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I am just about finished my little semi-knock down spray booth. It's 5x5 and will be totally enclosed with hinged doors for the knock down part. I will post pics when it's finished.

So I have my squirrel cage blower hooked up to remove the over spray and air and I think is is running a little slow at 690 rpm. I think it is about 200 CFM and only 0.25 SP. I'm sure I need to get it up to 1400-1500 rpm for me to get 400 CFM and 1.25 SP. I would think that I wouldn't need a high SP as I am not traveling any distance from the spray gun to the fan, the only resistance will be the filters as they get clogged.

I'm sure there will be questions regarding explosion proofing and such, I can assure you that I have thought about all this. I am having the blower housing and the motor enclosed (inside the booth but closed off from the fumes and spray area). The only concern is the lights. I know that I should get explosion proof lights, but I am placing them high above and behind the spray path and they are also in the path of the inlet air, so I think this should be ok.

I respect all you folks here and any help or criticism is always welcome.

Anyway, if anyone can help me out with the CFM requirement for the booth that would be great.Rod True38729.7934027778

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Rod,
I don't know the requirements. Here is what I do know. Most setups I've seen running explosion proof, expensive, gear, either use a 12 or 18 " fan.

Talking to C. Hoffman, he's had an 18 inch in use for many years, passing fire code in Minneapolis many times he sez. Plus he's still around to tell us what he does.

It would be an absolute shame to come here some day and read: You remember Rod or Bruce? Sprayed Nitro without explosion proof gear. RIP.

While inconvienient, some folks step outside and spray Nitro. Of course you have to pick your days. Never done that, myself.

While I've heard about the Big Bang Theory, I sure don't want to experience it first hand.Dickey38730.3317013889

I do my spraying outside, and have had good results. As Dickey said, "you have to pick your days". The occasional bug sands out without too much swearing.

Al Peebels

Seeing as how we on the "Wet" coast have had 25 days of straight rain, I don't realy have the option of outdoor spraying, If I want to set some sort of schedule. So, I have to spray inside or find someone locally to finish for me. But I would like to learn how to do the finishing as I think it is an important part of being a guitar builder.

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"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

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I rigged my booth (a small room that use to be a bathroom attached to my shop) with a ducted filter mounted in a window with fan and motor mounted out side it is only a 12" fan. The fan has nylatron bearings and ABS blades (all spark proof) the motor is an explosion proof 2hp 850/1150 rpm 2 speed. I run it at the low speed and have no issues. The ducting I built and use a 24 x 24 ac filter. My total exhaust system cost me about $250 total not counting my labor to build the ducting. But I also get NEMA motors at cost at work and that saved me about $75 on the motor The room is sealed with neoprene gaskets on the door edges and a neoprene wiper on the bottom. I have a filtered input duct ceiling mounted that is ambient pressure(non powered) this seems to work well for this size room 8' x 12' The overspray has on a few occasions built up a little in the room but clears quickly Even with all of this I make sure all ignition sources in the shop are secured before spraying nitro. I actually use KTM9 much more than nitro.MichaelP38730.6209606482

I believe a general rule of thumb is 100-125 linear feet/minute. Multiply that by the area of the air flow and you'll get the recommended CFM. Example, if you are spraying in to a 4'x3' booth you should have around 1200-1500 CFM.

120 LFM equals 2 linear feet per minute of air flow, which sounds about right. 200-400 CFM would barely accomodate a hobby booth for model cars.

And the motor is not the only thing that needs to be explosion proof or isolated from the air flow. If you do use a squirrel cage fan, it needs to be spark resistant. If you can't afford to set up safe ventilation, I strongly recommend just spraying outside.

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Just exactly how big of an explosion would one encounter if your nitro caught on fire? Would I just singe my eye brows or would my head get completely blow off?   

Graham, I've been on a few vapor explosions over my career. Usually it's "just" second degree burns and the severe pain only lasts a couple of weeks. If your real good though you can get the roof up a couple of feet and a wall or two on the ground.
If your not going to use explosion proof equipment at least use positive pressure. Think of a boat engine compartment... pump air in. If you use a duct so you have some distance prior to your booth, start the fan prior to spraying and leave it on till your way below the LEL (lower explosive limit).Nitrocellulose has a flash point of 21F and an autoignition temp of 338F (gasoline is 850F) so a spark or a hot bearing even static electricity would be enough to ignite it.
Then there's your insurance carrier...Mark

Well that's something to think about then. Wow.