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

[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.

• 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.

[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