How dangerous is this.. who else does it?

[Bart Sharp]

On this topic, let me pass along something I've heard and get your opinions. I have been told that the correct way to ventilate a spray booth is by having a fan or fans blowing in to the booth, not out. The inlet side of the fans is filtered to prevent dust being blown in, and air exits through filtered exhaust openings to keep airborn finish particulates from being spewed out. There are supposed to be two advantages of a "positive pressure" setup like this versus the "negative pressure" situation caused by having exhaust fans.

One, the fans are always operating in clean air (assuming they are turned on before the spraying begins, and that the entire inlet/outlet system is big enough to prevent fumes from building up). That should greatly reduce if not eliminate the potential for an explosion being set off by the fan motor.

Two, it helps reduce potential dust problems. Given that it would be difficult to guarantee that a booth were completely airtight, a negative pressure differential could pull dirty air through any little crack, not just nice clean air through the inlet filters. By positively pressurizing the booth with fans blowing in, no dirty air can sneak in. The only place the air enters is through the fans, and it is easier to ensure that they are properly filtered and sealed relative to the entire structure.

I've never built one, but it seems logical enough as it was explained to me that I'm pretty well convinced it's the right way. What say ye?

[Allen Bookout]

I am waiting for a response to Bart's post as I am just starting to set up a temporary booth now. Please hurry!

It sounds logical.

After thinking about it for a while I am thinking that it would work if your whole exit wall was filters. Otherwise it might just pile up overspray inside the booth.

[Bart Sharp]

After thinking about it for a while I am thinking that it would work if your whole exit wall was filters. Otherwise it might just pile up overspray inside the booth.

You would have to give some thought to the CFM that the fan is pumping in relative to the amount of air that could flow through the exit filter area that you have, but I doubt it would be anything as radical as having an entire wall of filters. Certainly wouldn't hurt, but I don't think it would be necessary. I'll bet there are tables out there for filters that would help answer this.

Like someone else mentioned before, I'm imagining the fan located up high behind the operator with the exhaust opening low on the opposite wall to keep the operator in clean air and take advantage of the heavier-than-air solvent vapors and finish particulates.

[Brad Olson]

You do have to be careful with a positive pressure booth.

Mainly, if you have a leak, the overspray will go somewhere you don't want it to go.

I personally would not recommend a positive pressure set up unless you shop is in a dedicated building and you do some serious engineering to make it work properly.

The usual way to design environments for hazardous fumes is to put them under negative pressure to a common controlled outlet. This way if there is a room leak, you know where the fumes are going.

For example Bio Safety labs are always under negative pressure to a common filtered exhaust. The reason being if something escapes, it goes to the filtration stack and not out the bottom of doors into the office space.

On the opposite side are Laminar flow environemtns which are under positive pressure. This is for cleanliness of the environment, but should never be used for working with hazardous materials.

[Allen Bookout]

That's good information Brad. Thanks!