How high is the risk of a dust collector fire from metal sparking?

[Dan Ness]

From reading different threads, it appears that there is a risk of a fire caused by metallic pieces (screws, nails, staples, etc.) being sucked into the dust collector when being used to sweep up the shop. From what I understand this is caused by sparking of the metal item when it hits the impeller (for single stage dust collectors). Since the impeller is right above a large bag of wood dust, even a small spark could start a devastating fire.

So my ? is. . .how high is this risk? Are there any documented cases of this happening? And if so, why are there dozens of companies out there selling floor vacuum attachments for DC without any warnings, etc.

When I first bought my single stage DC (soon to be upgraded to a cyclone), I also bought many attachments including the floor sweep wand, and the floor sweep gate, with the intentions of using it to clean up my shop floor, but ever since reading about this possibility, I have no longer use them.

So should I keep them in storage until I get a cyclone (if that is even safe?), or should I toss them and stick to my shop-vac/broom?

[glenn bradley]

I am sure the usefulness of a floor sweep is a matter of shop layout, DC power and any number of variables. I got rid of mine as I find that with good dust collection, the amount of "other" cleanup is so minimal that a broom and dustpan or the rare shop-vac-attack does the job. I also didn't have the floor space to spare to keep the path leading to it open ;-)

All that being said, I haven't seen any "official" studies or recommendations from the industry but, I have a home shop, not a pro shop and do not receive any professional trade magazines or other such. I do see "common sense" recommendations on the forums that any DC that passes the spoil through the blower should not be used for items other than chips and dust. Even the stray "chunk" of cutoff can ruin your day, let alone a bolt.

Let's hear from some of you that still use a floor sweep fixture ;-)

[Michael W. Clark]

If you are doing general floor cleanup, it is best to have a pre-separator before the fan. Cyclones are good at getting rid of metallic objects and sparks (burning particulate). From an industrial standpoint, DC fires happen more frequently than many realize. It is usually only documented, or makes the news, if there is a significant loss of property or someone gets injured. There are many instances where the system catches on fire, burns itself out or is extinquished, is repaired, then restarted. Cyclones are more forgiving (as long as the heat doesn't warp them) because there are no filters, internals, or electrical to replace.

How real is it in the home shop depends on how much metal you are picking up. The more you pick up, the higher the probability of having an event. I worry about pieces smoldering in the bag and catching on fire after I have left. I typically use a broom or shop vac for floor clean up since I do not have a pre-separator.

Mike

[Jamie Schmitz]

Can you put an rare earth magnet on or near the pickup attachment. I think static electricity is more of an issue.

[Dan Ness]

Well I was mainly concerned because every once in a while I would use the floor sweep and suck a screw or staple up into the system and hear the obvious clinging with the impeller. Then I started reading about the possibility of a fire starting. Especially a small spark that might ignite the fine powder/sawdust in a full bag. Didn't even think about the possibility of a hot chunk of wood getting in there.

Seems like more of a reason for me to get that cyclone/separator sooner rather than later.

[David Kumm]

I would not clean the crud off my garage floors after I went to cartridges. Dirt and sand are pretty tough on the fibers. Not applicable for those with a real shop. Dave

[Sam Stephens]

I would not clean the crud off my garage floors after I went to cartridges. Dirt and sand are pretty tough on the fibers. Not applicable for those with a real shop. Dave

yep. that's what a shopvac is for.

[Ben Hatcher]

Can you put an rare earth magnet on or near the pickup attachment. I think static electricity is more of an issue.
YES! We haven't had a good exploding duct work thread in a while. They're so much fun.

[Andy Pratt]

I'm in the camp that would rather deal with emptying the shop vac more often (and I run a hepa filter and bag in it all the time, you can re-use the bags if you are creative) than with all the floating dust created by sweeping. I feel like sweeping was a great way to clean up a shop floor until shop vacs were invented, then they became the best idea. If I had a smooth wood floor in the shop I might think differently but I'm on somewhat textured concrete right now and it takes way more effort to sweep than it does to collect it with the shop vac.

Only saying all this because I considered the floor sweep but didn't get it, and have never regretted it.

[Alan Schaffter]

Ok, here is the take, from someone who has had his cyclone-based DC connected in a "push-through" configuration for over 10 years without any issue. For those who don't know, a "push-through" is where the DC blower is upstream from the cyclone, so everything picked up by the DC hits my blower's 14" steel impeller before going to the cyclone.

Many know a little about me- I'm the guy who brought you the I-BOX, Lock Miter Master, etc.- so you know that I do a lot of tinkering and routinely work with both wood and metal. In over 10 years have I sucked up just about every kind of metal bits and pieces you can imagine- from nuts, bolts, metal cut-offs, to a drill press chuck key, and even a tape measure.

No fires, no near fires, no visible sparks, no smoke- NADA. I'm not going to say it can't or won't happen. I'm just giving you my experience and will let you make your own decision.

There are a number of reasons fire is less likely in a home DC system:

Metal on metal contact doesn't always create sparks
Air velocity in a DC is high and rapidly cools any sparks generated
Sparks, if any, are too short lived to ignite the dust and chips.
Air velocity reduces the concentration of dust in ductwork in home shops, further reducing the chance of ignition of the moving dust and chips. The conditions are very much different than high concentrations of slow moving or near stagnant dust in a grain elevator
There is insufficient heat created for ignition and sustainment of fire

I'm sure you'll find reports of fires in commercial and industrial settings, but even they can be rare. There was a SMC thread in recent years that cited one example, but if you read the report you would see that the fire actually started at a machine, and burning embers were sucked up.

[David Kumm]

The one commercial fire I've seen involved cardboard rather than wood. Similar set ups are used. If it were a big issue there would be a mandate on impeller material. Both steel and AL are used extensively now. Dave

[bobby milam]

I remember reading an article recently but can't recall where. It addressed this and basically said that since there is always that possibility manufacturers have to address it for legal reasons but no documentation could be found of it actually happening in a home shop. I'm not to worried about it but my first thoughts were to attache a strong magnet across the intake

[Alan Schaffter]

I remember reading an article recently but can't recall where. It addressed this and basically said that since there is always that possibility manufacturers have to address it for legal reasons but no documentation could be found of it actually happening in a home shop. I'm not to worried about it but my first thoughts were to attache a strong magnet across the intake

At typical DC air velocities of over 4000 fpm (or 67 fps or 45 mph) you won't likely find a magnet strong enough to catch/stop much of anything, especially a chunk of metal with weight and considerable kinetic energy (1/2 MV²)!

[bobby milam]

At typical DC air velocities of over 4000 fpm (or 67 fps or 45 mph) you won't likely find a magnet strong enough to catch/stop much of anything, especially a chunk of metal with weight and considerable kinetic energy (1/2 MV²)!

Hadn't really thought about it that much but you are right.

[Craig Behnke]

At typical DC air velocities of over 4000 fpm (or 67 fps or 45 mph) you won't likely find a magnet strong enough to catch/stop much of anything, especially a chunk of metal with weight and considerable kinetic energy (1/2 MV²)!

i've played with some expensive rare earth magnets that are so darn strong it's unbelievable. they are nothing like the magnets you played with as a kid. I wonder if those are strong enough.

[Michael W. Clark]

The one commercial fire I've seen involved cardboard rather than wood. Similar set ups are used. If it were a big issue there would be a mandate on impeller material. Both steel and AL are used extensively now. Dave

Hi David,
You are correct, both materials are used extensively in industrial fans. If you are dealing with a flammable material, a spark resistant fan is usually required (either by code or the insurance agency). Industrial fans are usually AMCA rated and different levels are spark resistance exist such that ferrous and non-ferrous materials can be used depending on the application.

I doubt most (if any) home shop DC fans are AMCA rated, let alone have AMCA spark resistant construction.
Mike

[Michael W. Clark]

Ok, here is the take, from someone who has had his cyclone-based DC connected in a "push-through" configuration for over 10 years without any issue. For those who don't know, a "push-through" is where the DC blower is upstream from the cyclone, so everything picked up by the DC hits my blower's 14" steel impeller before going to the cyclone.

Many know a little about me- I'm the guy who brought you the I-BOX, Lock Miter Master, etc.- so you know that I do a lot of tinkering and routinely work with both wood and metal. In over 10 years have I sucked up just about every kind of metal bits and pieces you can imagine- from nuts, bolts, metal cut-offs, to a drill press chuck key, and even a tape measure.

No fires, no near fires, no visible sparks, no smoke- NADA. I'm not going to say it can't or won't happen. I'm just giving you my experience and will let you make your own decision.

There are a number of reasons fire is less likely in a home DC system:


Metal on metal contact doesn't always create sparks



Air velocity in a DC is high and rapidly cools any sparks generated



Sparks, if any, are too short lived to ignite the dust and chips.



Air velocity reduces the concentration of dust in ductwork in home shops, further reducing the chance of ignition of the moving dust and chips. The conditions are very much different than high concentrations of slow moving or near stagnant dust in a grain elevator



There is insufficient heat created for ignition and sustainment of fire


I'm sure you'll find reports of fires in commercial and industrial settings, but even they can be rare. There was a SMC thread in recent years that cited one example, but if you read the report you would see that the fire actually started at a machine, and burning embers were sucked up.

Alan,
I'll second most of what you said especially about likely being rare in a home shop. However, I do think there is risk to sending metal objects through a steel fan. It only takes one incident to create a bad situation.
Sparks tend to burn out with the air velocities due to the ample supply of oxygen. The problem arrises when you get a big enough burning ember or spark that does not burn out before it gets to the dust strorage area. Industrial DCs have a lot of similarity to home DCs just on a much larger scale. They are running 4000 FPM+ as well but where you are pulling 2,000 CFM, they may be pulling 10 - 100 times that in flow.

The biggest problems arise where there is hot sparks or particulate being exhausted at the source. Cyclones help to get those sparks out because they are separating the particulate, there are no bags in them, and usually no dust storage in the cyclone. The storage that is sometimes used below the cyclone is in a metal container and property loss is minimal, if any, should it catch on fire. People often put a sprinkler/deluge system on the storage area if they have had experienc with fires in the past.

If the same fire happened below a bag filter, there can be significant property damage due to filter replacement and downtime. In the aluminum industry, cyclones are used in front of almost every bag filter to act as spark arrestors for this very reason.

You have a very nice system and I drool over it each time you post pictures. I have often wondered why you put the fan before the cyclone. The only possible advantage I am aware of is a cleaner duct arrangement, especially if you are venting to atmosphere.

[Steve Milito]

At typical DC air velocities of over 4000 fpm (or 67 fps or 45 mph) you won't likely find a magnet strong enough to catch/stop much of anything, especially a chunk of metal with weight and considerable kinetic energy (1/2 MV²)!

I would think that you would want to put the magnet at the entry point so that you would capture the object when it's momentum and kinetic energy was low.

[Jim Andrew]

The only fire at a cabinet company locally was caused by lightning hitting their steel building, and going down their electrical service. The owner showed me the breakers, they had copper embedded in them and when they were installed in the box could not be turned off. Guess they could be turned off, they just didn't stop the electricity from flowing.

[Alan Schaffter]

i've played with some expensive rare earth magnets that are so darn strong it's unbelievable. they are nothing like the magnets you played with as a kid. I wonder if those are strong enough.

The problems are:

If you attach or position the magnets near metal ducting, the magnetism will be spread over a large area- metal chips, etc. will cling to the duct walls, create turbulence, and reduce air flow.

The same thing can happen with PVC duct, especially if you try to use a metal grating to help catch the pieces of metal- that will also create turbulence and reduce air flow.


If you are concerned about it, the best option is to put the cyclone or other separator ahead of the blower.

[Alan Schaffter]

You have a very nice system and I drool over it each time you post pictures. I have often wondered why you put the fan before the cyclone. The only possible advantage I am aware of is a cleaner duct arrangement, especially if you are venting to atmosphere.

Thanks. At least five reasons for putting the blower ahead of the cyclone:

All single stage DC's work that way so I knew it would work.
It was much easier to mount and plumb. The blower intake is round and so is my ductwork. The blower discharge is rectangular and so is the intake on my cyclone. No awkward or SP-robbing rectangular to round or round to rectangular adapters were needed.


http://www.ncwoodworker.net/pp/data/1329/medium/P10100392.JPG


Reduced headroom requirements since the blower does not sit on top of the cyclone

Unsubstantiated, but reports from Bill P. and others say separation of fines is better with a "push-through" system.

By far the most compelling reason was that I didn't need an air tight dust drum with elaborate top. They can be a pain to get a perfect seal and a real messy pain to empty. Since my cyclone is always under positive pressure, I am able to attach a standard leaf and lawn plastic trash bag directly to the bottom of the cyclone with a simple band clamp. If it leaks a little, I'll just get some fine dust my DC room. When the bag is full I remove it, tie it off with a twist tie, and put it out for the trash- REALLY EASY and NO MESS!!! Even a 3 hp DC w/14" impeller doesn't generate enough SP to blow out the bag should the filters clog.


http://www.ncwoodworker.net/pp/data/1329/medium/P10100531.JPG

[Michael W. Clark]

Hey Alan,
Keep that seal on the discharge. It sounds counterintuitive, but you will get re-entrainment without a good seal on a push-through. Your cyclone looks better than most (larger and taller), which gives you more efficiency. You are correct, those clames about push through being more efficient are unsubstantiated and untrue.

Great looking system. Real estate always plays a major factor in equipment arrangement.

Sorry for getting off-topic.

Mike

[Craig Behnke]

thanks for the info Alan