Dust laden atmosphere - explosive?

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tillergirl

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SWMBO is going to buy me a workshop heater for Christmas but I remember reading somewhere that sawdust can be explosive. Since I am preparing 12 iroko floor boards, 5 iroko locker lids, cabin steps and sundry other things, all ready for numerous coats of varnish. Am I about the blow myself up? Is this a cunning plot after the perfect murder plot of death by cholesterol has proved itself too slow?
 
Quite possibly some explosion risk in a dust-laden atmosphere. I would be more concerned about the risk of breathing iroko dust. Apparently pretty toxic stuff so a good respirator and eye protection are required.
 
Point accepted and I am aware of that. In fact I guess the main dust in old varnish. I have found this which kinds of answers my own question.

"This last section is an important clarification in the standard, and one that should be emphasized. Sawing of softwood lumber, the major source for wood dust in a component manufacturing facility, does not create significant amounts of combustible dust. However, very fine dust (smaller than 80 microns) can easily
become airborne and settle on surfaces and in crevices that may be difficult to reach or to access from the production floor (exposed ceiling joists, for example).

However, NFPA 499, Section 4.3.1.1, states that the following set of conditions must be true and satisfied for ignition of a combustible material by the electrical source (a component saw, for example):

4.3.1.1(1) Combustible dust must be present;
4.3.1.1(2) The dust must be suspended in the air in proportions necessary to produce an ignitable mixture with a sufficient quantity of this suspension present in the vicinity of the electrical equipment; and
4.3.1.1(3) There must be a source of thermal or electrical energy sufficient to ignite the suspended mixture.

All of these conditions are unlikely to be present on the production floor simultaneously. Even if condition (1) is met, and combustible dust is created through your operations, it must be of a sufficient quantity and suspended in air. According to the Wood Dust Material Safety Data Sheet (MSDS), the lower explosive limit (LEL)—also called lower flammable limit (LFL)—in air is 40 g/m3.

This is the lowest concentration of a combustible wood dust in air capable of producing a flash over fire in the presence of an ignition source (arc,
flame, heat). Concentrations lower than LEL are 'too lean' to burn.

To give you a sense for what wood dust LEL looks like, NFPA 499, Annex A 5.2.2, states;

A5.2.2 “from a practical point of view, a room with a concentration of dust that is above the minimum explosive concentration [Criterion 1], results in an atmosphere so dense that visibility beyond 3-5 feet is impossible.”
Yet, while this is unlikely on your production floor, it is advisable to inspect your facility to ensure that there is not an enclosed area (like a dropped ceiling or crawl space) near your sawing operations that may be collecting the finer wood flour particles as they settle out of the air.

Under optimal conditions, the given minimum concentration for combustion to take place is 0.035 oz/ft3 (~36g/m3)—the equivalent to between 0.016 inches and 0.47 inches of dust accumulation on the entire floor surface of the room (depending on mean particle size, density and moisture content)."
 
I doubt whether the concentration of dust in your workshop would be right for an explosion to occur. I've (unintentionally!) created a wood dust explosion by tipping fine dust into a woodburner, so I'd imagine you need a high dust:air concentration.
Best to get rid of the problem at source, by connecting a vacuum cleaner to your planer or sander. A workshop filter will keep the general atmosphere clean, but I agree with Ken that a face mask or respirator is a good idea for any tropical hardwood (well, any wood at all I suppose, but the tropical ones seem to be worse - according to the very informative catologue from Boddys http://www.john-boddy-timber.ltd.uk/Pics/2007catlaogue.pdf )
Iroko is particularly associated with dermatitis. It sounds as though you're working on existing pieces rather than making new, but if not you could try Larch for your floor boards - a nice home grown softwood particularly good for floors as it doesn't splinter like some other species.
 
If you haven't blown yourself up by now, a heater isn't likely to make the environment any more dangerous. Static electricity from clothes, and heat and sparks from power tools, especially sparks emanating from the brushes inside an electric drill or other power tool are just as likely to ignite a combustible atmosphere.
 
I have been thinking about dust extraction; air filtering is just a tad expensive for the amateur use I give my workshop. After all I need to get rid of the dust for the varnishing phase. It's all refurbishing this year, not replacement so no need for new wood - anyway everything is 1" iroko - yes including the internal locker doors!

I have been working with iroko now off and on for 18 years and the skin is ok. When I replaced the decks though with a high grade Sapele ply, the dust from that was really aggressive. My hands actively tingled from the dust until I started wearing gloves and a barrier.

The extract I found suggests two risks: heavily laden air and accumulation of dust. I am clearly not going to get to the level of dust that report says is dangerous but the accumulations of dust could exceed the limits mentioned unless I get air extraction going.
 
Veritas make a seperater to use before your shop vac. While fitting my system to collect from the machines, I built one from a big plastic dustbin. Cut two holes in the lid, one in the centre to connect to the vac, the second at the edge at a tangent.This is connected via a gate system to the machines. All done with PVC waste pipe. Works a treat. Dustbin cost £10.
It stops most of the dust and chips, I only need to empty the shopvac if I foreget to empty the separater.
Andrew
The was a case quoted in the States of an explosion in a woodwork shop in sombodies basement. Conclusion was it must have been VERY thick in the air. Certainly beyond what you would fancy working in.-A
 
I envy you being able to work with Iroko; I can't go near the stuff without suffering severe skin itching! Making my new rudder a few years ago was sheer purgatory.
 
I don't think you've got anything to worry about on the explosion front, but I'd echo the concerns of other posters about the health risks associated with wood dusts etc.

There's a huge range of dust extraction equipment available to catch dust at source. I'm guessing that you'll be using mainly portable power tools (sander etc.) in which case a simple workshop vacuum cleaner would be a valuable investment. I'd recomend the Numatic range, which includes the robust but inexpensive Henry, as well as more powerful machines. As ever, Axminster Power Tools is a good place to start looking.

Even with extraction you won't catch all the dust, so a good mask is vital. The best I've found are the three panel design by 3M, widely available from Screwfix and others.
 
Agree on the health issue - went to the local yard to collect a piece of iroko. It had to be sawn for me, and even with dust extraction (obviously not very effective!) everyone nearby in the workshop was coughing by the time it was ready!
 
Thanks for all the advice. Can I repeat that I am aware of the health issues! And have been for 20 years! It was blowing up I was worried about. I shall check out the recommendations.

I have to echo oldharry. When I had the 'new' (but old and vast) plank planned up for the transom 18 years ago, the dust storm was enormous and defeated the dust extraction in the professional workshop.
 
Not just wood dust either. Remember the custard bombs we were not supposed to make? I believe windmills used to be a prime problem, fine ground flour being as good a dust as any for explosive risk.
 
I had the same problems and have found the oil filled radiators are the sest . The main problem was the dust settling over night . when you switch the heater on in the morning the dust had laid on the element and started to burn . So any inclosed heater would be good .
 
Re the flour, friends had a flour mill in Hertford. Hi risk, no inflamables allowed in, lighters matches banned. Rubber shoes the whole gambit. And this was a modern mill.
A
 
“from a practical point of view, a room with a concentration of dust that is above the minimum explosive concentration [Criterion 1], results in an atmosphere so dense that visibility beyond 3-5 feet is impossible." !
 
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