Big bang theory, revisited - Woodshop News

Big bang theory, revisited

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Here’s a fact: Shop dust is dangerous. But the likelihood of a dust explosion in a shop is incredibly minimal.

It’s common knowledge that a teaspoon of fine dust can explode. That's a classic experiment I recreated years ago in a high school science class: Take a teaspoon of fine dust – I used flour – blow it directly into a candle flame and poof, you've got a fireball. That's a perfectly provable example that dust explodes. However, you have to note two things about that example.

First is that the fireball created by that teaspoon of dust, while large and scary, doesn't have much combustive energy. The fireball is very widely dispersed and extinguishes in a fraction of a second due to the lack of sustainable fuel. Even if someone stood in the middle of that fireball, they'd probably not even get their eyebrows singed.

Could that fireball, however, feed off additional airborne dust to create an inferno? Sure, but only if the entire volume of air is proportionally carrying the same amount of dust as in the candle experiment. In the candle experiment, you force a single teaspoon of dust into a temporary loose airborne suspension in a very small area – the general vicinity of the candle flame, less than a single cubic foot of air – creating the fireball. To have the same thing happen in "real life" in a shop environment you would have to multiply that teaspoon of dust a tremendous number of times, and keep it in loose airborne suspension in a much, much larger volume of air. The fireball experiment works because that dust is brought together with the flame in the few seconds it's airborne before succumbing to gravity and falling to the ground.

My shop is 460 sq. ft. with a 10' ceiling – a volume of 4,600 cubic feet. To recreate your fireball effect, you'd need to disperse 4,600 teaspoons of dust in my shop and keep it in suspension to ignite the same way. A teaspoon of dust weighs about .17 of an ounce; 4,600 teaspoons weighs in at 782 ounces. That’s a bit less than 49 lbs. of dust. Touch that off and not only will my shop and home be history, but the resulting blast would probably do the world a huge favor and get rid of my whacko neighbor at the same time. But I simply can’t create 49 lbs. of dust in my shop, and I certainly couldn’t keep it in airborne suspension long enough to create an ignitable fuel source for such a blast.

I admit I’ve played fast and loose with the amounts and conditions in my example above, and in fact you can create an impressive fireball with just half a teaspoon of dust, meaning that you’d only need a mere 24-1/2 lbs. of dust to blow my shop and neighbor to smithereens, whatever those are. But the fact remains that there’s no operation I can perform in the shop – even the dust storm I created last week in my annual shop dust blow-out – could create even a single POUND of dust, much less keep it in perfect airborne suspension. In a grain silo? Sure, I could do that. But in a shop environment? Nope.

Dust is dangerous, no question about it, and breathing it constantly in a shop with no dust collection will turn your lungs to tapioca. Further, a shop with an inch of dust settled on everything creates a fire hazard on its own, and that shop will burn far more efficiently than a clean one.

There are lots of excellent reasons to control dust, but preventing dust explosions in small workshops isn’t one of them.

A.J.

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