Laser exhaust

[PlainName]

(I suspect it would have been simpler and cheaper to buy a CO2 laser, but I thought that would take up too much space...)

Built a box for the laser so I can exhaust the fumes. As it happens, I am stuck on PLA for 3D printing because I don't have an exhaust system, so I took the opportunity to fix both of these by having a 110x55 ducting fixed behind the boxes which leads out via a 120mm mains fan.

The laser box is made from a wooden frame and hardboard sides, except the front which is red perspex. A single 80mm fan feeds out the back to a 100mm duct which leads to the main ducting. The front perspex has a row of holes at the bottom for inlet.

The problem is I get some fumes coming out. Nothing like what goes to outside, but it's enough to worry me. The box isn't airtight but I figured air would be going in rather than coming out. Or maybe there is a leak in the ducting.

How can I find where the fumes are leaking? There isn't sufficient (at least in a concentrated way) to feel anything by hand or waggle a piece of tissue.

(The ducting is open from the laser to the 3D printer, but the external fan is sufficient to stop anything doing down that route - I can put my head in the printer and not smell a thing.)

[james_s]

It doesn't take much of a leak for fumes to get out. Do you have or know anyone who has a fog machine? If not, you can probably find a deal on one in a few weeks when the Halloween stuff goes on sale.

[PlainName]

I started this at just the right time, then!

Good idea. I think I might have a smokecloak around somewhere which could be resurrected. But I'd have to ensure it's not pumping in but being sucked in.

[Ian.M]

Cut the power to the duct fan and block off the duct branch to the 3D printer so only the laser enclosure fan is running, which will give you slight positive pressure in the ducting.  You can now check for leaks using the smoke from a joss stick held below the duct at the joints.

If you don't find any leaks, its likely the 80mm fan isnt scavenging the fumes from the laser enclosure effectively.  A gate or cap to block off the printer duct while the laser is in use may help, but odds are the airflow within the enclosure  is causing eddies back out the vents, so you may need to add some baffles to get unidirectional flow rather than circulation.   Again investigate with a joss stick just outside it.

[PlainName]

Wouldn't a positive pressure in the duct prevent air being sucked into the box, so anything in there would leak out through the (intentional and otherwise) holes?

likely the 80mm fan isnt scavenging the fumes from the laser enclosure effectively

Could be, yes. I can feel the air being sucked in at the front, but there might be a dead spot with no airflow in a corner, say.

Joss stick is a good idea 

[Ian.M]

No, because the 80mm fan at the back of the box providing the (miniscule) positive pressure in the duct will still be drawing air from the box.

Also, do the joss stick leak tests with the laser off.   The duct leak test should be with just the 80mm fan on, and the printer intake capped or blocked, and you may also need to cap/block the exterior outlet to get enough duct pressure to show small leaks. 

As you are looking for eddies and backflow, do the box leak test with all fans and duct openings operating normally.

Also, where is the replacement air drawn from?  You may be sucking in fumes from outside, near the outlet!

[PlainName]

You may be sucking in fumes from outside, near the outlet!

Yes, that had occurred to me - the outlet is right next to the window I am sitting by. However, it's not been opened for many years and I would notice the breeze from some of the storms we have here. Nevertheless, it's on my list to check

Thanks for the other instructions, which I will run through a bit later.

[Brumby]

Use a smoking taper or a cigarette and move it around while the system is operating.

You will soon see any undesirable air currents.

[PlainName]

My office now smells very nice, but I have to say that trying to spot white smoke against white ducting which is mounted on a white wall is the kind of test a Microsoft GUI designer would be proud of. Nevertheless I think I've spotted the problem (manifold attachment downwind of the extractor fan but still inside the office). Could be a pain to fix though.

[james_s]

Could you relocate the extractor fan to the end of the duct outside?

[PlainName]

No. It's not weatherproof and making it so outside, with power and everything through the walls, isn't going to run.

Its is one of these and I think the problem is where the exhaust tube attaches to the fan. It's shaped metal and there is no gasket between that and the fan body. Should've thought of that when I put it in, but I was concentrating on making the tube-to-duct joint airtight. That bit's OK

[PlainName]

Fixed. The issue was cause by the hole in the fan housing for the cable feed-through. I neglected to take a photo and ain't going to pull it all apart again just for that, so the attached image is a generic fan (clearly not a mains one!) which shows what the problem was. Being on the downwind side, it's an extra exit to blow air out of.