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Larry Robinson
02-03-2011, 3:12 PM
Last September, I built my own fume extractor. It seems to work well. There is hardly any odor and zero smoke. The only problem is my eyes start to burn after about a half hour of operation. I cut mostly Birch Plywood and Hardboard (lots of smoke). My wife says her eyes will burn when she is near the extractor, however, other people tell me there eyes are fine.
The extractor is a stacked design as suggested by Dan Hintz (great idea Dan). The bad air enters the top and uses filters that are 24” x 24”. First layer is a 1” Merv 7 filter, next is a 2” Merv 11 rated filter. Then a 12” deep Hepa filter. Finally a 3” deep bed of activated carbon. The air is drawn out the bottom with a HF 2HP, 1550 cfm blower and into my basement.
I also have a vacuum pressure gauge that helps me monitor each stage as far as pressure differential goes. The Merv 7 filter gets changed about a very 4 hours of operation.

My question is what stage would be responsible for the burning eyes; Hepa , activated carbon or maybe my blower is too big and drawing the air too fast through the carbon.

Robert Walters
02-03-2011, 3:37 PM
Your Merv and Hepa filters just filter out solids (particles).

It's the activated carbon that absorbs chemicals/molecules.

The more surface area the air stream has in contact with the activate carbon, the better the results.


A photo or two of your setup may help.

Dan Hintz
02-03-2011, 6:21 PM
If you're using the full power of the blower, you may need a thicker bed of carbon... you have to give the fumes time to bond with the carbon rather than sucked right past.

Larry Robinson
02-04-2011, 9:27 PM
Robert,

Thanks for your input, I will get some pictures posted once I get it working 100%

Dan,

Thanks, you confirmed my thought of adding more carbon. Just did not want to replace the Hepa filter for the wrong reason.

I noted others who built there own fume extractors limited their carbon bed thickness to 2-1/2" and they said it worked fine. I thought 3" would do it.

Will try 4" next and see how it goes.

Dan Hintz
02-05-2011, 10:39 AM
You shouldn't need to replace the filters, but I would suggest cleaning them out as much as possible. It will cut down on airflow, but not cause the eye burning issue you are experiencing.

Frank Defert
02-05-2011, 8:57 PM
A question for Dan. I too have built a fume extractor based on your design. It works well but not well enough.

The extractor has a surface area of 16 X 24 with a primary filter for particles, a 3in bed of activated charcoal and finally a Home Depot vacuum hepa filter for final cleaning. The dust extractor is 1 hp and 650 cfm, it draws extremely well and fully clears the laser. When I cut 3mm baltic birch a little smoke makes it's way all the way through the system along with some burnt wood smell.

The question is, how can I slow the extractor down possibly variable so I can achieve maximum extraction while slowing down the airflow over the carbon to maximize it's effectiveness?

Thanks .... Frank

Dan Hintz
02-06-2011, 10:35 AM
Frank,

You can slow things down, but are you sure smoke is actually making it out of the system? No matter how good your system is, there will always be a residual amount of smell hanging out inside the laser's cabinet, as well as lingering on the wood item itself. I enjoy the smell for what it is.

Frank Defert
02-06-2011, 1:33 PM
Dan,

There is a small amont of smoke making it's way out of the system at the hose end and there is always some smoke and smell within the laser. I try to minimize the smoke in the laser by allowing the exhaust system to run after the laser has finished the job. The cut wood pieces can also gas off for days so I have taken to storing them outside before they go to the customer.

I was reading above that if the ehaust were to slow down as it travels over the carbon the absorption rate of the carbon would be maximized. So really what I am trying to do is strike a balance between extraction and absorption.

Frank

Richard Rumancik
02-06-2011, 5:04 PM
It seems to me that slowing down the airflow by reducing the net CFM may be counter-productive. If your laser manufacturer is suggesting a certain CFM/pressure then I would tend to try to meet that as much as practicable.

A thicker bed of activated carbon should result in increased adsorption but may restrict airflow.

One way to decrease air velocity is to increase the amount of expansion in the filter chamber. e.g. if the cross sectional area of carbon is 1 sq feet, then increasing the bed size to 2 sq feet means that the air velocity would drop 50%. The carbon would be more effective as the amount of time the air spends in the filter is increased. It would also offer less impedance to the air. However, the downside of course is that the size of the filter unit increases. There might be some packaging tricks that could be done to keep it less than doubling in size.

Another way of looking at it is whether you should put more filtration in series (thicker charcoal) or add more filtration in parallel (increase bed size). I'm leaning towards increasing the bed size. But I admit I have not actually tried to build one.

Smell is the first warning sign of air contamination. Even though wood is a natural material it does not mean that the products of combustion are not hazardous. Burning eyes certainly should not be ignored.

Frank Defert
02-08-2011, 7:28 PM
Richard,

I checked the owners manual for my machine and the exhaust requirement is a minimum of 400 cfm @6" of static pressure. My machine is rated at 650 cfm, so there is more than 50% in excess of the actual requirement.

I understand that increasing the bed size / or thickness of the carbon would slow down the air and allow for more absorption. But then slowing down the speed of the air should achieve the same result?

I used to be able to exhaust to the outside of the building so that's why the large cfm extractor. This is no longer an option for me, so I am looking for a way to slow down the extractor to reduce the available cfm while having enough flow to clear the laser.

If after slowing down the extractor, hopefully with a variable control, and having enough flow to clear the laser there is still smoke and or smell I will increase the size of the carbon bed or add a second bed. I would then have to gradually increase the air flow to compensate for the increased carbon and strike a balance between air flow and absorption.

So my question is how do I slow down the speed of the extractor, hopefully with a variable controller, to reduce the cfm of my system? The extractor is a direct drive 1hp 650cfm General dust collector and the carbon bed is 16 X 24 X 3.

Thanks,
Frank

Dan Hintz
02-08-2011, 7:57 PM
Frank,

Is that 650CFM @ 6" or simply the advertised 650CFM (which generally means at 0")? Huge difference...

Also, how much time have you put into that charge of charcoal? If you've been using it for a while, it may be time for a replacement charge. Did you weigh the original charge as I suggested in my original build thread?

Russell Ludwick
04-18-2011, 1:05 PM
Can anyone link Dan's original article. i couldn't seem to find it in my search. I have been doing a good amount of wood and I would like to build a better system. I think I am going to follow laser genius Dan's advice and build a good home made filter.

Dan Hintz
04-18-2011, 1:24 PM
Can anyone link Dan's original article.
http://www.sawmillcreek.org/showthread.php?143725-DIY-air-filtration-unit-for-lt-200