DIY air filtration unit for <$200

[Dave Sheldrake]

I'm venting outside Dan but smoke on the home laser / test machine is something I'm trying to cut down. I have a 150mm inline from the machine to a sealed box with the unit like Rob has flowing in to a second 150mm inline then out to atmosphere. I'm wondering if adding a hepa in front of the carbon can will reduce the smoke?
I could just go buy a commercial air cleaning unit but I like tinkering

cheers

Dave

[Glen Monaghan]

Seems like a really good deal in what way?

I got one of these incidental to acquiring the 8" fan unit I was getting. These things come with either 1", 1.5" or 2" thick layers of what is supposed to be activated charcoal (it's definitely charred, but I don't know how to verify that it's activated carbon).

All I can say for sure is that the stuff is charred pieces on the order of 0.5" chunks rather than granules, and even the 2" thick versions don't allow for much "contact" time with the air getting sucked through. Consequently, my empirical test showed these things are inadequate for removing odors, especially when cutting acrylic.

You can get 55 pounds (at least twice as much carbon as these canisters hold) of GC 4x8S activated carbon granules from General Carbon for $99 plus shipping. I was looking at commercial filter units at the NBM show and they typically pull air through at least 10" of carbon for odor removal with small to medium sized laser machines.

[Glen Monaghan]

By "great on acrylic" I assume you mean removing visible smoke, rather than the odor?

These things are generally unsuitable for filtering smoke because they have relatively HUGE chunks (about 1/2" diameter) of charred material, layered about 2" thick. You will have rather poor smoke removal and fairly quickly clog up the charcoal with what smoke particulates it does filter, ruining whatever odor removing ability it may have.

[Robert Silvers]

When I'm engraving/cutting heavy wood (large wood plaques and such), I get zero smoke in the room after a couple of hours of straight work. First it hits a furnace filter, then the 2" bed of carbon, and finally the HEPA. I purchased one of those Honeywell Electronic filters thinking I could trap a lot of smoke that way and increase my carbon life... I skipped it after I determined the amount of airflow would make it all but worthless, and I'll likely install it in the home HVAC next remodel session.

Does the furnace filter capture a lot of stuff? Does it get very dirty? Do you know the MPR or MERV rating for it?

[Robert Silvers]

A good deal in terms of the packaging (metal container). But if commercial laser filters are 10 inches of carbon, then these units are not good enough, and it sounds like raw carbon contained between two screens is the way to go. But Dan says that his 2 inches of carbon seems good, right?

[Glen Monaghan]

That's what he said. I just received some of the General Carbon product but haven't had time to test it. However, the stuff in that canister was basically ineffectual when I tried exhausting through it while cutting acrylic, and only somewhat helpful when cutting BB ply. I used a 4" thick 19x16 whole house filter followed by a 4" thick 19x16 HEPA filter for smoke, which worked great, and then the canister for odor, which was a fail as mentioned.

[Dan Hintz]

I'm wondering if adding a hepa in front of the carbon can will reduce the smoke?

A HEPA will definitely reduce your smoke (that's what they're there for)... but you'll fill that filter up in a heartbeat. You'll get the most bang for your buck if you let the HEPA get rid of just the final remnants... a thick layer of activated carbon should cut down on your smoke significantly.

Does the furnace filter capture a lot of stuff? Does it get very dirty? Do you know the MPR or MERV rating for it?

No MERV rating (at least one I care about). I have two layers of it... the top one is to catch bulk material (like physically viewable chunks of material that came off of the vector table during cutting), and the other is mostly there to hold up the activated carbon. Smoke and such will go through them, for the most part.

A good deal in terms of the packaging (metal container). But if commercial laser filters are 10 inches of carbon, then these units are not good enough, and it sounds like raw carbon contained between two screens is the way to go. But Dan says that his 2 inches of carbon seems good, right?

The 2" I put in mine seem to do a nice enough job, but then again I didn't want to put in a thick, thick layer and have air flow drop significantly. In my next design iteration, I may increase the thickness to 3" just for completeness.

[Dave Sheldrake]

By "great on acrylic" I assume you mean removing visible smoke, rather than the odor?

More the odor, I don't have too much trouble with the acrylic smoking it's more the smell (that my wife happens to like but I hate). The can filter works ok for that (I'm extracting outside anyhows).At low power (60 watts or so) there is quite a bit of smoke from the acrylic but once it goes to 160+ watts the smoke almost vanishes.
My work filter system is huge and vents clean air (it gets inspected by the HSE on a regular basis because of the UK's "clean air act") so I really just like to toy around at home to see what can be done.

I'm off to town to see what bits I can get

cheers

Dave

Looks like I'm going to be pulling mine to bits and seeing if I can add loose carbon then the hepa.

[Glen Monaghan]

A HEPA will definitely reduce your smoke (that's what they're there for)... but you'll fill that filter up in a heartbeat. You'll get the most bang for your buck if you let the HEPA get rid of just the final remnants... a thick layer of activated carbon should cut down on your smoke significantly.

As I read the literature, a true HEPA filter is actually overkill for most smoke particles. High-MERV non-HEPA filters (especially electrostatic types) appear sufficient for removing most smoke, are less expensive than HEPA, and won't clog so quickly. Consequently (and being a cheap b@st@rd), my build is using a High-MERV as pre-filter and then HEPA for the really fine stuff. Also using fairly large and deep filters to minimize air flow restriction and maximize life, compared to the much smaller canisters some people are talking about.

As for using activated carbon to filter the smoke, General Carbon says running smoke through the carbon is a quick way to clog up the pores and prematurely destroy the carbon's effectiveness. They also noted that non-visible vaporized compounds from cutting acrylic and other plastics tend to coat and quickly clog activated carbon as well. They recommend using the activated carbon as a "polisher" after other filters, more as an odor remover than a particulate remover.

As Dan noted previously, having the carbon last in the filter stack means you could have fine carbon blowing out the exhaust for a while when you change the granules out, so he put his HEPA after the carbon. My build is using 4 identical 16x19x4" filter bays in which the filters can be arranged/rearranged as desired. One for prefilter, one for HEPA filter, one for activated carbon filter, and final open/optional bay for a possible post filter or, more likely, second carbon filter (I intend for each carbon filter to be 1/2 the 55# bag of GC's granules, so using both means I wouldn't have to store half a bag of carbon somewhere else, and presumably means I could go about twice as long before needing to change out the carbon). Because the bays are identical, if using only one carbon filter, I can drop the HEPA filter from 2nd to 4th bay for a while after changing the 3rd bay's carbon, to catch carbon dust. Or, if I go with 2 carbon bays and change the granules, just swap the HEPA and 2nd carbon until the dust clears, then switch them back.

The 2" I put in mine seem to do a nice enough job, but then again I didn't want to put in a thick, thick layer and have air flow drop significantly. In my next design iteration, I may increase the thickness to 3" just for completeness.

No idea what your fan specs are, but the GC4x8 granules have a pressure drop curve that ranges as follows:
0.3" water per foot of carbon granules at ~21 fpm superficial airflow velocity
1.0" water per foot of carbon granules at ~48 fpm superficial airflow velocity
2.0" water per foot of carbon granules at ~75 fpm superficial airflow velocity
4.0" water per foot of carbon granules at ~110 fpm superficial airflow velocity
5.0" water per foot of carbon granules at ~125-130 fpm superficial airflow velocity

[Zach Radding]

I know this thread is a little old, but thought someone searching through the archives, might like to see what I did for a DIY fume filter. It cost a bit more than $200 (actually double that), but I posted a step-by-step on how to build it here:
http://www.instructables.com/id/Buil...ume-extractor/

[Jerome Stanek]

I am thinking of something like that also. I cut a lot of acrylic.

[Dan Hintz]

what I did for a DIY fume filter. It cost a bit more than $200 (actually double that)

Kudos to an interesting design, Zach. Very modular in nature, and I bet if you removed the fan from your parts list, the final cost will be a lot closer to my $200... $60 for the filter, $100 for the carbon, and $40-50 in miscellaneous.

That said, I do notice you're not using a HEPA filter in yours, and that can be a big deal when handling a lot of the junk that comes from burning materials like acrylic and wood. Also, if I view your design correctly, the laser fumes will hit the filter before the carbon... that will clog your filter very quickly when you're dealing with high-particle materials like wood. To be fair, mine suffers from a similar malady where the carbon gets gunked up fairly quickly (the HEPA comes after the carbon on mine), but that's much cheaper to replace.

It's funny you should bring this thread up, as I was just thinking of a new design this morning, more of a stackable setup than inner-to-outer, but I still wish to improve upon my earlier design (both in terms of efficiency and in ease of replacement for each section/cartridge). I was leaning towards an aluminum extrusion design this time around, though the details still need hashing.

[Glen Monaghan]

I went with a whole-house filter that is nominally 16x19x4 inches to serve as a relatively inexpensive sacrificial prefilter. I also found a 16x19x4 HEPA-class filter so decided to go with 16x19 for the activated charcoal (AC). Because of the amount of AC in the bag I bought, I opted to go with a 16x19x5 tray of AC (which is actually more like 4.5" of AC because I used an adjustable-sized foam furnace filter on the bottom of the tray to retain the AC), then decided to allow for an optional second 16x19x5 tray of AC, both because I had enough AC to do it and because the commercial unit I looked at appeared to have about 8" of AC so I thought I might need or at least want it.

I find that a single AC tray is fine for cutting BB but have been using both AC trays because using just one lets too much smell through when cutting acrylic. Sufficient space was available so I chose 4 inch thick, deep pleated 16x19 pre- and HEPA filters to provide a large surface area for lower restriction and longer life, and made all four filter bays big enough to handle any filter so that I could play around with different combinations if desired. I've placed the HEPA after the AC to minimize possibility of carbon fines making their way into my lungs.

I had laminated sheet stock left from building a cabinet for the laser to sit on (and to hold some drops in progress, fixtures for often-repeated jobs, a few tools and cleaning supplies, etc.), so I used that to make a matching filter tower that sits beside the cabinet. Opted for a down draft arrangement, with inlet being smooth 8" duct going to laser (transitioned there to match laser's 4" port) and outlet being a 12x12 inch grille at the bottom of the tower as shown in the image.

[Zach Radding]

Hey Dan, The general modular design is not mine. It was highly "inspired" by the Electrocorp design, but modified to make it easy to build DIY. The filter I am using claims to be HEPA. The pre-filter takes a good portion if the particle load before the HEPA. We'll see how long it lasts....

[Dan Hintz]

Glen,

The stacked design is what I'm designing around this time around, but I'm going to attempt one without an outer cabinet to reduce weight. I'm thinking each stack will have tubes on the corners that threaded rod will fit through. Once you have everything stacked up, a wingnut at the top will snug every layer together.