Spray booth explosion

It's certainly within the realm of possible. I use a kitchen hood on my spray booth, but only spray acrylics, which admittingly are thinned with alcohol. But alcohol doesn't explode very well, it burns fine, but evaporates so fast that any atomized molecules disappear long before they hit the motor, so it's not been an issue. I know I was more concerned about using incandescent bulbs, so I put in a fluourescent fixture instead.

I certainly wouldn't think about using lacquer or enamel based paints in it, and strongly suggest no-one else does either.

I think the possibility of a spray booth explosion if pretty small, but still large enough that I don't want it to happen to me! I'm thinking about converting my booth to a downdraft and will be looking at using the proper type of fans for that, since they're easier to mount in the first place.

Actually, Bgrigg, there's not a lot of difference between Isopropanol and laquer thinner in terms of hazard for our purposes.

The MSDS for Isopropanol indicates a LEL of 2% and an UEL of 12.7% by volume

The MSDS for lacquer thinner indicates a LEL of 1% and an UEL of 25% by volume

Seems to me that as long as your fan motor sees an paint/thinner mixture in air of less than 1%, whether you spray enamel or acrylic, explosion shouldn't be an issue.

Anybody else have any thoughts on this? 

I think the simple answer is that due to the low volumes of solvents sprayed with an airbrush and given relatively high airflow volumes, the solvent/air mixture is going to be quite "lean".  I don't think that concentrations will approach even 10% of the LEL of the mixture. (Out of a statistical sample of 28 modelers, the average amount of paint and solvent sprayed on a kit was only about 59ml, total.)

Having said that, I think the potential for a fire/explosion incident increases once the spray cans come out.  Spray can volumes cannot be controlled; it's either all or nothin'. 

But there are a lot of other variables that have to be considered, too. Spraying that 1/196 USS Neversail with a spray can is a lot different than a 1/48 Mustang.  Spraybooths differ, too.  A badly designed homemade booth can add significantly to the hazard potentials associated with painting.  Good design and the right airflow go a long way toward reducing that potential.  That's why I keep harping on maintaining at least a 100fpm face velocity, and using a fan with the motor out of the airstream.  Correct paint filtration is a must, as well.  Fires in industrial spray booths are not uncommon.  They result from paint residues that have built up on the fan blades and ductwork over time.  That's probably unlikely with a homemade booth, unless you've been using it for the past 15 years without a filter, and haven't bothered to clean it out on a regular basis, or once again the booth is designed poorly.

If you're interested, I'll try and do a little research as time permits, and give you a more quantitative answer.

BTW, alcohols can vaporize and cause explosions in correct concentrations just like any other organic solvent!  Not all acrylics use alcohols, but in those that do, or their thinners, don't be lulled into thinking you're home free.  Be careful.  A well-placed fire extinguisher can go a long way toward adding to your mental stability (Did I say that right?)

Hope this helps some.

Gip Winecoff

gulfstreamV wrote:

Isn't the type of fan used part of the equation? A brushless or shaded pole motor is of the best choice for this application?

It most certainly is!  I don't believe I've ever seen a commercially built spray booth where the motor was in the path of the airstream.  Certainly a brushless or shaded pole motor, as you mentioned is the best choice.  I also tend to think a centrifugal fan (e.g., "squirrelcage") is one of the better choices if you're building your own, but again, a fan mounted in a box is worthless unless it can EFFECTIVELY entrain and capture vapor/pigment concentrations and push them through exhaust ducting to an outside location.

I think another thing to remember is that air and solvent vapors moving through ducting creates a static charge.  Keeping motors or anything that could spark out of the airstream is important, as well as ensuring motors, etc. are adequately grounded.  Letting the fan run for a while after painting is complete will allow time to exhaust any residual vapors and reduce the chance for any accumulation to occur, provided the fan is strong enough to do its job, and the booth is designed such that any dead spaces are eliminated.  Typically that's done with a baffle or small diffuser panel.

Gip Winecoff

Gip Winecoff

Obviously Gip has a ton more experience than I do, and I certainly don't wish to get into an argument with him about the hazards of paints, solvents and our hobby, for I would certainly lose. However I don't consider the concentrations of airbrushed acrylic paints and isopropyl alcohol to be any cause for concern. Alcohol evaporates incredibly quickly. Rub some on your bare arm, feel how cold that is? That's the alcohol sucking the heat out of your body during evaporation, do note how quickly the cold sensation goes away. That's the alcohol evaporating leaving extremely small trace amounts on your skin. Alcohol does burn, but it's quite safe, which is why caterers use it to keep steam trays warm at public events.  You are more likely to burst into flame due to a stray static electrical shock at the local gas station, than have your homemade spray booth explode while painting with Tamiya acrylic paints and isopropyl.

Bill, I think you are forgetting something (at least that's how I'm reading what you wrote): The vapors are the most dangerous. Generally speaking, a flammable material is more rapidly combustable as a vapor than as a liquid. For example, liquid gasoline burns, but gasoline vapor explodes (assuming sufficient oxygen). The more easily a flammable liquid evaporates, the greater the danger of combustion.

Unless conditions are right for an explosion, however, the greatest danger is often not the initial ignition, but the fire that may follow.  

As for the quantities involved being small, I wouldn't bet on it. Smart people do stupid things—why should the dumb ones have all the fun?

Now excuse me, I have to extinguish the cat… 

Ross,

Oh no, I'm not forgetting! And it's not just vapors, it's the "correct concentration" of vapors that are the most dangerous. Something that just isn't going to happen in the hobby spray booth, or at the very least not going to happen in MY spray booth. This is why I mentioned the alcohol burners that caterers use. I've lit them, and they light from the vapors, not the flame touching the liquid alcohol. But they don't EXPLODE! and you have to hold the lighter quite close to the burner before it lights. Holding a lit match a foot above the burner isn't going to do much. Isopropyl dissipates so quickly that by the time it reaches that lofty match, it just doesn't exist anymore in a dangerous state.

I recall the use of alcohol in F1 racing in the Way Back Then days, and the dangers were the explosive nature of the substance, and the fact that it burns almost invisibly. Mind you the average pit station held over 1000 gallons per pit and there were often 30 or 40 pits. That's a lot of alcohol. If you are attempting to use 1000 gallons in your spray booth, you should say "Y'all watch this" before each use! And please, use a camcorder to catch all the action!

My booth is built around a 36" wide kitchen hood and is 18" deep and 18" high. That's 6.75 cubic feet of volume. Given that I mostly use a 1/4 oz. color cup with my Badger 150, and thin about 60/40 paint to thinner, that's a pretty small amount of alcohol in "use". I am in more danger from the AB needle, than an explosion.

Bill,

Ah, the joy of stoichiometry! I'm not worried about you—just the dumb kid who tries this at home…

Some solvent vapors have a rather wide range of stoichiometric ratios with air. Not sure about common alcohols—but I don't want to find out the hard way that I've achieved stoichiometry… (Ain't that a marvelously euphonious word!)

If I recall correctly, those alcohol burners used in food warmers have the alcohol in a gel-like material that controls the rate of evaporation, which makes them much safer.

I agree that the amounts used while thinning and spraying are likely to be tiny, but Gip's comments about vapor pockets, especially in home built spray booths, are very germaine.

And then there are those of us who frequently to occasionally use rattle cans to prime a model (or paint something else). Under those circumstances, as already mentioned, the amount of flammable material available to create havoc is substantially greater.

And common sense isn't.

Serious stupidity has some of the qualities of great genius… 

Ross,

Email me your home address so I can send you 25 cents for stoichiometry! I should probably send you four bits since you used that and euphonious in the same sentence.

Gip's comments are always germaine, and he is coming down on the side of caution, which is always the best idea. I sometimes tend to cut things a little close to the edge, and have the scars to prove how fragile the edge can often times be! 

Bgrigg wrote:

Ross, Email me your home address so I can send you 25 cents for stoichiometry! I should probably send you four bits since you used that and euphonious in the same sentence. Gip's comments are always germaine, and he is coming down on the side of caution, which is always the best idea. I sometimes tend to cut things a little close to the edge, and have the scars to prove how fragile the edge can often times be!

He He He.

Keep'em coming guys.  It's quite funny.

So far I have had good luck with the ceiling fan in the kitchen and opening a window.
The kitchen table has taken on a look of the 60's, the cats, well I'd rather not mention them.
For some reason the wife and kids complain about headaches and when they blow their nose the buggers are funny colors.  I love my enamels...  well it's probably just that I got stuck on them since I started using them around 1967.

Whatever you do, don't light a cigarette after spraying for awhile. Zippo's have really big flames for some reason while painting.I used to have facial hair. 

Other than that, it's all good.

Bgrigg wrote:

Gip's comments are always germaine, and he is coming down on the side of caution, which is always the best idea. I sometimes tend to cut things a little close to the edge, and have the scars to prove how fragile the edge can often times be!

I resemble that remark! I've come down on the side of caution often enough to have permanent scars… (It's not the trip up, or the trip down that hurts—it's the landing where you miss the caution and hit the rocks and trees…)

Well folks, I threw out the line hoping that someone would bite on doing the math.

Frankly, I would be very hesitant to display my calculations because I just know that someone out there would simply look at the end result without working through their unique situation. Could be dangerous given the wide range of variables.

For what its worth, my approach is as follows;

Determine the average distance from the spray nozzle to the fan motor.

Set the compressor to the minimum pressure you would use. Fill the spray cup with a measured amount of solvent (the usual amount you normally fill), then first, with maximum liquid spray measure the amount of time your to empty the spray cup. Next, refill the cup and position a target at the average distance mentioned above and spray a pattern (it will be circular) without moving the nozzle.

Do it all again with the maximum pressure you would use.

From the distance and the circular pattern you can determine the volume of the cone for both air pressures.

Consider the worst possible scenario, assume that the entire contents of the cup are held in the cone, and determine the amount of solvent in the volume at both pressures.

If the result falls below the LEL of the solvent you might begin to relax.

Bear in mind that with the fan in operation, the air volume is moving past the motor at a rate determined by the CFM of the fan and static pressure of the system. Clearly this reduces the solvent density in the booth.

Keep in mind too, as others have indicated, the need for filters to keep paint buildup from the fan blades and motor, and the use of appropriate face masks.

These calculations are simple and straight forward and should indicate what the margin of safety is for any given set up. 

Well folks, I threw out the line hoping that someone would bite on doing the math.

Frankly, I would be very hesitant to display my calculations because I just know that someone out there would simply look at the end result without working through their unique situation. Could be dangerous given the wide range of variables.

For what its worth, my approach is as follows;

Determine the average distance from the spray nozzle to the fan motor.

Set the compressor to the minimum pressure you would use. Fill the spray cup with a measured amount of solvent (the usual amount you normally fill), then first, with maximum liquid spray measure the amount of time your to empty the spray cup. Next, refill the cup and position a target at the average distance mentioned above and spray a circular pattern without moving the nozzle.

Do it all again with the maximum pressure you would use.

From the distance and the circular pattern you can determine the volume of the cone for both air pressures.

Consider the worst possible scenario, assume that the entire contents of the cup are held in the cone, and determine the amount of solvent in the volume at both pressures.

If the result falls below the LEL of the solvent you might begin to relax.

Bear in mind that with the fan in operation, the air volume is moving past the motor at a rate determined by the CFM of the fan and static pressure of the system. Clearly this reduces the solvent density in the booth.

Keep in mind too, as others have indicated, the need for filters to keep paint buildup from the fan blades and motor, and the use of appropriate face masks.

These calculations are simple and straight forward and should indicate what the margin of safety is for any given set up. 

Bgrigg wrote:

Ross, Email me your home address so I can send you 25 cents for stoichiometry! I should probably send you four bits since you used that and euphonious in the same sentence. Gip's comments are always germaine, and he is coming down on the side of caution, which is always the best idea. I sometimes tend to cut things a little close to the edge, and have the scars to prove how fragile the edge can often times be!

gulfstreamV wrote:

Bgrigg wrote: Ross, Email me your home address so I can send you 25 cents for stoichiometry! I should probably send you four bits since you used that and euphonious in the same sentence. Gip's comments are always germaine, and he is coming down on the side of caution, which is always the best idea. I sometimes tend to cut things a little close to the edge, and have the scars to prove how fragile the edge can often times be!  OK, I know that stoichiometry is similar to osmossis or dispirsal of molecules in a chemical reaction or something like that. But what the heck does germaine mean?

D'oh! I meant germane! Germane:  adj.   Being both pertinent and fitting.

Germaine is a female given name...

Hey! We're working without an editor here!!

Bgrigg wrote:

gulfstreamV wrote:  Bgrigg wrote: Ross, Email me your home address so I can send you 25 cents for stoichiometry! I should probably send you four bits since you used that and euphonious in the same sentence. Gip's comments are always germaine, and he is coming down on the side of caution, which is always the best idea. I sometimes tend to cut things a little close to the edge, and have the scars to prove how fragile the edge can often times be!  OK, I know that stoichiometry is similar to osmossis or dispirsal of molecules in a chemical reaction or something like that. But what the heck does germaine mean? D'oh! I meant germane! Germane:  adj.   Being both pertinent and fitting. Germaine is a female given name... Hey! We're working without an editor here!!

Bgrigg wrote:

gulfstreamV wrote:  Bgrigg wrote: Ross, Email me your home address so I can send you 25 cents for stoichiometry! I should probably send you four bits since you used that and euphonious in the same sentence. Gip's comments are always germaine, and he is coming down on the side of caution, which is always the best idea. I sometimes tend to cut things a little close to the edge, and have the scars to prove how fragile the edge can often times be!  OK, I know that stoichiometry is similar to osmossis or dispirsal of molecules in a chemical reaction or something like that. But what the heck does germaine mean? D'oh! I meant germane! Germane:  adj.   Being both pertinent and fitting. Germaine is a female given name... Hey! We're working without an editor here!!

Maybe you were thinking of a German leafy vegetable, related to the French version, romaine… 

or rather, I was—I made the initial spelling error! (My only excuse is that I learned Latin at an early age, and it forever confused my spelling of English.)  Gott sei dank für Deutsch!

Triarius wrote:

Bgrigg wrote:  gulfstreamV wrote:  Bgrigg wrote: Ross, Email me your home address so I can send you 25 cents for stoichiometry! I should probably send you four bits since you used that and euphonious in the same sentence. Gip's comments are always germaine, and he is coming down on the side of caution, which is always the best idea. I sometimes tend to cut things a little close to the edge, and have the scars to prove how fragile the edge can often times be!  OK, I know that stoichiometry is similar to osmossis or dispirsal of molecules in a chemical reaction or something like that. But what the heck does germaine mean? D'oh! I meant germane! Germane:  adj.   Being both pertinent and fitting. Germaine is a female given name... Hey! We're working without an editor here!! Maybe you were thinking of a German leafy vegetable, related to the French version, romaine…  or rather, I was—I made the initial spelling error! (My only excuse is that I learned Latin at an early age, and it forever confused my spelling of English.)  Gott sei dank für Deutsch!

but...

The funny (sad?) thing is I actually caught your error and then went and duplicated it! English is by far the most difficult language to spell, as it consistantly ignores it's own rules. Like the "I before E except after C" rule, which has no weight when science can disprove it so easily...

Or "OUGH", how do you explain to a non-English speaker how to speak or spell consistantly when it has seven different ways to pronounce it: thorough, through, though, tough, bough, cough & nought?

OK, OK, which one of youse guys is callin' me Germaine?  I tink weef had about enough of da frivolities, ya' know?  Germaine was my first wife's name. She died from eatin' da poison mushrooms.  Come ta tink of it, my second wife, she died from eatin' poison mushrooms, too.  My third wife, though, she died from a serious head wound when she fell down, but dat's cause she would not eat da' poison mushrooms...

Oh, yeah...I was going to post some results of some calculations I did for savill (the OP), but I got a big attack of the stupids and left everything at work.  Sorry.  I'll try to have something posted tomorrow.  The numbers are interesting, and not exactly what I expected...

Gip

savill wrote:

Keep in mind too, as others have indicated, the need for filters to keep paint buildup from the fan blades and motor, and the use of appropriate face masks.

I had worked in an automotive shop which used a spraybooth for years. There was no filters and no paint build-up on the fan blades or motor. As long as the exhaust port of the booth is of far enough distance from the sprayer (airbrush in this instance) and the fan/blower is moving the proper CFM's, then the finely atomized paint particles are "usually" dry before it reaches the fan. With it moving the proper CFM's it kept the paint "dust" from settling on the blades.

Heck, that fan would almost suck my pack of cigarettes out of my shirt pocket.

Just thought I'd throw that tid-bit of info out for whatever it might be worth. 

Oh, the booth was approximately 4 feet wide, 3 feet tall, 3 feet deep with roughly a 30± inch fan cut into the back wall that blew straight outside.

Bgrigg wrote:

Like the "I before E except after C" rule, which has no weight when science can disprove it so easily...

OK, I definitely got a chuckle out of that!