Acrylic exhaust filter problems

[John Fehr]

Hi Everyone,

I'm starting to cut acrylic, and while I'm exhausting outside, the blower is inside, so leakage occurs. My neighbour is also pretty close so I'd like to reduce the smell and hazardous fumes as much as possible. I originally used the built-in filter with my versalaser cart, which has a pre-filter, heppa filter, and activated charcoal. I wanted to use an external exhaust because you can't use the advanced panel with the computer-controlled exhaust.

Someone wrote an article describing how to make a sub-$200 filter that should work with acrylic, but since I have no supply of activated charcoal, I was stumped, until it hit me. The versalaser cart has a nice activated charcoal filter, and an easy to refill container if it ever ran out.

So, with the help of a good friend, I built a box with a pipe to the blower at the bottom, then a shelf for the charcoal above, then a shelf for a pre & heppa filter, a pipe to the laser above that, and then a lid. The two pipes are quite solidly in there... No leaks there. The lid isn't sealed well, but since the air is being sucked out of the box, air shouldn't escape there; if anything outside air will be sucked in, so there shouldn't be a leakage.

After all that, I tried it, and

#1) I noticed a lot lower air flow. I'm using the DC3xx, and its quite noticably weaker at the laser end of the filter. I ordered a DC250SEMB to compensate (1350 cfm vs 850 cfm) so hopefully I'll get somewhere close to my original airflow when its hooked up.

#2) It still stinks in the room with the exhaust fan, as well as outside at the vent.

Any ideas what else I should do? The filters are just laying down on their shelves, would it help to put some kind of rubber sealant for them to lay on? Should I have the filters order reversed so that the laser air is sucked into the bottom, through a pre-filter/heppa, and then up through the carbon? (IE is the suction packing the carbon so much that air isn't being sucked through, but rather around the edges where its sitting on the platform?)

Any help would be greatly appreciated.

John

[George D Gabert]

I do not know about the order of the elements, but a higher CFM blower may not help you because it is restricted flow. I think what you need is a blower with a higher suction head pressure to draw more. Or move the blower in front of the filter and use a higher output pressure blower.

Regards
GDG

[John Fehr]

But if I put the blower in front of the filter, we have positive pressure inside the filter, so the air would leak out. We also still have the problem with the blower itself leaking slightly.

Isn't higher CFM equal to higher suction head pressure? I know its my old blower is 1.5HP, and the new one is 2.0HP, so there should be more pressure.

I do not know about the order of the elements, but a higher CFM blower may not help you because it is restricted flow. I think what you need is a blower with a higher suction head pressure to draw more. Or move the blower in front of the filter and use a higher output pressure blower.

Regards
GDG

[Doug Griffith]

As a side note, extruded smells much worse than cast. Plastic liners smell worse than paper liners.

Also, my external exhaust runs up the side of my house and ends about 2 feet above the roof-line. It works well. I mounted a 'loop' on the eave facia so I can slide the pipe up into it and remove when not needed.

[Dan Hintz]

I assume you're talking about my article for the sub-$200 build. The place to order activated charcoal from is listed in the article, and it will cost you less than $100 for a 50-pound bag. The charcoal was half of the total cost of the filter.

Also, higher CFM does not necessarily equate to high static pressure, though as you move up in HP you often get more of both. That said, my initial design was working just fine with a Harbor Freight 1HP red-can DC.

[John Fehr]

Hi Dan,

Great article! I was going to attempt to build it when I ran into the charcoal supply problem, and I realized I already had a box of about the size you mentioned in the article from the computer-controlled exhaust cart I was using previously. If that runs out I'll definitely look at the supplier that's mentioned in your article.

Are you talking about the 1 hp mini dust collector (http://www.harborfreight.com/1-hp-mi...tor-94029.html) at harbor freight? 914cfm is more than my DC3XX puts out at a fraction of the cost. I might just cancel my order for the DC250SEMB if the 1 HP does the job.

One thing I noticed different on yours is that mine is a top to bottom airflow system, where yours is bottom to top. Do you think the compaction of the carbon is causing the restricted air flow?

Did you have to do any special sealing around the filters so that air doesn't just go around the filters instead of through them? The carbon I have is about 3" thick, plus the box its in is designed in such a way that the air comes in from the top on one side of the box, and out the bottom on the other side of the box, so the air actually has to go through about 6" of carbon.

I assume you're talking about my article for the sub-$200 build. The place to order activated charcoal from is listed in the article, and it will cost you less than $100 for a 50-pound bag. The charcoal was half of the total cost of the filter.

Also, higher CFM does not necessarily equate to high static pressure, though as you move up in HP you often get more of both. That said, my initial design was working just fine with a Harbor Freight 1HP red-can DC.

[Joe Pelonio]

I had leaks on my installation with the fan inside, but it was easy enough to fix with silicone and duct tape over all the joints.
My 400 CFM fan is about 4' from the laser, then another 3' to the wall. Another thing that helps is to keep a window open so that you get a flow of fresh air going into the laser, preferably on a wall far from where the exhaust exits the room. The only smell I get now is when I open the laser after I finish cutting and that can't be helped.

[Larry Robinson]

I would test all the connections on the discharge side with soap suds. Even the smallest leak can produce a noticable oder.

[John Fehr]

But with proper filtration (similar to Dan's $200 setup) all the odor's/fumes should be gone, right? Dan is exhausting back into his house, so any leaks after the filter box shouldn't produce any odor. Any leaks before the filter box shouldn't matter either, since there's negative pressure, and the leak would be in, not out.

I did buy some foam rubber so I'll see if that seals the leak around the filters themselves...

I would test all the connections on the discharge side with soap suds. Even the smallest leak can produce a noticable oder.

[Larry Robinson]

I forgot to mention, make certain you check the blower housing as well. I found a couple of leaks in the seam area and on the housing where the air is discharged.

[John Fehr]

Hey Larry,

I just have the blower input attached to the laser output, and the blower output attached to the outside venting hose. I notice a lot of air coming out of the blower when its running, but how do I know if that's from the motor or from the laser exhaust?

John

I forgot to mention, make certain you check the blower housing as well. I found a couple of leaks in the seam area and on the housing where the air is discharged.

[Dan Hintz]

I forgot to mention, make certain you check the blower housing as well. I found a couple of leaks in the seam area and on the housing where the air is discharged.

As John pointed out earlier, if the air has passed through the filter before hitting the DC, leaks don't really matter (from a safety standpoint, anyway).

[Larry Robinson]

John,

Put soap suds on any seam of the housing, bolt holes, or where the discharge of the blower is. I noticed a definite odor close to the blower. I took a paint brush with some soap suds and found three seperate leaks. Put silicone on these areas and all is well.

Dan,

There is still positive pressure in the blower and if not sealed, you can get leaks.

[Dan Hintz]

Dan,

There is still positive pressure in the blower and if not sealed, you can get leaks.

Sure... but the point was the nasty stuff has been removed from the airstream before you ever get to the positive-pressure portion of the DC (the exhaust). I smell a faint odor of wood after a long plaque burn, but I'm also venting everything right back into the room. Leaks after the filter aren't that concerning.

[Larry Robinson]

Sure... but the point was the nasty stuff has been removed from the airstream before you ever get to the positive-pressure portion of the DC (the exhaust). I smell a faint odor of wood after a long plaque burn, but I'm also venting everything right back into the room. Leaks after the filter aren't that concerning.

.

Dan,

You are correct.