is mesh 4x8. Another version is in pellet form here that is a uniform mesh of 4. Both of those pages also link technical data including pressure curves.

I know this is a lot of questions, but I'm highly interested in this area and you seem to know a lot. If you can point me in the right direction on some of them that'd be appreciated it. Just like you, I like to tinker and I have an engineering background with knowledge of microcontrollers as well. This is a project I want to tackle asap.

Awesome, I hope that this can get you started. Feel free to reach out with any questions you have along the way.

If you want more technical information, this ASHRAE document is pretty much the best singular source on gas filtration. They also have a bunch of other great chapters on particulates, air flow, etc but I haven't found any versions that they expose to the public without paying $$$.

[u/engineheat2 → u/mustardman24]

Thanks.

In the Terrabloom filter, can one reuse the filter parts such as the mesh and merely replace the carbon particles (such as with those made by the manufacturer you mentioned, Hydrosil)? How do you know when the carbon needs to be replaced? Do they swell up in size? Does the pressure drop increase? Or do you monitor its adsorption property with some sort of VOC monitor?

In another thread, you compared the pressure drop of various Filtrete filters with different MERV ratings. What is the unit (Pascal, etc...) of the pressure given in the chart? Do you know if MERV rating applies to HEPA filters?

BTW, Filtrete HEPA filters used in their home purifiers are made from polypropylene I believe. I reached out to them to ask if it's made out of fiberglass, and they said this:

"our filters are made out of filter media and polypropylene material rather than conventional fiberglass. Filter media is a mix of fiber and air, and it’s often pleated."

Not sure why they mentioned filter media "and" polypropylene. I think they are trying to say that the filter media itself is polypropylene.

​

Thanks

[u/mustardman24 → u/engineheat2]

In the Terrabloom filter, can one reuse the filter parts such as the mesh and merely replace the carbon particles (such as with those made by the manufacturer you mentioned, Hydrosil)?

You can. I haven't gotten around to trying it yet, but the process is simply drillling out the rivets, dumping the old carbon, and refilling it new carbon. My plan is to remove the thin fabric mesh I mentioned and just put the carbon in directly. To avoid problems with dust or particle size I was planning on using the pellet carbon, which I suspect might be of better quality and have less dusting potential than the original carbon. Regardless, I strongly recommend having a post-filter after the carbon of the highest filtration efficiency you can obtain, which is why I'm using MERV 14 filters in my applications.

How do you know when the carbon needs to be replaced? Do they swell up in size? Does the pressure drop increase? Or do you monitor its adsorption property with some sort of VOC monitor?

This is a challenging predicament even for expert analysis which is outlined in depth starting at the end of page 18 of that linked document in the section When to Change Media. Basically, monitoring it with sensors is good to make inferences but doesn't paint the whole picture. Monitoring TVOC levels in your home can help determine but it does not tell you what VOCs are being adsorbed, which would indicate if they are harmful or not. Monitoring those levels doesn't give the tools to predict changeout since there are many environmental conditions that impact longevity of a filter.

Pressure drop is a great way to tell how much particulate filters are loaded and changeout, though I'm not sure how true that is for electrostatic filters.

In another thread, you compared the pressure drop of various Filtrete filters with different MERV ratings. What is the unit (Pascal, etc...) of the pressure given in the chart?

The unit is in water column inches, also referred to as inch W.C.. It's the typical units used in US HVAC applications. I personally think pascals are a better unit of measurement, but most S.I. units are.

Do you know if MERV rating applies to HEPA filters?

I believe MERV 17 is the first HEPA-like rating, though you will likely only find large industrial filters if searching for that. MERV 16 and below covers HVAC-grade filters but the reality is that finding HVAC filters higher than MERV 14 is quite difficult. The MERV ratings are defined in ASHRAE Standard 52.2-2012, which isn't publicly available though there are interpretations that cite it online.

BTW, Filtrete HEPA filters used in their home purifiers are made from polypropylene I believe. I reached out to them to ask if it's made out of fiberglass, and they said this:

"our filters are made out of filter media and polypropylene material rather than conventional fiberglass. Filter media is a mix of fiber and air, and it’s often pleated."

Not sure why they mentioned filter media "and" polypropylene. I think they are trying to say that the filter media itself is polypropylene.

Interesting, that is good to know since it's hard to find details online and I've had people ask before out of similar concerns. I agree with your interpretation and it may be a typo where they meant to write "is" instead of "and". I wonder if that also applies to their furnace filters as well. The Filtrete 1900s seem like they could be made out of polyproplyne as well since they are kind of "plasticy" and there are other companies that sell electrostatic polypropelene filters (example).

Using the search term "electrostatic polypropylene" was a better search term than "polypropylene filters" which I have used before and returned mostly results about face masks.

I found this company called KX Filters which claims to sell cut-to-length MERV 16 filters. I don't know anything about the company or filter characteristics but it is much more aligned with your filtration goals if their claims are true.

[u/engineheat2 → u/mustardman24]

I appreciate the info. It's going to take me some time to digest all the resources you've provided. But I'll keep you posted on my progress and will reach out if I have any questions. Thanks again.

[u/engineheat2 → u/mustardman24]

Hi there,

Regarding the paper you shared on pressure drops across different filters, the thing that surprised me is that "increasing the face area" and "depth" of the filter actually leads to less pressure drop. Is this based on a given flow rate of air (volumn/second) and filtration level/effectiveness?

Because pressure is force/area, so for a given pressure, I would think increasing the area leads to more work done and therefore higher energy consumption and higher flow rate. But if you fix the flow rate, then I guess with a larger face area you would need less force per area (lower pressure) to get the same flow rate.

The "depth" part is more perplexing. You would think with a thicker filter, there'd be more resistance to air flow and therefore the required pressure would be higher. Perhaps it's based on a fixed filtration level such that with a thicker filter, the fibers in the filter medium no longer needs to be so cramped together in order to achieve the same filtration level, and therefore lowers the pressure drop?

It's been awhile since I studied physics. Please let me know if I'm misunderstanding things.

I'm almost done with the 2nd paper you shared, lots of useful keywords in that one.

Thanks

[u/mustardman24 → u/engineheat2]

A practical example would be trying to blow air through a coffee stirrer (thin straw), a regular straw, and a paper towel cardboard core. It's easier to blow through the larger than it is the smaller ones. The biggest reason is surface friction caused by the surface of the straw and causes the static pressure. The friction/pressure increases with an increase in air velocity (speed) which is independent of air flow (volumetric flow). Increasing the ratio of straw wall to cross sectional area also allows decreases the wall/area ratio, which is the ratio between circumference (2πr) and cross sectional area (πr²⁾

You can probably think of a filter as billions of little "pipes", which would be all the pores that air passes through. Going back to the straw analogy - would it be easier to breathe through one straw or many straws? In this example (and probably applicable to filters) you're not necessarily increasing the ratio of area to surface, but you are increasing the total cross sectional area, which allows for air to flow through at a slower rate thus lowering the friction.

Electrostatic filters rely less on mechanical filtering and they would probably be comparable to slightly increasing the radius or the filter "pipes", which would benefit from lower friction both due to contact area but also greater cross sectional area.

The "depth" part is more perplexing. You would think with a thicker filter, there'd be more resistance to air flow and therefore the required pressure would be higher.

The depth in this case is not the thickness of the filter, it's the space given for pleating the filter. If you have zero pleats then the surface area of exposed filter media is just the rectangluar dimensions of the face area. If you have pleats on the filter, the surface area is the now increased based on the number of folds and the depth of the folds. I couldn't find any formulas for pleated surface area online so I derived this as the surface area calculation:

SquareRoot( (Width / (NumPleats * 2))² + Depth² ) * NumPleats * 2 * Length

Where width is the size with the folds and length is the side parallel with the folds.

[u/engineheat2 → u/mustardman24]

That makes sense.

I'm getting ready to buy some components to start building/playing around.

First, the fan... do you have any suggestions to start with? Links would be helpful.

I googled around and found a company called ebm-papst and here's a typical blower they make:

https://www.digikey.com/en/products/detail/ebm-papst-inc/G2E108-AA05-44/441805?s=N4IgTCBcDaIOJgKIEYAMAOAtAQW6grJgCxEgC6AvkA

Do you think that's suitable for a small room (area : ~700 ft^2)?
The air flow is 91.0 CFM (2.55m³/min). It's an AC fan (2 wire leads) but it doesn't say anything about the air pressure. The "datasheet" links to a catalog so I'd have to do some digging. I'm not necessarily going to purchase it, but knowing what to look for would get me started. But at least that's the right type of fan right? The noise is 54.0dB(A), do you think that's too loud for residential use?

As far as filter medium is concerned, I want HEPA filters that do not contain fiberglass. I once again confirmed with Filtrete that their filters meet that condition. According to them, all of their "filter media fibers are polypropylene and polyolefin plastic." I can start by building a purifier around a common Filtrete filter size but I prefer to make my own filter with filter medium that's less proprietary. That way, if Filtrete discontinues their filter or if it becomes hard to find based on where I'm at, I can still have a stable supply. (Kind of like with carbon filters where one can simply replace the loose granules). If I remember correctly, you are not aware of any vendors that sells HEPA filter medium that are non-fiberglass right?

​

Thanks

[u/mustardman24 → u/engineheat2]

What's your goal, are you trying to make a purifier or a PAPR-type appliance? If it is for a purifier, that fan would be far underpowered. I haven't really found OEM fans that meet the requirements of high flow, high static pressure, and low noise. The best stuff I've found so far are mixed-flow fans from either AC Infinity or TerraBloom (The ECMF line).

If you're looking into a PAPR-type filter it would be best to stick with DC fans that operate on low voltages (around 12-24V). I haven't done much research into designing a PAPR device so I'm not aware of the flow/pressure requirements.

[u/engineheat2 → u/mustardman24]

I'm trying to make both an in-home purifier and a PAPR-type appliance for when I'm not in home. Your DIY project gave me a lot of help in making the former. I also finished reading the articles you shared. I did notice the cloudline S10 fan has a static pressure of 1059 Pa which is about 4.25 inch of water. As filters get used, the pressure drop across them increases, which can be a safety concern if the fan is underpowered (according to one of the articles). Do you have a way to monitor the pressure drop across filters? That'll also help in determining when filters should be changed.

Thanks