DIY air filtration unit for <$200

[Glen Monaghan]

I just looked at the design again and realized that there is no fan in the stack, so they probably are using an external fan after the stack, in which case you'd need the duct hole rather than a register grate in order to connect the stack to the fan. I'd still include an airflow deflector inside the stack to smooth the 90 degree transition, and probably a larger duct with final reduction to fan inlet diameter right at the fan. As the design is drawn, there appears to be enough room that you could simply include an inline fan in the stack, which is what I did, placing it at the bottom to keep the center of gravity lower. There's something to be said, though, for using an upflow rather than downflow arrangement, because then you might be able to eliminate a 90 degree bend on the inlet side and the outlet could be straight up through a grate in the top cover, again eliminating a 90 degree bend, plus really big particles could fall to the bottom of the stack without clogging the prefilter.

[Glen Monaghan]

In another post on this thread, I mentioned blowing carbon dust out of the AC before you install it. Then, after installing it, use a "sacrificial" filter for a few minutes while running the unit to clean out additional carbon dust released by the process of installation. The AC shouldn't continue shedding carbon dust during normal operation. Just make sure you don't do anything like washing the AC since impurities in the water will likely ruin the AC's ability to adsorb odors.

[Scott Shepherd]

Thanks Glen, I guess I'll pony up for another filter and give it a try. Worked good for the first day

[Raphael Weil]

Yep, what Dan said, they are using multiple thin activated charcoal (AC) filters stacked to achieve better odor reduction.

This design is very similar to what I made, only I'm using 16"x19"x4" filters and, rather than routing slots into the sides for trays, I screwed angle aluminum strips to the sides to create shelves that hold the filters. I have four filter "bays", each four inches deep, with the idea that I'd have a 4" thick whole-house filter for prefiltering, a 4" HEPA filter for final filtering of particulates, and either one or two 4" activated carbon trays for odors (especially acrylic). I found that one 4" AC filter was adequate for wood but definitely needed the 8" to control odors from cutting acrylic. Although 8" of AC has proven adequate to control odors for most of my purposes, it wouldn't hurt my feelings to have even a bit more because I definitely noticed the acrylic odor when I did a job that required over two hours of cutting acrylic.

Also, you'll improve efficiency a bit if you provide a large curved airflow deflector in the bottom of the box to smoothly turn the air 90 degrees from the final filter to the exhaust port. Assuming you are exhausting back into the room rather than piping the exhaust somewhere else, replace that dinky circular exhaust opening with as large of a register grate as fits. Likewise, you'll gain a bit of efficiency if you substitute 6" or 8" ductwork for the 4" inlet ducting, along with the appropriately enlarged inlet hole. Finally, with the inlet at the top, it will likely be above your machine's exhaust port and so will have duct coming up and turning 180 degrees to come back down into the top of the stack. Better to move the inlet to the side of the stack with a single large final 90 degree bend in the inlet duct, and then provide a large curved deflector inside the box to turn the air 90 degrees down onto the prefilter. This both provides a larger, less restrictive bend for that final 90 degrees, and also better expands the intake air across the prefilter.

I'll say this, if this isn't one of the best forums on the internet, I don't know what is. You guys (and not just you Glen) really provide a huge wealth of info.

Glen is your stack order prefilter -> AC -> Hepa? Also your deflector idea sounds amazing. What material did you use for that, some sort of flexible sheet metal or plastic of some kind?

I just looked at the design again and realized that there is no fan in the stack, so they probably are using an external fan after the stack, in which case you'd need the duct hole rather than a register grate in order to connect the stack to the fan. I'd still include an airflow deflector inside the stack to smooth the 90 degree transition, and probably a larger duct with final reduction to fan inlet diameter right at the fan. As the design is drawn, there appears to be enough room that you could simply include an inline fan in the stack, which is what I did, placing it at the bottom to keep the center of gravity lower. There's something to be said, though, for using an upflow rather than downflow arrangement, because then you might be able to eliminate a 90 degree bend on the inlet side and the outlet could be straight up through a grate in the top cover, again eliminating a 90 degree bend, plus really big particles could fall to the bottom of the stack without clogging the prefilter.

In my case I'm going to vent out a window after all the filtering. Might be a bit paranoid, but I think it's a must in my case. The window is 4-5 feet off the ground, so your upflow idea is exponentially more efficient for me. What I don't completely understand is where you place the fan in the upflow version. Does it have to be inline and elevated? Can it still be on the ground before the stack? You can tell I don't know that much about this. I look forward to learning and then one day being the one to help people on here.

[Tony Lenkic]

Many commercial air purifiers install charcol last (after HEPA). There must be a good reason they do that.
My Quatro unit hase them in this sequence:

[Raphael Weil]

Many commercial air purifiers install charcol last (after HEPA). There must be a good reason they do that.
My Quatro unit hase them in this sequence:

Can I ask what you pay a year to maintain/swap out all the filters?

Any chance they put the Hepa before the AC to make it more expensive to maintain? I know that seems shady, but if the Hepa is expensive they probably don't mind having you swap it out a bit more often than is ideal.

[Tony Lenkic]

Less than $ 500.00 / year but that is relevant to usage.

[Glen Monaghan]

Raphael, yes, I settled on prefilter -> AC -> HEPA. Originally, I put the HEPA before the AC but thought it was clogging too quickly, and it's the most expensive section to replace. With HEPA after AC, I find that a light-colored dusting of particulates accumulates over time throughout the 8" of AC. Not sure what it is specifically, but I wonder if it's mostly dust and solidified vapor from acrylic; regardless, that was probably clogging the HEPA filter.

I intended to bend two or three flexible plastic sheets into arcs, roughly quarters of circles, arranged concentrically in the air stream to more smoothly turn the air through 90 degrees. A quick smoke test showed that simply injecting air into the boxy shape and letting it find its own way "around the corner" created turbulence and dead spots. I started by simply bending a suitably sized thin sheet of plastic and slipping it into the space to create a smooth 90 degree arc from the top of the inlet area to the far side of the filter stack. I never did decide on a good way to install and hold a second or third concentric deflector that would divide the air flow and more smoothly direct it across the full face of the prefilter, so it's still just the one deflector on the inlet and one on the outlet.

In general, it's better to have the fan at the end of the run so that the entire system is subject to negative pressure internally, meaning that any leaks will suck in outside air and contain the exhaust products. If the fan is not at the end, then any part of the system downstream of the fan is under positive pressure internally, meaning that any leaks there will allow exhaust products to escape back into your space. That's particularly unpleasant if it happens prior to filtering, less problematic if it happens downstream of the filters. So, for an upflow version with internal fan, you'd ideally have the fan at the top of the stack just prior to the filter's exhaust port so that only filtered air is in the pressurized filter exhaust duct. Just be careful with stability since putting the fan at the top raises the stack's center of gravity. My stack is on casters and, even with the fan near the bottom, it's a bit tippy if you hit a rough spot or something that the wheel won't easily roll over.

[Dan Hintz]

My understanding is that the pre-filter is for the large particles, the hepa is really to eliminate the bulk of the particles, and then the activated carbon is really to clean/deodorize/filter the last portion before venting. Is my understanding correct? In which case having one last filter after the activated carbon is probably pointless?

I would skip the last charcoal filter in favor of a standard filter between the HEPA and charcoal. Other than that, you have it right.

I'll ask for some technical help, I made one, put the prefilter, then the AC, then the hepa. Ran black acrylic for about 2 days straight, noticed the air was barely moving. Stopped things, looked at it, the hepa filter was black. I weighed the AC and it was essentially the same weight. My guess is that it sucked up the dust from the AC and clogged the hepa filter. How do you stop the charcoal dust from clogging the hepa filter prematurely?

I blow out my charcoal before charging the system to prevent just this sort of issue. There's entirely too much dust in a bag of charcoal, so you need to get rid of as much as possible, else it's just in the airstream the first time you turn it on.

[Dan Hintz]

Many commercial air purifiers install charcol last (after HEPA). There must be a good reason they do that.
My Quatro unit hase them in this sequence:

The idea there is to remove all particulate matter from the stream before it hits the charcoal... sounds good, until you realize he HEPA filter gets jammed up pretty good with particulates that can't be easily removed. Charcoal gets replaced and it's cheaper than a good HEPA, so I now prefer that filter order. To be fair, I ended up ith that order due to poor planning on my part during the original design, and my intention was to "fix" the problem on my next rebuild. After some serious use, however, I saw what was happening and decided to leave well enough alone.

[Zach Browning]

I'm looking to build a similar stacked filter for my laser, which is going to be located in an area where external venting is not a possibility. I also need it to be as quiet as possible, so a dust collector motor is not going to work. I saw another thread where they used two vacuum motors in a sound dampened box, but the last post on that thread was over a year ago. I PM'd the OP of that thread about what model motor he used, but haven't heard anything back. What kind of CFM should I be looking for? Most vacuum motors I've looked at push around 90-100 CFM, but does that scale so that two motors running at half speed would push the same 90-100?

I also found this http://www.supplyhouse.com/Fantech-F...8-Duct-502-CFM
The spec sheet says it does ~500 CFM at 0" SP, but drops to ~100 at 2" SP, not sure what kind of restriction the filter would apply.

All this airflow terminology is new to me, so any input would be greatly appreciated.

[Dan Hintz]

I'm looking to build a similar stacked filter for my laser, which is going to be located in an area where external venting is not a possibility. I also need it to be as quiet as possible, so a dust collector motor is not going to work. I saw another thread where they used two vacuum motors in a sound dampened box, but the last post on that thread was over a year ago. I PM'd the OP of that thread about what model motor he used, but haven't heard anything back. What kind of CFM should I be looking for? Most vacuum motors I've looked at push around 90-100 CFM, but does that scale so that two motors running at half speed would push the same 90-100?

I also found this http://www.supplyhouse.com/Fantech-F...8-Duct-502-CFM
The spec sheet says it does ~500 CFM at 0" SP, but drops to ~100 at 2" SP, not sure what kind of restriction the filter would apply.

All this airflow terminology is new to me, so any input would be greatly appreciated.

Impeller speed versus airflow is much closer to a square law than linear... and combining two motors never works the way people intend (certainly not double the airflow). CFM at 0 SP is a worthless figure as you will never have 0 SP. Expect at least several inches of water SP in your system, and 100 CFM just isn't going to clear the cabinet very fast (lots of smoke will linger).

[Raphael Weil]

Hi all,

I was wondering if I could get one last check up on design, I'm picking my blower now.

My design has a first right angle out of the laser to my filter stack 2-3 feet away. Another right angle into the stack with a deflector to help redirect air upwards (my stack is bottom to top airflow). Then it runs through 4 feet of pre filters/filters/carbon/hepa up into the blower. So this is where I'm currently stuck. Based on everything I've read, it sounds like an 8" blower in the 600-700 cfm range is going to be what gets the job done (I'm assuming louder than I'd like even sound proofed). So my question is, if I am using an 8" blower, should I stick to 8" size throughout the design and convert from 8" to 4" at the laser? Am I negating the benefit of the 8" duct if I convert to 6" at the area that leads to the laser? I don't actually know if 8" -> 4" is a connector that's out there. I had originally planned on 6" to 4".

I have my eye on this, just not sure if it's total overkill, and if I could somehow pull this off with a quieter 6" (based on research on here, there is no 6" blower with enough cfm's)

http://www.hydroshop.ca/en/climate-c...rtex-1039.html

[Glen Monaghan]

The smallest opening or duct in the system will be something of a limiting factor, but a duct is more limiting than an opening of the same diameter (due in large part to friction along the duct wall). Ideally, your duct work will be at least as large in diameter as your fan, so 8" duct will be better than 6" duct and much better than 4" duct. Although it would be better to have a smoothly transitioning 4" to 8" adapter (or possibly 4" to 6" immediately followed by 6" to 8" if that were easier for you), a simple step adapter really won't be noticeably worse in this application. I originally created my system with an 8" fan that looks similar to the one you referenced and so used 8" duct between the laser and filter stack. (That said, I wasn't happy with the air flow through my prefilter, 8" AC stack, and HEPA filter and so have since replaced the 8" fan with a 12", the largest that fits my layout.) Where the 8" duct meets the laser's 4" port, I closed the end of the 8" duct with an 8" duct tee cap in the center of which I had cut a 4" hole to fit the laser port. Sealed all the joints with aluminum duct tape. Alternatively you could cut an arc of aluminum flashing and fashion a 4" to 8" funnel shape for what should be a more efficient home made adapter.

[Raphael Weil]

The smallest opening or duct in the system will be something of a limiting factor, but a duct is more limiting than an opening of the same diameter (due in large part to friction along the duct wall). Ideally, your duct work will be at least as large in diameter as your fan, so 8" duct will be better than 6" duct and much better than 4" duct. Although it would be better to have a smoothly transitioning 4" to 8" adapter (or possibly 4" to 6" immediately followed by 6" to 8" if that were easier for you), a simple step adapter really won't be noticeably worse in this application. I originally created my system with an 8" fan that looks similar to the one you referenced and so used 8" duct between the laser and filter stack. (That said, I wasn't happy with the air flow through my prefilter, 8" AC stack, and HEPA filter and so have since replaced the 8" fan with a 12", the largest that fits my layout.) Where the 8" duct meets the laser's 4" port, I closed the end of the 8" duct with an 8" duct tee cap in the center of which I had cut a 4" hole to fit the laser port. Sealed all the joints with aluminum duct tape. Alternatively you could cut an arc of aluminum flashing and fashion a 4" to 8" funnel shape for what should be a more efficient home made adapter.

Wow how many CFM was the 8" fan you used? That makes me a bit nervous. This 10" beast allegedly runs at 52db and cranks out 800 cfm. I'd hope to manage to still draw 450 by the time it makes it to the laser. Maybe wishful thinking.

http://www.aftproducts.com/Centrifug...l-800-CFM.item