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This is very interesting to me, Back in the day, I worked in very large cabinet manufacturing plants. We sprayed plenty of solvent based products including lacquers. Anyway we had huge conveyor systems that after the spray booth and flash off areas, there were huge ovens. I am talking like 40 x 100 feet. In these ovens there was a HUGE gas fired burner with like a 2 foot flame flying out of it. The burner would cycle on and off depending on the temp the thermostat was set at. There were/are several fans also in there circulating air and also exhausting fumes. Let me tell you, you could not go in this oven without being overcome with the fumes, to me it had to be flammable. I have never asked anyone, but I am assuming they were using the gas fired burner to produce positive pressure in the oven, so as long as the fumes never got to the flame, it would not blow up. I have always thought, If a guy used lets say a house furnace, and could make the fan run continuously and then introduce the flame, it would be the same principle. Obviously you would have to have filters to the outside and somewhere for the air/fumes to go out, like a traditional positive pressure booth, or the fumes would eventually back up to the flame. Obviously, the air the furnace used to burn, could not be exposed to the fumes. If the cold air came from the outside, it would only raise that air a certain temp. Could be trouble in sub zero type weather Someone smarter than me has to have an opinion on this but there has to be literally hundreds of these ovens in the US curing flammable coatings. LOL. I think this principle could work, just don't want to experiment with it.......

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OK, You are right; a real paint booth setup uses a direct fired make-up air unit (usually natural gas)in conjunction with a dedicated and interlocked exhaust fan. As you've hypothesized, these units use 100% outdoor air. We've installed around a hundred of them over the years. There isn't an explosion hazard if the ventilation rate is sufficient; i.e. the flamable vapours are exhausted and the positive airflow is in the opposite direction. Contaminated air cannot get to the burners.

In this particular circumstance, I wouldn't recommend a direct fired heater.


Dave