PDA

View Full Version : Exhaust placement question



Chad Fitzgerald
11-03-2013, 8:32 AM
I need an opinion on which way you guys think would be best.
Currently, my exhaust runs down from my laser then 90 deg. for about 10 feet out the wall of thebasement. Works great but, it is out the west wall. I live on a hill with pretty consistent west NW wind. in the summer if my office window is open it will come back in, in the winter when not running it cold air gets in(i do have a flapper vent to slow it down but). Im using harbor freight red blower motor which works great.
My two choices for changing it are:
1: straight up about 12 feet into the attic are and just let it exhaust up there. (my attic is huge) my shop is an old dairy barn. basement is where the animals were, shop is in the hay loft with 12' ceilings. attic is the rest of the way to the roof. Lots of ventilation up there, its an old barn.

or
2: down into the basement, a 90 deg. then about 30-35 feet to the east wall.
I would prefer number 2 just because it would be easier to access, the power is already there, etc.
If I did 2, would i put the blower at the end of the 30' or would it be better to put it half way?

Any thoughts are appreciated.
Thanks Chad

Joe Pelonio
11-03-2013, 10:31 AM
I would not do the attic thing, you will find that it sends out fine debris all over the rafters anf also that the fumes come back in through the ceiling. I had a leased commercial space where I went through the drop ceiling 25' to the front, and it vented just fine with the fan between the laser and where the vent went into the ceiling. You just have to make sure all joints are well sealed. Ideally, you would place the fan outside so it's sucking rather than blowing so any leaks are sucking in fresh air rather than blowing fumes into your work space.

Glen Monaghan
11-03-2013, 10:32 AM
I'd much rather exhaust to the outside than into an old attic, "just in case"... However, for that long of a run, you'd be best using something much larger than 4" pipe, starting as close to the back of your engraver as possible.

Chad Fitzgerald
11-05-2013, 8:57 AM
makes sense as far as the attic goes. So if i go down into the basement(about 4' drop) then a 90 degree bend, straight for about 30-35' out the east side wall. Will it matter (as far as suction) where in the line i place the blower? i prefer to have it at the end just before it goes out the wall, but am wondering if half way would provide better results(15' sucking and 15' pushing the air)??
Glen, the exhaust port on the laser is 4", the port on the blower is 4", will a larger pipe in between make a difference?? to me that doesnt make sense but that is why im asking. I just dont know enough about this. My thinking is i cant pull 6" of air through a 4" opening????
thanks
chad

Joe De Medeiros
11-05-2013, 11:31 AM
makes sense as far as the attic goes. So if i go down into the basement(about 4' drop) then a 90 degree bend, straight for about 30-35' out the east side wall. Will it matter (as far as suction) where in the line i place the blower? i prefer to have it at the end just before it goes out the wall, but am wondering if half way would provide better results(15' sucking and 15' pushing the air)??
Glen, the exhaust port on the laser is 4", the port on the blower is 4", will a larger pipe in between make a difference?? to me that doesnt make sense but that is why im asking. I just dont know enough about this. My thinking is i cant pull 6" of air through a 4" opening????
thanks
chad

If you want to put the blower in the middle you can run 4" to the blower then use a 4" to 6" transition and run 6" the rest of the way, this way you reduce the static pressure on 1 half of your system. As far as sealing, you can get a latex duct sealer that you paint over the joints, it stay flexible and gives you a better seal.

Joe Hillmann
11-05-2013, 12:00 PM
I would suggest not running it into the attic, the smoke WILL find a back inside. I used to have a blower in an area between two roofs, the area was wide open and the end of the pipe was only about 6 inches shy of sticking out past the edge of the roof and. Somehow the smoke was able to get its way into another attic area that was separated by a brick wall then travel 80 feet to the front of the building, into the balloon framing and down into the where it would collect into a haze. If you vent it into your attic you will have much worse problems.

I would also put the blower as close to the end of vent as possible. In fact I would put it at the very end and mount it to the outside of your barn that way you won't have to worry about the noise. I would also make the run between the blower and laser 6 inch pipe, even if your blower and laser only have 4 inch outlets, it will reduce friction. And make sure you. Instead of a 90 degree angle two 45's would reduce the flow less. And seal every single joint, where the pipes butt together, on the long joints where the pipes are joined when they are rolled up and snapped together, and if you are using adjustable angles tape all those joints as well.

Glen Monaghan
11-05-2013, 4:02 PM
Chad,

Every pipe has a "resistance" or "impedance" to air flow through it. Smaller diameter means greater resistance/impedance, larger diameter means less resistance/impedance. Think of how much harder it is to suck a drink through a skinny cocktail straw versus a fat "big gulp" straw. Hook a pinky-thick "watering" hose to your exterior water tap and measure the flow rate, then replace with a fat "gardening" hose and see how much greater the flow rate is!

Your blower is rated for a given CFM at various "impedances" or loadings (usually stated as inches of water or similar). The 4" laser and blower ports remain restrictions or loads of their own, but the pipe that interconnects them is a separate variable that adds _to_ the ports' impedances. Connecting the ports with 35' of 4" pipe will impose considerably more impedance to air flow to your system (and thereby lower the CFM moved) than the same length of 6" or 8" pipe. Likewise, using smooth walled piping is much better than using the flexible aluminum stuff which in turn is hugely better than that spiral wire dryer vent stuff. It even makes a measurable difference to replace your 90 degree bend with two 45's... keep the path as smooth and straight as possible, and especially so the longer the run gets.

Put your blower at the far (exhaust) end of the setup for the oft-cited reason that it keeps the entire duct under negative pressure so any leaks will result in clean air getting pulled in with the laser exhaust, whereas putting the blower at the laser end puts the entire duct under positive pressure so that any leaks will result in exhaust leaking into your duct space and possibly back into your work space. Be sure to properly seal every joint/junction in the ducts.

Kev Williams
11-05-2013, 4:22 PM
I don't know how many times I've found the inlets of my blowers (which are outside) almost completely plugged with pieces of tape, plastic baggie or whatever else gets sucked in that I didn't know about. But even nearly plugged they evacuate my laser's smoke and fumes just fine. Also, I'm expanding my basement shop into my garage, and I just put one of the small "red" dust collectors up in the garage attic, and connected 3" cheap black ribbed outdoor drain pipe to it, to use as an engraving chip vacuum. I have 30' of pipe connected to it, and even with ribs and small diameter the thing sucks air like gangbusters! On the exit side I find no noticeable airflow difference with the inlet hose connected or disconnected...

Other than money burning a hole in one's pocket, I fail to see the need for 6" pipe to suck up laser smoke...

Glen Monaghan
11-05-2013, 4:34 PM
Maybe the difference is in the blowers and filters. I'm using a quiet in-line blower that is probably rated much lower than the "red devil" and you apparently have no filtration while I have a HEPA and several inches of activated charcoal filter in line that further degrades air flow, so I get a noticeable drop in airflow with 4" pipe. Bottom line, the larger pipe will have less impedance, as will straighter runs and smoother pipes (just read up on most any wood crafter type article on dust collection systems). More power (and noise) can make up for restrictions, and/or you may find the reduced output insignificant for your situation. Not so for me.

Chad Fitzgerald
11-05-2013, 6:39 PM
ok now it makes sense to me. thanks for the info.
one last question, i have pvc pipe now, seems to work fine, do i need to go to metal duct or can i stay with pvc.

Henri Sallinen
11-06-2013, 3:31 AM
PVC pipes can build up an electrical charge that could theoretically spark up and set the gases in the exhaust pipe on fire. If you would use metal instead, there would be no such worries.

Dan Hintz
11-06-2013, 6:11 AM
PVC pipes can build up an electrical charge that could theoretically spark up and set the gases in the exhaust pipe on fire. If you would use metal instead, there would be no such worries.

Man, this stuff gets passed on every few months (but usually on the woodworking side of things). There are generally no explosive gases resulting from lasering, other than the potential for some acrylics in condensed spaces.... but that would only happen if you had no airflow, and if that was the case, no static.

Dave Sheldrake
11-06-2013, 8:12 AM
Henri,

I think the biggest risk would be from the big scary laser setting stuff on fire if the machine is left unattended :) The chances of a static caused fire (unless you are working in a saturated Oxygen environment) is so close to zero the number is almost un-calculable.

cheers

Dave

Al Launier
11-06-2013, 8:56 AM
If, as you say, it's working well exhausting to the outside, except for the "wind" problem, try elbowing it on the outside with a run of duct to exhaust it away from its current location to discharge the flow so it doesn't blow back into your house. If that doesn't work properly, then I would recommend re-using this "trial" duct & go with option #2.

Henri Sallinen
11-06-2013, 10:33 AM
Man, this stuff gets passed on every few months (but usually on the woodworking side of things). There are generally no explosive gases resulting from lasering, other than the potential for some acrylics in condensed spaces.... but that would only happen if you had no airflow, and if that was the case, no static.


Henri,

I think the biggest risk would be from the big scary laser setting stuff on fire if the machine is left unattended :) The chances of a static caused fire (unless you are working in a saturated Oxygen environment) is so close to zero the number is almost un-calculable.

cheers

Dave

Like I said, theoretically ;) But thanks for the corrections!

David Somers
11-06-2013, 11:53 AM
Henri,

Dan and David are right. A static caused fire from what you produce from a laser engraver would be truly surprising. You can produce flame and spark on the units, but the concentration of flammable particles in your exhaust pipes is going to be pretty darn thin and not likely to be a problem at all.

To be fair, OSHA has 280 spark caused dust fires and explosions registered through 2009, with about 119 fatalities and 700+ injuries. https://www.osha.gov/dep/combustible_dust/combustible_dust_nep_rpt_102009.html

Keep in mind the conditions though. Flammable dusts, from sugar and wood, etc....in concentrations that permit flashover in a confined space like a pipe, are just not happening with what you produce from a laser engraver. I would think the worst condition would be if you had joined your piping with that of piping from your dust collections system for your wood shop. Nooooow you might have an issue. Sparks from the laser entering a dust laden stream of air from your dust collector? Murphy would be just waiting to step in. Of if your pipes joined a dust collector that fed into a collection bag or bin of some sort. Now you could conceivably be introducing hot materials into a large bin of fine flammable particulates and shavings. Also a recipe for disaster. Or, even worse....you were trying to develop a production line for laser engraving sugar granules!! Whoa! Sugar dust is actually a big contributor to the OSHA stats about things that go boom. Fortunately, I haven't seen anyone discuss engraving sugar granules on the forum so with luck, we are safe. (that could be an interesting niche market though!)

In terms of sparks, you can get sparks in a PVC pipe due to static, or do to something that got ignited and then got drawn into the pipe, which is why OSHA specs metal/conductive piping for a dust collection system. It can be grounded to prevent static sparking. But for a spark to be an issue you have to have enough volume traveling through the pipe with the right conditions to create static, and you have to have a dense enough airborne mix of flammable dust to ignite and flash over. Your lasers just aren't creating those conditions.

So do think about static with your wood dust collection systems. And do think about fire potential in any dust collection containers/bags you have. But for a laser being vented to the outside with a dedicated pipe? Shouldn't be a worry. By the way....I did check this with our safety officer to be sure.

By the way...if you are in the US, and are a business, you should be thinking about OSHA. They are harsh on dust related issues in a business environment.

Dave