Metal Dust Collection

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Jan 9, 2008
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I am in the last stages of finishing up my extended shop. I want to include some type of dust collection for the 2x72 as well as the surface grinder. I was looking at these dust collection cabinets, but they are pretty spendy. Then I was thinking (not as rare as some might say) that I have a 5 HP dust collector for my wood shop 15' away. I know enough not to suck metal into my sawdust for many reasons, but,..........what if I could still utilize the vacuum of that system to draw through a separator/spark arrestor that wouldn't allow any metal to get drawn into the wood? Has anyone done this, or know of a system I could purchase for this?
I'm only interested in pursuing this if it can be done 100% safely.
 
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Jan 18, 2010
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I'm trying to emulate the dust collection system by @van_barnett on IG. There's several pictures and videos showing how/what he did. his knives are also inspiring to look at.
 
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Aug 2, 2010
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I'm trying to emulate the dust collection system by @van_barnett on IG. There's several pictures and videos showing how/what he did. his knives are also inspiring to look at.

ooo! I have some serious shop envy of Van! :eek: ;)


~Paul
My YT Channel Lsubslimed
... (It's been a few years since my last upload)
 
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Oct 28, 2015
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I have seen some people using a steel dust deputy with a steel barrel with some water in it to catch the sparks...I would still clear out the bag of the dust collector before grinding any metal though
 
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Nov 24, 2016
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I would still fear the possibility of a spark getting into your dust system and causing a fire. My system exhausts directly outside which eliminates most potential of a spark related fire. My system is 3/4 hp 110v and seems to be plenty. Good luck with your endeavor.
 
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Mar 8, 2006
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Well at least you'd be starting with a big enough dust collector...

The short answer is it's not going to be 100% safe. And although I can think of a way to hook it up being mostly safe, the expense would probably exceed the cost of buying another dust collector just for metal. You would have to design a system that would eliminate any possibility of cross contamination between the wood and metal dust while using the same fan on both systems. The fan would have to come after the cyclones and filters, which would mean some serious plenum building and some big blast gates.
Even with dedicated dust collectors just for metal, the manufacturers reference "flame events" as a possibility. When I put dust collection on the grinders in the metal working area, I used both spark arrestors and cyclones before the fans, and had flame retardant MERV 15 cartridge filters custom made. A spark would have to make it through the arrestor and the cyclone to start a fire in the collection bin, or go through the fan and into the filter plenum, which would ruin the filters, but not burn down the shop.
Air traveling in a dust collection duct is typically traveling under laminar flow conditions, a spark in the duct will be traveling at the same speed as the air around it and quickly becomes surrounded by warm oxygen depleted air. The spark stays hot, and does not "burn out" because it is isolated from oxygen. Spark arrestors stop sparks by disrupting the laminar flow and slowing the air. As the air mixes around the spark, oxygen gets to the spark and consumes it, and cool air cools the ember.

Spark arrestors are surprisingly expensive and many have lead times that imply they are made to order. I went with Nordfab since the units were available at a somewhat reasonable price and lead time. Spark arrestors differ in how they disrupt the airflow, and the manufacturers will recommend specific lengths of duct before and after the unit, 10 duct diameters being pretty typical. So for the 6" duct I used, 60" or 5' on each side. In my case I wanted the dust collector a bit closer than that to the grinders so I bought a viewing spool and did some testing. I found the 10 duct diameters on the outlet was critical to eliminate 99.9% of the sparks, but the length on the inlet side could be reduced a bit and the units still worked. YMMV with different brands, I have a different spark arrestor from another project, and a lot of sparks get through unless there is at least a 10 duct diameters length of duct on both the inlet and the outlet. A cyclone can also be a spark arrestor, (for smaller systems the steel Dust Deputy surprised me with it's effectiveness) but given the design of the dust collectors I bought, the only way to test the cyclone would have put the filters in jeopardy, and each spark arrestor cost only slightly more than a new set of filters would have...

BTW the dust collection cabinets with the 4" inlet like the Jet don't move enough air to be worth buying for a grinder. I would suggest another 5 Hp dust collector if you want to use both grinders at the same time, 2 or 3 Hp if you would only be using one machine at a time
 
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May 23, 2017
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I have one of the cabinet style ones. Out of the box it was inadequate. 1.5hp motor, but an undersized filter and 4" line. I modified it to a 6" line running straight to the grinder, and replaced the filter with a pipe venting straight outside. The highly unscientific rest of "how much it pulls my hand in" seems like close to double the flow of how it came. Its amazing how choked up those things come.

Knowing what I know now, I wouldn't buy another. I'd sooner get a 2-3hp woodworking collector and have it dedicated. I'm incredibly happy with my system, but I could have had even more flow for less money going a different route
 
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Jan 9, 2008
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Thanks guys. Mahoney, that was a great explanation. Much appreciated.
I gotta sleep at night so am seriously considering canning this idea and exausting directly outside.
6" ducts minimum. I use an 8" main duct in the wood shop.
 
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May 23, 2017
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If you're not in a place where you'd freeze to death in the winter, I'd definitely go with an outside exhaust. No chance of fine dust getting back in the air, and it will act like a much more powerful blower. Filters are amazingly restrictive, I'd need at least 2-3hp filtered to equal my direct venting 1.5hp system.

Something like a harbor freight or grizzly 1.5-2hp woodworking collector would be a good starting point
 
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Mar 8, 2006
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Exhausting outside is a good option if you are not heating or cooling the shop, and of course if a stray spark won't start a fire. Consider prevailing winds and where the make-up air will come into the shop, you don't want the fine dust coming back in.

6" duct is a good size for a grinder, you can move enough air to get the fine dust. You need 5,000 feet per minute velocity to keep metal and abrasive dust moving in the duct. For 6" duct that's 1,000 CFM airflow. If you will have a spark arrestor and/or a cyclone in the system, you'll need 2 Hp to get the job done without filters. Properly sized filters don't add much back pressure to a system, but almost no dust collectors come with properly sized filters

One more thing, wood working dust collectors are designed to have intake restriction from ducting and back pressure from filters. When we run them with short efficient duct runs and no filters, it is possible for the fan to move more air than the manufacturer thought it would be able to, and as a result the motor will pull too many amps. Counter-intuitive I know, but more air being moved means more amps, block the airflow completely, less amps. So check your installation with an amp meter and make sure you won't burn up the motor.
 
Joined
Jun 4, 2017
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Working on getting my evacuation system going.
https://imgur.com/a/7S8qO

I have mocked it up, I will exhaust out the window of my garage right next to the harbor frieght 2hp dust collector fan. I will need a longer hose than came with it though, it is a 5" connector and hose. I will put water in the bottom of the pvc for metal grinding, got idea of that from fellow member. Hose from collector to blower is metal dryer duct. Yes I will move the propane tank next to the blower ;)

I am only a hobbyist, making about 15-20 knives or so a year, but tired of dust all over everything in the garage.

Any opinions on this setup??? Don’t want fire of course, I haven't ground on it yet, but i can tell some hot metal will make it into the hose as hard as this thing pulls. I will watch for this and could even disconnect hoses when done grinding each time as infrequent as I actually grind lots of metal.
 
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Mar 8, 2006
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866
Now you will have dust outside the garage, and it's possible/probable that a few sparks will go shooting out the duct as well. Depending on the neighbors and how much flammable vegetation there is outside, this may, or may not, be a problem. Have a buddy grind a little steel while you look for sparks outside. If the sparks are getting through, you could get a cyclone or spark arrestor to put in the system. Also, think about where the air will come from to replace the air you are exhausting. If the air will go from the exhaust back over to the open garage door, it will carry the very fine dust back in
 
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Jun 4, 2017
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Those are good points Mahoney. I think I will build a proper spark arrestor, just don't want it to suck up water into my fan. Also I will keep the big garage door closed while grinding, as you are right fine dust might come back in that way. also may try a bucket outside to try and catch some. more work to do on it over the next few weeks
 
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Jun 17, 2015
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I have a pretty good size dust collector somewhere in storage. I should dig it out because dust in the shop is really beginning to bug me. I will certainly set it up as an outdoor exhaust.
 
Joined
Jun 4, 2017
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167
Here is an update on me. Decided to put a spark bong in the system and did some research on here, built a traditional design, here is a schematic I got off the forum https://i.imgur.com/wg3qBjx.jpg
Bottom line is my 2hp dust collector pulls way too much vacuum without throttling it back at least 2/3rds with a cutoff it pulls water through the system. Here is pic of it I just tested https://i.imgur.com/0RoEPIK.jpg. I think this design is better for a shop vac power level, it just didn’t seem like enough suction to control dust being throttled back that much. Hate I have such a good vacuum and not use it the power

Right now I am at a loss for a spark bong that won’t pull water with full power on the 4” system, ideas?
 
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Jun 4, 2017
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167
I tried my first design out and it doesn’t pull any water out of the bottom at full power. https://i.imgur.com/NbTa041.jpg
However it does pull some sparks directly in the hose given the design, but some heavier ones go to the bottom https://i.imgur.com/AwiVHlJ.jpg

Any thoughts are welcome, I will be venting out the window, possibly venting all into a bucket with water outside will be safe enough, and keep an eye on my hoses.
 
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Jun 4, 2017
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Ok, try not to laugh, but think I came up with a solution- corn syrup. I used 2 bottles of this from Walmart, $1.88/e https://i.imgur.com/3mlTxgT.jpg

I tried to think of something less viscous than water so it would not get sucked into the collector, but was non flammable and easily disposed of, so thought of plain old corn syrup, and it works at full throttle! Some of the syrup moves over to one side but does not get in the air. I will have to use 2 buckets, one for mostly metal grinding with syrup, and one for handles though. You can see some metal in the syrup after just a few hits on the belt. https://i.imgur.com/VHPxgVf.jpg

This will enable full suction and keep dust down as much as possible, but reduce some fire risk. I will still vent out the window, regardless.
https://i.imgur.com/0EeOLXe.jpg
 
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