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View Full Version : Rugged outdoor laser cutter air scrubber with humidity control



Keith Colson
05-30-2014, 12:40 AM
So I visited some filter suppliers and materials suppliers this week. I got some horrendous prices but found some fair prices after hunting and calling around. I found the standard filter size here is 592 x 592mm so my whole design is based on that.

I thought I would run it by you guys with the experience before I start ordering and cutting materials (which will be mid next week I hope). The bottom section is fire resistant with metallic cardboard.

So here is a quick cad job with some annotations. The main dog house is made from 20mm Corelight PVC composite (totally waterproof). I have added a humidity sensor which will turn on the light bulb so I can drive moisture out during winter. There is also an air driven outlet flap that will gravity close when no air is running. Bryce is there so you can visualise the size, 1.2 meters tall.

Yes, the centrifugal air pump is before the filter. I know that running it after has some advantages but I am not changing my blower configuration which is already nicely installed. At least the pump is outside!

Can anyone recommend how much carbon I should put into the tray? I have designed it so I can change the tray thickness easily. This is the only custom tray, everything else is off the shelf including the tray frames.

Here are some pictures which explain things better. My extraction/blower is 1100 cfm through 200mm ~8"duct.

Cross section
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view1
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view2
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I might get some change from a 1,000 bucks which I think is pretty good. Constructive feedback and ideas welcomed.

Cheers
Keith

Adrian Page
05-30-2014, 7:43 AM
If there is any condensate it might fill your light socket. Maybe move it to the side of the box? The filters must be very expensive... it looks like you should get change from 200 bucks.

Adrian

Glen Monaghan
05-30-2014, 1:16 PM
How much carbon would depend on the carbon, what you're cutting, and your goal. I originally tried activated carbon produced from approximately 1" chunks of coconut husks and, subjectively, it did little for me. Switching to an equal thickness of activated carbon pellets sized on the order of 1/8"x1/4" made a huge difference. My filter stack (4" thick whole house filter as prefilter, followed by activated carbon and 4" thick HEPA filter trays) is used to recirculate conditioned air inside the laser room so I needed to virtually eliminate perceptible odors and particulates (smoke) from the laser exhaust. For me, 4" of carbon was helpful but inadequate. It took 8" to make cutting acrylic possible without significant odor.

No idea what you are cutting or what your environmental needs are, but a lot of people exhausting outside don't filter at all, unless there are neighbors complaining or possibly pollution-type regulations involved.

Robert Walters
05-30-2014, 2:44 PM
Light bulb...
You might want to use one of these "cage" type fixtures:
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The main thing about the carbon is you want as much surface area as you can get;
1. To do it's job in filtering, and
2. Not to be too much of a resistance to air flow.

Don't ask how I know about air flow restriction thru carbon pellets, it's not a pretty story =(


You said 1100 CFM, but at what static pressure?


24" x 24" = 576 sq inches.
Let's say 4" depth, gives you 2304 cubic inches.

Now, if you knocked your box on it's side...

24" x 48" = 1152 sq inches.
4" depth = 4608 cubic inches.

and you would have a larger surface area for air to enter and be distributed more evenly.


Also, if your shop is heated/cooled...
You might want to recirculate the exhaust from the dog house back into your shop like Glen mentioned.


If your shop was 20ft x 20ft x 8ft tall = 3200CF:
At 1100CFM, That means you potentially can suck out all that nicely warmed/cooled air in under 3 minutes!
Plus, if you are exhausting that much from the room, you have to have some inlet air from outside too.

Adrian Page
05-30-2014, 2:50 PM
Agree with Robert on the replacement air. It would be a good idea to install a 4" or bigger air duct from outside to feed the laser... rather than suck it in through random cracks and holes in the building. I have a fresh air duct to my homes fireplace for the same reason. Works great.

Adrian

Keith Colson
05-31-2014, 3:43 AM
Hi Guys

Thanks for the feedback so far. I have made some changes. The light bulb has gone as it looks like a weak spot. The whole key as the title says is to make some thing rugged. I have ordered some 15 ohm 50 watt resistors (3 bucks each). Running off a 19,5V laptop power supply (which is cheap) will give me 50 watts of heat with no bulb to blow and condensate on to etc. I will also put the laptop power supply inside the box so its heat will also contribute to the heating. I can run my humidity sensor off that supply too. I will add an external led so I can see when the heating is on.

290380

My current charcoal tray is 80mm thick or 3.15" so I will start there and see how it goes. I don't need to knock off all the smell as it is outdoors. I just want something that knocks of a majority of smell and smoke.

My laser cutter is in my garage and the sectional door has plenty of gaps to pull in fresh air "when it's closed". I don't want to bring the filtered air back inside.

The stuff I am cutting is 75% acrylic up to 6mm think, 10% acrylic >6mm, 10% thin plywood and MDF. The rest is prollly polycarb, ABS and other stinky crap.

Static pressure - I have no idea or way to measure it. All I can say is I have 200mm ducting on a 1.1kw centrifugal fan. Here is a photo of what is outside so far.

290381290382

Cheers
Keith

Brian R Cain
06-01-2014, 6:11 PM
How much carbon would depend on the carbon, what you're cutting, and your goal. I originally tried activated carbon produced from approximately 1" chunks of coconut husks and, subjectively, it did little for me. Switching to an equal thickness of activated carbon pellets sized on the order of 1/8"x1/4" made a huge difference. My filter stack (4" thick whole house filter as prefilter, followed by activated carbon and 4" thick HEPA filter trays) is used to recirculate conditioned air inside the laser room so I needed to virtually eliminate perceptible odors and particulates (smoke) from the laser exhaust. For me, 4" of carbon was helpful but inadequate. It took 8" to make cutting acrylic possible without significant odor.

The flow rate that fume passes through the carbon is an important factor. If you have it too high, the carbon doesn't have the chance to absorb the chemicals. The filtration systems you can buy don't usually have anything like this depth of carbon but as Robert says, have enough surface area to suit the fan they use. In other words, a system that will cope with a 500 CFM requirement has filters that are around 15" square. Go up to 1,000 CFM and the filters are around 24" x 30". They usually only have a couple of inches depth of carbon though on the standard ones or for high acrylic use they have others with double this depth.

Keith Colson
06-01-2014, 9:40 PM
Thanks Brian, it sounds like the 3.15" of thickness for my 24"x24" filter fits your comment quite well as a starting point. Thanks for the feedback.

Cheers
Keith

Glen Monaghan
06-01-2014, 11:27 PM
My memory of the air filtration unit I examined at the NBM show last year was that it had on the order of 8 inches of carbon, although that may have been to provide the 1 year life they claimed for it.

Brian R Cain
06-02-2014, 7:22 AM
Did you actually examine the filter or just see it in a unit? A lot of these filters are a combined HEPA and Carbon unit. The unit is around 8" tall but there is a void in the bottom to help spread the fume and the HEPA part is around 4" deep. Typical life can be around a year, but not in a production environment and it's entirely dependent on what you use the laser for. Nobody can give a definitive filter life without knowing precisely which materials the machine is being used with and how thick the materials are. Obviously you will get double the volume of chemical in the fume if you cut 1/2" acrylic compared to 1/4". If all you do is engrave acrylic the amount of chemical is relatively small and filters would last for several years.

Generally, the HEPA filter gets blocked first causing the system to alarm and the whole unit needs replacing long before the carbon is saturated. Materials that produce a lot of smoke are the culprits. Card is among the worse, which often surprises people.

I had a school find their filters getting blocked after a couple of weeks, but on investigating what they'd been cutting, it turned out they had only been using plywood. The glue vaporises to particles small enough to pass through the pre-filter and then coat the HEPA filter with a layer of glue that prevents air from passing through.

Filtration systems can be expensive to run if you don't give any thought to what you put through them. The only thing you can really say is that they do what they are supposed to do and catch all the crap that you might have been breathing in. In some instances, they are the only option, but I was always careful to let my customers know that although they might get a year between filter changes, there is no guarantee, and the only way they will find out is through usage. So long as filters are costed as a consumable when you price jobs, you can live with it. Ignore this cost and you will be forever cursing when they need to be replaced.