I am interested in a spray booth, and wondered about the genuine explosive risk of painting with acrylic paint mixed 50:50 with isopropyl alcohol.
So I did some calculations. Chemistry might be a little rusty. Does this look right? Advise if it doesn't!!!
Problem:
- at what volumetric flow rate would a 50:50 (acrylic paint:isopropanol) mixture have to be sprayed to generate a concentration of isopropanol vapour in air that is 10% of the Lower Explosive Limit of isopropanol?
- at that rate, how fast would some standard cup sizes have to be emptied?
- does it therefore appear safe to use an ordinary range hood with minimal risk of sparking off an explosive mixture?
Assumptions for this calculation:
- using a Nutone 24" Economy Range Hood for ventilation of booth, 180 cfm rating, mounted low in a spray booth to catch the heavy vapors
- using 100% pure isopropyl alcohol
- an ignition source can safely be present if the flamable vapour is at a concentration of 10% of the lower explosive limit
- all of the isopropanol will instantaneously evaporate into a vapor once sprayed from AB
- homogeneous mixing in the spray booth air as it enters the blower
Isopropanol properties
(http://www.osha.gov/SLTC/healthguidelines/isopropylalcohol/recognition.html)
- molecular weight (MW) = 60.09 g/mol
- Lower Explosive Limit (LEL) = 2%
- Vapor Density = 2.1 (yes, twice as heavy as air, so it will sink)
- specific gravity = 0.78 g/cm3
Basis: 1 minute
Calculations:
180 cfm fan = 5097 litres per minute of air
5097 litres of air * 2% (LEL) = 101.94 litres of isopropanol vapor in 5097 litres
10% of LEL (10 fold safety factor) = 101.94 litres * 10% = 10.194 litres of isopropanol vapor
molar volume of a gas at NTP (normal temp of 25 deg C & atmospheric pressure) = 24.4 litres per mol
10.194 litres / (24.4 litres per mol) = 0.4178 mol of isopropanol vapor (or liquid)
0.4178 mol * 60.09 g/mol = 25.10 grams of isopropanol vapor (or liquid)
(25.10 g)/(0.78g/cm3) = 32.19 cm3/minute of isopropanol liquid
@50% mixture, mixed paint rate = 64.4 cm3/minute
64.4 cm3/minute would be the rate at which a 50:50 mixture of paint would have to be sprayed to reach a level of 10% of the lower explosive limit of isopropanol.
How fast does that equate to some standard cup/jar volumes?
1/16 oz cup = 1.1848 cm3
1/3 oz cup = 9.848 cm3
4 oz jar = 118.3 cm3
at a rate of 64.3 cm3/minute, the above cups would have to be emptied into the air in...
1/16 oz cup = 1.1848cm3/(64.4 cm3/minute) = 1.1 seconds
1/3 oz cup = 9.848 cm3/(64.4 cm3/minute) = 9.2 seconds
4 oz jar = 118.3 cm3/(64.4 cm3/minute) = 1 minute 50 seconds
My Conclusion
- I doubt that I will spray a paint mixture at a rate greater than 64.4 cm3/minute with an AB (but correct me if I'm wrong!)
- I think that 10% LEL is a good target for safe painting into a simple range hood that is not explosion proof and may be an ignition source
- I therefore think it appears safe to use the range hood for ventilation with little risk of explosive atmosphere
Thoughts??
Kanuck
Calgary, Alberta