good afternoon, all. i am looking for a spray booth for spraying enamels in my hobby room which is a spare bedroom. i have read several posts but they seem to contradict each other. is there a booth that you have been using which works to remove the overspray and the smell to outside? Experience is king and i hate to put out money for a booth that only works with acrylics. Thank you for your feedback
Two things youāll want. A fairly powerful fan to pull fumes. And second the fan type should be a āsquirrel cageā type. Where the fan motor is not in the direct air path. With enamel paints there is a chance that the fumes could spark the motor if in direct path.
Okay, first off, I ask EVERYBODY to forgive the chemistry ālesson.ā Second, I beg @tempestjohnny to understand this isnāt about his post. Third, Iād like to share with everyone some research I did when I built my own paint booth.
Iāve 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 Iād 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 (āhoney, I burnt the basementā) 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)
- 72Ā°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, Iāve 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 itās a huge part of the Testorās enamel MSDS and it has a very low LEL, but itās 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 doesnāt blow them somewhere to accumulate (like the closet youāre working in).
- Your temperatures and pressures may be different (although with this math, neither will have any significant impact unless youāre airbrushing on the surface of Jupiter)
- Your air flow will be different. If itās FASTER (>100 CFM), youāre even better off. If itās SLOWER, you need to consider carefully what youāre doing. I mean, even at 25 CFM it would be one Testors bottle in 16 seconds, but that āevacuation vs. accumulateā 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 wonāt evaporate in 4 seconds, so I didnāt 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 arenāt 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. Heās also not wrong to caution about fans, because safety is always important and I considered the same issue when building my spray booth. I donāt know him except through here, but Iām sure heās a swell guy and this is not to disparage him at all. I just didnāt get to this post before he did. Please donāt hurt me!
7 Likes
This is a great post @gomeral - and echoes a similar discussion that one often sees in woodworking groups about dust extraction/collection systems. In theory a static spark in dusty air can cause a flash fire or even an explosion. It is a serious concern in industrial spaces.
But like youāve found, in the low concentrations of dust per volume of air that a hobby or even a small pro shop can generate, the risk just isnāt there.
This isnāt to say you should empty an entire rattlecan into your little Amazon spray booth. Or that other factors, like faulty wiring, clogged filters, etc etc canāt be fire hazards on their own. Recently a well known YouTube woodworking wood shop had a dust collection duct fire, but it wasnāt due to explosive dust concentration, it was due to the screws used to assemble the ducting penetrated into the airflow and had created a small clog which caught a spark.
Iām in the early mental stages of rebuilding my airbrush booth, so this is a good discussion. Thanks!!
2 Likes
Absolutely great info, @gomeral! I read the whole thing! 
Thanks for taking the time to share.
Fantastic break down, I had already heard that it was difficult or near impossible to reach levels where the paint fumes at the amounts we use in this hobby could cause a flame from a spray booth motor. So having the actual breakdown and math behind it is nice to have.
Do have one question, more of a double checking of math really, if I build a spray booth and use multiple fans the CFM is additive? (IE if I use 3 50 CFM rated fans it would be the same as using 1 rated at 150 CFM)
Thanks, guys. I was a little concerned when I posted!
I want to add some additional information, since I had a little time during my lunch break to look up some values and do some math. Different solvents have different LELs, molecular weights, and densities, so an āeasyā comparison between xylene and other solvents can be done in Excel. For the community, a list of other VOCs/solvents in order of least-to-evaporate-per-minute to most (essentially, least to evaporate to reach explosive limit to most to evaporate):
- 107 mL/min - Butanol (mw 74.12, LEL 1.0%, sg=0.81)
- 144 mL/min - Ethylbenzene (mw 106.17, LEL 1%, sg=0.87)
- 144 mL/min - Xylene (mw 106.16, LEL 1%, sg = 0.86)
- 150 mL/min - Mineral spirits (low flash point) (mw avg = 140, LEL 0.7%, sg=0.765)
- 179 mL/min - Isopropyl alcohol (mw 60.1, LEL 2%, sg=0.786)
- 189 mL/min - MEK/Butanone (mw 72.11, LEL 1.8%, sg=0.805)
- 225 mL/min - Acetone (mw=58.08, LEL=2.6%, s.g.=0.78)
- 262 mL/min - Butyl acetate (mw=116.16, LEL 1.7%, sg=0.883)
At least one of these was a surprise to me - I would have expected acetone to be near the top of the list! But the lower explosive limit is almost 3 times that for xylene and others.
Anyway, hopefully this little exercise is helpful to someone. I reiterate that this is my math and maybe Iām doing something wrong, but this calculation is specifically to determine āhow much would have to evaporate to reach the explosive limit at 100 CFM?ā Some of the smelliest and likely easiest to evaporateā¦it still takes a lot!
Also, @Ranger_Chris61, I would say (a) good question, and (b) yes, CFM is additive. Iām considering the flow rate in the booth and not only where the spark might be. If you have 100 CFM flowing through the booth filter and out through multiple vent fans, the concentration at the sparky place would be determined by the flow/mixing in the booth and at/around the filter, not just at each fanās entrance. I suppose itās fair to add, then, the caveat, āthis all assumes perfect mixingā but I donāt think thatās unreasonable for vapors like this. Well, I suppose to be ultra-rigorous I could do velocity calculations and estimate mixing coefficients, butā¦yeah, no. Certainly not today! 
Anyway, once the air/vapor flow is combined, you achieve the %VOC mix. Any quantity of that mix that you remove from the booth would either be at the LEL or not.
Hope this helps!
2 Likes
Very pleased you have taken the time to enlighten ignorant folk like me! This is a very useful post that I will peruse
I am looking at building my own for all the reasons you mention regarding enamels and things like Mr Leveller etc
Darn you people and your valid and well-considered comments!
This has been stuck in my head for a while, so I pulled up a Tamiya rattlecan MSDS. They use dimethyl ether as the propellant, which as a gas makes it hard to compare to āevaporation rateā of the liquids. But, it would have to be present at the LEL percentage and since it is already a gas, if youāre pulling 100 CFM and the LEL is 2%, youād have to be spraying 2 CFM of DME.
I donāt have any idea how much propellant is in a Tamiya rattle can, so I measured the dimensions of one (AS-12, of course) and took a wild guess that itās at 200psi. If that is the case, and the entire can is nothing but DME, there would beā¦0.08 cubic feet at standard conditions. (The can is 2" diameter by 3" tall.)
Working backward to calculate the pressure if there were exactly 2 cubic feet of compressed DME in there, I getā¦5,400 psi. I have my doubts. Even if this were the case, youād have to blow through the entire can in one minute. I still donāt see an issue unless you decided to spray directly into the fan (donāt do it).
Finally, paint cans typically work by having a compressed gas at the top and a liquid at the bottom, using a dip tube to pull the paint up to the spray nozzle. So flammable propellant isnāt really an issue until you get to the end of the paint, except for whatās dissolved in the paint itself. Play it safe and donāt de-pressurize your can by spraying the gas into your booth.
This has gotten ridiculous. I promiseā¦no more!
3 Likes
Apologies, @mecherraa, for inadvertently hijacking your thread. The answer to this question is basically āany of them should be fine.ā
1 Like
I didnāt go as in depth as @gomeral did, but I can confirm all his work. We are not producing enough volitle vapors to be concerned with a fire risk. And I have been using a 400CFM grow room fan for 7 years now with no issues. My fan is an axial fan with the motor right in the middle of the airstream.
Iāll share another story that is similar.
Washer fluid. It has some alcohol in it so there is this big warning that it could be flamable. Well, in an emergency this one time, I used it to put out a fire.
Ok, now back to the paint booth and getting rid of smellsā¦
It is impossible to get rid of all the smells without venting to the outside. I donāt care who you are or what you did, it is impossible. I have done everything I can to bring my paint system into my house because I live where it gets cold and I paint in the basement and donāt have a good way to vent outside.
The fumes cannot be removed. So I have to paint in my garage. In the winter I have to spend a lot of energy to heat my garage up. Turn on some fans to blow the air outside, and only then can I paint. So in the winter I canāt get a lot of painting done. This applies only to high odor lacquers and enamels though. I use non-toxic acrylics for parts painting inside.
Anyways, you have to vent outside. And it doesnāt really matter what kind of paint you are using. Lacquers, or Enamels.
Another place you can vent if your room is in the right location is you can vent into your attic. I have a friend that does this. He actually created a two stage vent system. He uses a bathroom vent fan at the booth, and then he has a shop vac in the attic that sucks the fumes up there.
Just a thought.
3 Likes
No ill will taken. Your reply is a great learn