How to Maka a Quiet Exhaust System?

[Tim Earling]

Hello,
I just purchased a Craftsman blower (650CFM) for my Epilog Helix. The blower is pretty quiet but when I hook up the duct hose (4" diameter) it is very loud when I duct it out of my garage. My goal is to reduce the noise level of the air being ducted out.

I have read on the internet about silencers and they seem like they would work well, but they are pretty expensive and I don't want to spend a lot of money right now. So I am trying to build one myself.

I was thinking about buying a car muffler and attaching it to the duct before it is vented out of my garage. Also thinking about constructing my own muffler, but not sure what to use. Can anyone give me any suggestions on this?

Another thought I had was to step down the duct from 4" to 3" to 2" - I am not sure if this will help or increase the noise. The length of my duct from the blower to the vent in my garage is about 15 feet long.

[Mike Null]

If you're going to do all that stuff you should have bought a 300 cfm blower to begin with because that's what you're going to wind up with. Take a look at securing your duct so as to eliminate vibration, then consider a wrap for it.

[Tim Earling]

Mike,
You are right. I should have gotten something smaller but the specifications for the laser (Epilog Helix) called for a 600 CFM fan.

[Dan Hintz]

Tim,

Mike's point is by the time you finish using a long 4" pipe with an automotive exhaust in the middle, you might as well have gone with a low-CFM part and be done with it. You're creating a high-resistance path, so you're going to lose a lot of CFM. On that note, ignore the manual's "requirements" and just put together something that works.

I question the use of the Craftsman blower to get what you need for two reasons... first, Craftsman is notorious for over-rating their blowers by using the peak turn-on value (which means bupkus in the real world), and second, even a true 3/4 HP is probably pushing the limits of what you would need for a good fume collector with a very short run (and again I question the capability of a 3/4HP fan to push 600CFM at anything other than 0" of H20). How long is the 4" run?

I would rethink your plans before getting too far into them.

[Tim Earling]

Dan,
My 4" hose run from the exhaust of the blower to my vent is a good 15 feet or maybe more. This is my first machine and set up for something like this so I don't expect to get right the 1st time around. My biggest issue is to have a quiet system.
The craftsman blower seems to have a lot of power and is it super quiet (the main reason I bought it). I attached a photo of how it looks.


Picture copy.jpg

[Mike Null]

Tim

If you're going to house it on that shelf I'm guessing you're building in noise from vibration. That blower needs to be mounted on something stable even if it's the floor. Then be sure you don't have vibration from the ducts. For every el you have in your line you will reduce efficiency by 10% or more (a curve is better than an el). Rather than using a smaller line out I'd think about a 6" line out.

we have a couple of pretty knowledgeable members regarding air flow so maybe they'll weigh in here.

[Dan Hintz]

That long of a run at 4" is going to kill you, and Mike is right about it being on a shelf. I have a cheapy Harbor Freight version sitting on concrete and it's still noisy as a banshee. I don't want to imagine what it would sound like sitting on a flimsy shelf.

[Lee DeRaud]

My 4" hose run from the exhaust of the blower to my vent is a good 15 feet or maybe more.

If I'm reading that picture correctly, that's an exterior wall, right? I'd seriously consider putting the blower on the floor where the shopvac is sitting now with its outlet plugged directly into a hole in the wall and one of those flapper-style clothes-dryer duct fittings on the outside. Then the inlet of the blower faces the Epilog and you need maybe 3' of hose total.

[Robert Walters]

If you are going to have a 15ft run, make sure it's internally "GLASS SMOOTH" wall.
The corrugated will increase the air flow resistance.

You could go up to an 6" or 8" exhaust hose with minimal resistance.
Every bend/curve in the hose will increase the resistance too.

[Mike Lassiter]

I suggest you run all the duct you can in metal clothes dryer duct (4") and limit the ribbed flexible hose to the bare minimum. Secure the metal ductwork rigidly to prevent vibration.The hose will reduct the air flow from the smooth walled metal pipe, and increase noise partly because it increases air turbulence inside it with all the ripples in the inside wall the air is flowing over. The motor should be on a solid surface as other's noted, anything that vibrates will add to the noise. I have a 3-1/2 hp Penn State blower sitting inside a closet that pulls air from a metal 6" pipe running about 28' to the laser. The blower screams with the closet door open. I insulated the closet for sound deadening when we finished the inside of the building to help the noise. The air flowing thru the laser when the top is opened and the fan on does make some noise, but the main culprit is the blower; which I isolated from the work area as best I could. The only flex hose I have is a short length connecting the metal ductwork to the blower itself and the laser. That was done to prevent the blower vibration from transferring to the metal ductwork. My fan was rated I think at 1500 cfm ( ULS recommended 1100 cfm I believe due to the large laser we have, and also because of running it in "pass thru mode" with end cabinet door open).
Personally I don't think moving air itself is that much of a noise maker as one would think. Central vacuums used in homes make a good example of my point. The fan or blower is the biggest cause of noise, and typically put close to the point of use. That was why I put mine blower as far as possible from the laser. (12x32 building). The central vacuums are much quieter (blower is not running in area of use) than portable ones you use. See how much noise your clothes dryer makes outside when it is running. I realize it is less air flow, but point being your blower will be much louder inside the work area than the air blowing outside will be.

[Tim Earling]

Thanks for the feedback. I mounted the blower onto the shelve. It does not vibrated much - which surprised me. I can easily move it back on the ground - I put it on the shelve to allow more space. If I take it down and put it on the flower, the distance to the vent would be 8 ft.
This may be a dumb question, do I lose a lot of suction power from the back of the laser if the OUT/Exhaust duct is longer vs shorter?
Thanks again for all your help.

[Gary Hair]

Tim,
You want the exhaust system to draw the exhaust from your laser, not blow it away. If you have any air leaks at all, and you will, the draw system will simply draw in air where the "blow" system will expell exhaust into your room. The best situation is to have the blower outside and draw the exhaust to the blower. In your example the exhaust is drawn from the laser for part of the way and blown from the blower for the rest - that's just looking for trouble. I have my blower mounted to the wall in my garage and it vents directly outside, there are enough leaks in the blower housing itself that I get exhaust smell in the garage, I can't imagine how bad it would be if the blower were in the middle of the duct somewhere.

Gary

[Mike Lassiter]

Tim, I think technically you lose air flow with longer pipe on either side and elbows hurt the most.What I thing you should focus on is, do you have enough air flow to pull smoke out of your laser when using it? If you are doing that then you have enough air flow to do what you need to do. You should see smoke being "sucked" away and not lingering inside the laser when running it. If the smoke doesn't clear out with a very obvious draw pulling it out and clean air in you are not getting enough air flow. The smoke inside the cabinet not being drawn out quickly could affect laser operation (smoke clouding the lens). Laser is kind of like us - we both need a good steady supply of fresh air.

[Mike Lassiter]

Gary makes a good point. his post appeared after I submitted mine. I taped all seams with metal duct tape in the metal duct work I have. Couldn't do that where the flexible hose connects to the ductwork or laser and blower, but as he notes, any small pin hole will allow air to enter on suction side, and to excape back into your work area on exhaust side of blower.

[Linda Tetreault]

We built a small shelter on the outside of the building, ran the duct work straight from the back of the laser to the dust collector, used about 6 ft of tubing, the cord from the blower comes in the same space so we can turn it on/off without going outside. It has worked great for us for 5 years & most of the noise stays outside.