Configuring an exhaust system using outside air

[Bill Bangs]

Does anybody have experience setting up an exhaust system that uses outside air (instead of conditioned inside air) as an air source for a laser? While I hope it is as simple as configuring duct work with sufficient capacity to the front of the machine, I’m very interested if there are any challenges that might not be obvious.



We have a 60 watt Epilog Helix 24 which is using two Vortex VTX1000 fans to pull air through a carbon chip filter system. We are considering a new industrial space for our workshop. As we understand it, Denver building code requires that the exhaust system does not create a negative airflow from other rooms. It also requires any air that is added to our laser room to be conditioned. Consequently we would prefer not to continuously pull external air, condition it, then pump it outside again. Ideally we would like to duct unconditioned outside air directly to the front of the machine, pull it through the laser/exhaust system, and push it back outside.



Any guidance is much appreciated!


Thanks,
Bill

[Scott Shepherd]

I would think that would be a disaster on rainy or high humidity days. You'd be pulling moist air right through your metal machine and optics. That would be my concern.

[Dan Hintz]

Exactly what Steve said. I'll also add, any reason why you don't want to filter (you're already pulling through carbon) and exhaust back into the room? No need for makeup air then.

[Bill Bangs]

Thanks Scott and Dan

Scott - good point about rain. Fortunately humidity in Denver is quite low compared to other areas of the country. In reality, our swamp coolers probably add more humidity to interior air than we usually experience outside. Nonetheless, we would definitely want to shield any air intake from rain and possibly stop cutting during the occasional heavy rain storm to avoid any short-term high humidity.

Dan - good question about venting inside. I forgot to mention that we cut 1/4" baltic ply that is painted and taped. Even with multiple washable furnace filters to catch particulate and 2" of activated carbon downstream we have not been able to scrub the air to a degree that anyone would want to breath it. We have experimented with ways to cut before painting or cut painted wood without taping in an effort to remove some of the offending exhaust. Our challenge is the post cut processing and handling for both those methods is too time intensive. Any ideas about other ways to economically scrub the air would be appreciated as venting inside is an interesting concept.

[Scott Shepherd]

Scott - good point about rain. Fortunately humidity in Denver is quite low compared to other areas of the country. In reality, our swamp coolers probably add more humidity to interior air than we usually experience outside. Nonetheless, we would definitely want to shield any air intake from rain and possibly stop cutting during the occasional heavy rain storm to avoid any short-term high humidity.

I'd be thinking about any precipitation, snow included. Any time you'd have that, I'd be concerned, and even now, more than before, since you mentioned what you were cutting. I'd hate to introduce outside humidity into that type of product line, much less my laser.

These numbers below are numbers that would concern me for all of the above reasons...

310488

That's a much higher number than I'd be comfortable with.

[Don Corbeil]

As Steve posted, even though there's a fairly low relative humidity on average here in colorado, the fluctuations are always there.
Think of the summer monsoon season, which hits all of colorado, where the humidity can shoot from 7% to 60% in an hour.

[Bill George]

I doubt if the Local Code requires the Make Up Air to be air conditioned when brought in. Restaurant grill area MUA units are not. Its just outside air filtered and brought in right over the cooks head. It will be heated in the winter, but no AC. What we did on some jobs is open the economizer (if equipped) damper to be opened to a minimum outside air position when the exhaust fans other than kitchen hood is powered on. So whatever came in the economizer or outside air/return air system went thru the H-C system, in most cases a roof top unit. Sure it wastes energy, but the other alternative is a heat wheel system. Either one only work when the exhaust fan(s) are powered on.

You don't dump the outside air directly into the laser machine, its mixed with the supply air for the room.

[Bill Bangs]

Good point Scott - thanks for the included image.

[Bill Bangs]

Interesting point Bill - I'll check with the GC to understand the situation a bit better and see what is feasible.

[Samuel Espy]

In addition to the problems associated with humidity differences already mentioned, temperature difference (between inside and outside) would cause enormous issues as well. Really bad idea to draw cold air across your optical and mechanical system when it is at room temperature. Obviously same issues occur when much hotter outside. Besides expansion/contraction issues you don't ever want your optics much colder than ambient as vapors will condense much worse on them--like the "sweat" on the outside of a cold drinking glass in the summer.

[Bill George]

In addition to the problems associated with humidity differences already mentioned, temperature difference (between inside and outside) would cause enormous issues as well. Really bad idea to draw cold air across your optical and mechanical system when it is at room temperature. Obviously same issues occur when much hotter outside. Besides expansion/contraction issues you don't ever want your optics much colder than ambient as vapors will condense much worse on them--like the "sweat" on the outside of a cold drinking glass in the summer.

The method I described is one way that will not happen.

[Glen Monaghan]

I'd say only using 2" of AC is the reason you are not wanting to breath the filtered air if odor is the problem. Furnace filters only catch the really big particulates, most of the fine smoke and essentially all of the odor pass right through those. You'll need a final HEPA filter to remove fine particulates (both visible smoke and invisible fines), and almost certainly more carbon to take care of the odor. (Also, there are treated carbons that work better than virgin AC on certain specific VOCs/odors). The other variable I found is carbon pellet size. The big chunks of coconut-based carbon in those "herbal filters" are worthless for this, while tiny granules are too constricting on air flow. I'm using an intermediate size called 4x8S.

The commercial filtration machines I looked at had 4" to 12" deep AC containers, though I gathered the deeper ones were because they were trying to increase filter life and increase efficiency by allowing for more contact time with the AC granules as the air was getting sucked through at high speed. So, I planned for flexibility and built my setup with a stack of four 4" bays, the first being a deep pleated MIRV-13 prefilter, the last being a HEPA filter, and AC in the middle. I initially tried a single 4" deep bed of AC and found it so-so for cutting wood and simply not tolerable when cutting acrylic. Adding another 4" of AC (8" total) made a huge difference. No problems cutting wood at all and I don't normally notice any acrylic cutting odors until I open the lid at the end of a job (helps to leave the lid closed a while after cutting ends, but the cut acrylic still retains and emits some odor), but I do still notice the smell if I cut acrylic for more than maybe 20 minutes. Works for me because I rarely have to cut that much acrylic, otherwise I'd investigate whether treated AC granules are available for that application.

The down side to using deeper AC beds is that, for the same bed area, deeper beds restrict/reduce air flow more than shallow beds. I make up for that by using much larger filter cartridges with a lot more surface area than provided by the little cylindrical filters some others here have been using. Mine are 16"x19" (x4") because I could get both HEPA and MIRV-13 filters in that size, and I made my AC trays to match.

Interestingly, I recently had a plumber here installing a sink in my shop. While he was working on one end of the shop, I was 30' away cutting acrylic at the other end. After maybe 15 minutes I was getting a headache from the PVC cleaner and glue the plumber was using. Done with my cutting, I went over and saw both the cleaner and glue bottles sitting there with caps off. I asked if he would please close the bottles because the smell was getting to me. I then asked how he could stand to work with that stuff without wearing a respirator mask, since it seemed like the smell would soon destroy all your brain cells. He shot me an incredulous look and said he was just used to it and the smell didn't bother him, but the stink from whatever I was cutting was giving _him_ a serious headache...

[Kev Williams]

It would seem to me that if this 'Denver Code' was enforceable, there wouldn't be a wood shop anywhere near Denver....

[Bill George]

Nor a Restaurant. The only kitchen I ever saw BTW with AC and the Make Up Air unit with an AC coil was TGI Fridays. There may be more but we got out of a lot of the restaurant and bar business because we had problems collecting the bills.