'68 Coronet R/T wrote:minimum of 100 feet per minute (FPM) of air flow past the operator. For most booths this equates to needing a volume source of air of from 8,000 to 10,000 cubic feet per minute (CFM).
This is so true. As you say, without the right flow you're left standing in a fog of overspray which very quickly overpowers the capacity of your mask to filter.
Forgive me if I convert to metric here. I really find it difficult to work in Imperial measurements these days (although I have always been 5'10" tall and have no idea what that converts to, without a calculator, - go figure).
100 fpm is pretty close to our standard which says 0.5m/s. However, my idea of a standard booth is 4.0m wide by 2.7m high (13' x 9'). To get the amount of air movement required, multiply width by height, so 4 x 2.7 = 10.8 sqm x 60 sec/min x 60 min/hour = 38800 x 0.5 = 19440 cubic metres/hour. At 1 m3/hr = 0.59 cfm this is 11470 cfm which is quite a bit more than your 4 x 2000cfm fans will clear. You really need 6 fans. Obviously, this changes with dimensions of the booth, but your point about air movement is agreed and it is usually a lot more than most people think.
Positive pressure booths here rank about the same as premeditated murder. Makes sense to me, pushing those isocyanates into the rest of your workshop is exposing anyone in the workshop (including yourself) to a deadly poison. Negative pressure is the way to go, with an exhaust that gets the fumes away from people. Standard here is for the flue to be 3m above the highest point of the roof and directing the exhaust straight up. That's a bit hard to achieve in a non-commercial building - probably 3m above ground level would be better. I know negative pressure can introduce issues with dust - just need to be careful about sealing up any cracks.