Okay, first off, I ask EVERYBODY to forgive the chemistry ‘lesson.’ Second, I beg @tempestjohnny to understand this isn’t about his post. Third, I’d like to share with everyone some research I did when I built my own paint booth.
I’ve heard for many years that high VOC paints (volatile organic compounds) can be dangerous to spray because of the potential for flammable solvents to explode if sparked while mixed with air. When I built my el-cheapo spray booth, it was with the intent of doing mostly acrylic paints, but knowing that occasionally I’d want to clean an air brush to spray a solvent-based paint. So I looked into this, being the chemical engineer (inquisitive), homeowner (insurance), and husband (“honey, I burnt the basement”) that I am. I did the math to consider how much volatile solvent would need to be in the air to be an explosion hazard if I were running a spray booth with a sparky-type fan. I chose the following assumptions:
- A very conservative 100 CFM for a spray booth fan flow rate (the $80 specials on the Bezos supersite run ~160 CFM)
- Xylene as the offending chemical (it has a 1% lower explosive limit - LEL - the lowest concentration in air that becomes explosive)
- 1 in of water column pressure in the ductwork of the fan system (a very slight pressure to move air out of the booth)
- 72°F room temperature
- ideal gas law (low pressure)
With these assumptions, I did the following math - for transparency and to allow people to check me, I’ve made mistakes before:
- PV=nRT (ideal gas law) rearranges to n=PV/RT, where n = the molar mass of material in question
- P = 1.00246 atmosphere (1+ 1" of water column)
- V = 100 CFM x 1% = 1 CFM
- R = 0.730241 atm-ft3/lbmol-R (ideal gas constant)
- T = 72 + 460 (rounded off) = 532 R
- n = (1.00246)(1) / ((0.730241)(532)) = 0.002582 lbmol
- molecular weight of xylene = 101.16 lb / lbmol
- mass = n x mw = 0.002582 x 101.16 = 0.274 lb xylene = 124g xylene
- density of xylene = 0.864 g / mL
- 124 / 0.864 = 144 mL xylene
So, to reach 1% (LEL) xylene in the air being evacuated from the spray booth, I would have to be evaporating 144mL of xylene per minute. Assuming it were 100% xylene, that would be an entire Testors 10mL bottle evaporating in just over 4 seconds. I decided not to worry about an explosion-proof motor in my spray booth fan.
Now, like any good engineering consultant, I want to provide caveats:
- I chose xylene because it’s a huge part of the Testor’s enamel MSDS and it has a very low LEL, but it’s not necessarily the lowest. Pure ethanol is lower (~10% lower).
- The assumption here is that the fan evacuates the vapors (like out the window) and doesn’t blow them somewhere to accumulate (like the closet you’re working in).
- Your temperatures and pressures may be different (although with this math, neither will have any significant impact unless you’re airbrushing on the surface of Jupiter)
- Your air flow will be different. If it’s FASTER (>100 CFM), you’re even better off. If it’s SLOWER, you need to consider carefully what you’re doing. I mean, even at 25 CFM it would be one Testors bottle in 16 seconds, but that “evacuation vs. accumulate” issue become more important.
- Not once did I calculate nor have I mentioned how fast a chemical CAN evaporate. Vapor pressures, temperatures, air flows, etc. all play a part in this. I know from common sense that if I open a jar of Testors, it won’t evaporate in 4 seconds, so I didn’t get into this level of calculation.
- This is my math. Yours might be different. Your situation might be different. I am not advising you on how build, use, maintain, or otherwise be in the same room with a functioning spray booth.
Also, some additional information for those that read this far and aren’t yet annoyed with me: VOLATILE FUMES CAN BE HAZARDOUS TO YOUR HEALTH. This math was purely speculative about explosion concentrations, not breathing. Use your own judgement and consider a half-face respirator if you spray volatiles indoors. Also, consider the wife/kid/dog/cat that may be in the room/house with you and not wearing a respirator.
Finally, I want to be clear that @tempestjohnny is not wrong that enamel paints contain a lot of volatile solvents that are flammable. He’s also not wrong to caution about fans, because safety is always important and I considered the same issue when building my spray booth. I don’t know him except through here, but I’m sure he’s a swell guy and this is not to disparage him at all. I just didn’t get to this post before he did. Please don’t hurt me!