Hey there, here is a model of my paint booth layout in my shop. I did not design this booth, it was already there when I bought the place. I am wondering a couple things. Is it ok for the air to be recircualting back into the booth, after being filtered of course? Right now I have been hanging poly tarps, blue cheapo's from menards, from the track on two sides of the booth. I am wondering if these are ok, or if I should be using something more low particle friendly, like the 3M sheeting for car covering. I wonder if I need to be particular to what I am using so it is not giving off FM or any other kinds of particles or fibers. No idea what the CFM is of the fan, but you can feel it a little bit, but not too much. The motor/fan is mounted up in the attic, and the motor is out of the flow of fumes, so I believe it to be safe. It does pull some air, it moves a kleenex if you put it in the air path, but the air exchange in the room I don't think to be all that great. I don't know if a way to check the CFM to see what it is and if I need to increase it or if it is fine for now. I don't paint a lot, maybe once or twice per year, but may start to do more once I get more experienced. Any insight from the expects is greatly appreciated. Thanks.

I don't mean to sound harsh but that's really a bad, dangerous design on so many levels it's hard to know even where to start. For one thing, just because the fan/motor is out of sight it doesn't mean that with enough vapor concentration you can't create an explosion. The poor airflow is probably keeping you alive on that point, right now.

Unless those filters are very high-tech active (carbon) filters, recirculating the air is doing nothing. You can't filter out ISOs with a fiber/paper filter.

I would not be painting in there until it was re-engineered.

For the wall covering question, yes the automotive masking plastic would be better.

Thanks so much Chris, for the reply and being harsh. I want the truth, not sugar coat. Best would be to change it so the exhaust is sent to the outside, and fresh air from outside is brought into the booth? Or is it not quite that easy? Yes, they are just the fiber filters at this point. Good thing I've only painted in there twice. But I did use the booth to do some powdercoating at one point, but not any longer. I want it to be right, and safe. I don't believe the fan to be anything "paint ventilation" specific. Looks like it was a cage fan that had some of the tin work wrapped around it and a motor mounted to the outside of the tin work. I wonder if it needs to be replaced with something made for paint ventilation...

Are you saying you have 2 solid walls and 2 curtain walls? If that is the case it is more of a prep booth than a spray booth. It can be used a spray booth but it is not compliant. It would also need to be setup as a positive pressure booth. Meaning you pull fresh air from the outside, run it through a filter to get the dust out, then pump it into your space from the top vents. This will push all the dust and fumes under the curtain and into the rest of the building, but keep your paint job clean. It also means the fan doesn't need to be explosion proof as it is not exposed to the fumes, only fresh air.

They make special curtains for this application.

Correct, it has 2 hard walls and 2 curtain walls. It is in the corner of my shop, and is normally used as shop space, until time to spray, which I've done very little of up to this point, which is a good thing apparantly. Then the curtains come in and it get's cleaned up. I just wonder if I bring in fresh air, filtered before booth entry, can I use the exhaust setup I have if I exhaust to the outside of the building? I can still positively pressurize the booth, but instead of blowing it out into my shop from under the curtains, just use the exhaust setup I have and get it out of the building. It doesn't need to be elaborate because I don't need to paint a car a week, just now and then mainly my own stuff. I thought this was a better setup than any garage setup with a couple fans in the overhead door, but this one seems to be a train wreck at the moment. And I don't want to blow up..

You can try it, but a positive pressure booth will send the overspray wherever the least resistance is; generally they work better when the room is sealed well and you can control the path of the exhaust.

Correct that the fans do not need to be explosion proof, if just sucking in fresh air.

So maybe I can get some fresh air intake fans to make it positive pressure and use either my existing exhaust fan and exhaust ductwork or a new exhaust fan with my existing duct work to get fumes to the outside? I understand I need to change the duct work to exhaust outside. Chris, if not that, then what would you do with a setup like this to fix it right perhaps without junking it and buying a complete new booth.

For a DIY booth there are some fundamentals to consider:

1. Air turnover per minute
2. Velocity of air through the booth
3. Input & output sizing

You want a minimum of 4 complete turns of air per minute, at a velocity of approx 100 feet/min.

Imagine a room 10 x 10 x 10 = 1000 cubic feet. If we wanted to turn that volume of air over completely four times per minute, you would need to move 4,000 CFM through the room. You could do that many different ways. If we had one huge 10x10 exhaust duct and a huge 10x10 fan, you could move the air 40 feet per minute (around 1/2 MPH) through the room -- barely detectable breeze -- we didn't meet the minimum velocity, although we did meet the air turnover number.

If you only had a 2' x 2' exhaust and 2' x 2' fan, the velocity would be 1000 FPM ( approx 11 MPH) -- now we are painting in a pretty good wind, which is not too nice, either (you can actually disrupt your spray pattern).

So you need to balance it -- in this case a couple of 2' x 2' fans and matching exhaust that can do 2000 CFM each would be a good compromise. There's a lot more that goes into this, like eliminating local dead spots, avoiding short-circuits, etc -- but that's why real booths are so expensive.

Good calculator here:

http://www.comairrotron.com/airflow-unit-conversion

Sizing the input & output is important because you can have a 10,000 CFM intake fan in a positive pressure booth - but if your exhaust port is too small, all that fan is going to be doing is fighting static pressure in the booth and you won't move much air. Simple answer is to make the exhaust duct around 2x the (cross-sectional) size of the input side...then you should have enough headroom. Same basic idea for negative pressure - if the intake is too small then the exhaust fan will be inefficient (sort of like trying to suck a swimming pool dry through a drinking straw).

For your situation, I think you can figure out how to configure this correctly but the challenge is going to be that fan you have now -- there's probably no easy way to figure out the CFM rating.

Jim, one of our moderators here, built a very nice positive pressure booth and on a small budget:

https://www.autobody101.com/forums/view ... 12&t=10122

There are tons of threads on this topic as well, check out the search (esp. 'advanced search') - lots of guys have gone down this path.

And at the other end of the spectrum my positive pressure booth consists of a 10X10 tent, three 20" box fans with filters taped to them, and a lot of duct tape.

Thanks a bunch guys. I appreciate all the guidance. I will be making some changes to it before anything gets sprayed.