Ducting

[Shawn Handley]

Greetings,

So the exhaust on my laser set up is not doing what it should be doing. I have 3 fans with 3 90 degree angles and about 20 feet of that split seam duct pipe.

It was working great, then it failed because air started leaking through the 10 foot length of metal seamed pipe. I taped and sealed everything but after a few months, it leaks again.

My question is, would I be better off with just flexible metal pipe throughout the whole setup? I have a major fan at the bottom right after the Y split in my Legend, then a vertical run of about 6 feet with a fan in the middle to help push the air up and around the next bend, then one 90 and a 10 foot length, then another fan like the vertical one, and a quick bend up and out the roof. I have flexible duct between the engraver and first fan, and flexible from the 1st fan and the next fan but the rest is all the metal duct.

[Joe De Medeiros]

if all you want to do is seal the duct, there is a acrylic latex duct sealer that you brush on, when it dries it stays flexible, I don't know where you are located, but in Canada you can get at any HVAC suppy like Don Park. Flexible duct is the worst thing you can use for long lengths, it increases the friction loss and therefore reduces fan performance.

[Dan Hintz]

Your 3-fan setup is probably your biggest failure in the setup... each fan acts like a restriction to the others. You're better off going with 1 larger fan. Get away from the 90 degree angles, too... try to go for two 45 degree sweeping bends separated by a foot or more, and only use bends when absolutely necessary. The fan should be at the end of the run, not the beginning or in the middle... pulling air rather than pushing it means leaks don't end up pushing fouled air into your shop, it can only suck clean air in.

[Shawn Handley]

Thank you both for your replies.

The only reason I don't think that the 90s and fans are a problem is because there is more leakage with the major fan on. It is acceptable with just the two fans on and the major fan not running. I can cut plastics and not small a thing with just the two on but if I turn on the third, I can smell it almost right away. I checked for leaks and it only comes through the seams. I checked the pipe on the roof and it's blowing pretty hard (I do not wish to check cfm at the beginning and at the end of the setup so please do not ask). The 90s have to stay. It's the only way through the ceiling and stuffed trusses. I wish I could change to a 45 but lets assume we can't. It makes a lot of sense that the major fan will push so hard that the other fans are resisting. I do have one fan at the beginning and one at the end but one in the middle. I'm afraid I am not going to be paid to trial and error otherwise I would pull each one out and see if it makes a difference. I put in the other two fans because the major one didn't do it's job (tuned out to be stuffed full of crap).

I will try the acrylic latex. Unfortunately we don't have a Don here in BC. I'll keep my eyes peeled.

Thanks again to both!

[Mike Null]

I second Dan's suggestion but I would use a Y connect from the lasers to the fan. A Y connector with blast gates will give you a far more efficient result.

[Bren Kano]

What type of tape have you been using to seal the duct? Try aluminum foil tape if you haven't already.

[Shawn Handley]

I started with duct tape and of course it failed :P So then I used aluminum foil tape. It took muuuuch longer but it failed too.

[Bren Kano]

Adhesive fail? Applied on the best of conditions they should last years. Sounds like you have a lot of unwanted back pressure in your system..

[Shawn Handley]

It is very likely there is back pressure; however, the shop it is used in will go from 40 degrees Celsius to -5 degrees Celsius (104-23 F) over a year (during off hours of course. We do monitor the temperature when someone is in the shop). I've seen adhesive fail at a much smaller delta temperature. I'm sure this is a combination of issues. The pipes are clean and clear too. I figure someone would suggest that but I clean them once every two months or so.

[Dan Hintz]

Adhesive on that tape should have no problems holding at those temps (at least most of 3M's tapes are rated down the low)... ductwork in the attic is subject to those temps every year in some states.

Excessive backpressure would likely be due to having more than one fan. I simply see no purpose (and only harm) to adding more than one in the line... if the one originally installed didn't provide enough inches of water at the desired CFM, increase the duct diameter (if that's the limiting factor) or replace the fan with something more powerful.

[Barb Macdonald]

AbigailRun.jpgAbigail.jpg

[Dan Hintz]

Pics are on the small side to see any real detail, but I see a bit of unnecessary restriction right behind the laser with two 90 degree turns right before the 'Y'. Raise the blower off of the floor level with the outlets and make that run a straight horizontal shot... at the very least, make it a straight angle downwards. I can't tell what the output of the DC does, pics too small. If you have fans after the first DC as mentioned earlier, it's a problem. Also, having the DC so close and "pushing" the air is not only a poor use of the DC (they want to suck, not blow), you're blowing fumes from even the smallest of leaks back into the shop.

[Mike Null]

Try shortening the connector duct between the laser and the blower. Whatever amount you shorten it will unrestrict the air flow from the machne to the blower.