The issues are how much cubic

The issues are how much cubic feet of air you want to move to provide sufficient airflow and how long the vent pipe is going to be, which determines how much static pressure you need to overcome.  There are several web pages that discuss this issue, here's one: 

http://www.briansmodelcars.com/tutorials/tutorial.asp?tutorialid=23&curpage=1

I built one using an old furnace blower fan unit and 4" exhaust pipe, but it only is about 16" in length so i didn't have much static pressure to overcome.

Ken L.

Just to poke some fun at your

Just to poke some fun at your logic. If you where modeling Z scale or TT you could get by with 6 x 6 spray booth..  The problem is  the user is  1:1 scale with  1:1 scale arms and  body to match. He's holding a 6 " air brush full of paint, this hose  dangling, in one hand.  In the other hand hes got this  6" pincer holding his 5" box car or diesel body.  He has to be 3" or 4" away to spray properly.  Hum!..    Going small is not good unless you stand at arms lenght  in front of the booth and hope the fans pull like H***.

Just to compare, what is your kitchen fan pulling ? Something  like 350 or 400 CFM, maybe more.  Into what size vent ? and not more than  12"  or 14" usualy.   Now apply this to your booth.  You need high output to get the fumes and particles in the wind and out as quick as possible.  Particles floating around will settle on your work. Thrashing 12V PC fans, thru a furnace filter into a 2 " pipe.  

Depending on the paint you plan to use, plastic tube  could get attacked in the long run. Metal vent, 4"  installed and forget about it.  Safer too if  anything bad (God forbid)  should happen,  like fire.

If you shop around you can get used Dayton blowers  that pull 350CFM for very reasonnable. PC fans have a habbit of being cheap, bearings wear, then they scream, then stop dead.  My 60$ Used Dayton is still going strong and pulls fine.  I just looked quick on Ebay  and you can get Dayton 460CFM for 43$ buy  now.  These are virtually indestructable and cost a mortgage new.  Mine did not look great. Took it apart, repainted it and use it.   During a very hot spell we even used it as a fan.

Marc Fournier, Quebec

2" Pipe Not Good

Fans move air well but do not compress air well. Big difference here. With a 2" pipe you will have the air moving capacity of a 2" diameter fan. A 2" fan won't move sufficient air to clear the spray booth. Compare the CFM of your airbrush to the fan CFM. You need a fan and appropriate sized duct with a CFM several times higher than your airbrush rating. The length and shape of duct will play a big role also. Straight is good, lots of bends is bad for air flow.

Also take into consideration the booth opening area. If the opening is 14"x14" (kind of small) then there is 196 sq. in. of opening. A 3" muffin fan has an opening of 7 sq. in. This means the air speed will be 28 times slower at the opening than at the face of the fan (196/7) without any ductwork. Put a 2" duct on the fan and you reduce its effective area to 3 sq. in. Now the air speed at the opening becomes 65 times less than at the face of the fan. See where this is going? You started with an insufficient CFM fan and made it 65 times less effective with the duct.

The 4" HVAC metal duct and a 400+ CFM squirrel cage fan is the way to go. Mimic the production units that are available.

DO NOT USE COMPUTER FANS to Vent Paint Overspray

Unless you plan to NEVER use Oil or Solvent Base Paint Don't use a DC Fan as they cause 100 Times more sparks than an AC fan. I am a finish Carpenter and have used and owned a number of Large room size paint booths. AC has a much lower chance of armature spark then DC has. 

Paint Booth Fans are sealed and shielded and never come in contact with paint or fumes and the fumes are what cause Explosions.

Extreme Volume of Air vs minor Volume of Solvent is the best chance of avoiding Explosions. If you build a 14"X14" spray booth with a 4 inch unshielded DC fan pulling the over spray through a 4" tube make sure your fire insurance is paid up as well as your medical insurance. Because your building a small BOMB if you use the booth for anything other than Water base paints.

Yes is sounds easy to put a couple small DC fans behind a small booth and suck the air out, only problem is when it explodes and blows a couple fingers off and burns all the hair off you face and head it is too late to worry.

This may seem extreme but It can happen. so look up on the internet Paint booth exhaust fans and see the difference as well as the requirements and look at your family Fire insurance to see that using unshielded fans in paint spray booth will VOID your home Fire insurance if they cause the fire that killed you and burned down your family home.

This sounds like over kill but it is the worst case scenario and can happen. Just do a little more than think a cheep spray booth will work and find out what is actually required to build one SAFELY.

We can't afford to loose Model Railroaders so always think safety first in all your work habits.

I suggest you spend a few dollars like Mark suggested by buying & useing one of the Daton 300-800CFN spray booth  fans. They are designed to be fire proof. Actually any of the 300-400CFN fans will have plenty of pull for a small 14" tall X 18"Wide X 18" deep spray booth with a 12 inch lazy susan to turn your project as you spray.

This is as small as I would ever use in order to reduce the over spray down to close to "0" as well as being a Safe booth to use. Just because your project is small doesn't mean that you can use a small booth personally 24W X 24 H X 24Deep is what I recomend for modeling from Z-S scale.

Dan

By the Numbers

First, Dan is spot on. Solvent vapors and sparks are a recipe for disaster. Whatever fan you use make sure the motor is not in the air stream. That effectively rules out the computer muffin fan idea. Would it blow up? Probably not. Can it blow up? Absolutely.

Let me throw a couple more numbers at you using industrial equipment as a benchmark. A common small spray area configuration found in many factories is a CTOF booth. Closed Top Open Face. Essentially what you are building. A small CTOF with a 12' wide by 8' tall opening will usually have an exhaust system somewhere around 10,000 CFM. Using this example the booth face = 13,824 sq. in. 10,000 CFM across a 13,824 sq. in. area = 1.38 CFM per sq. in. Your 14" x 14" opening has 196 sq. in. 196 sq. in. x 1.38 CFM = 270 required CFM exhaust.

The production hobby spray booths with their 300+ CFM exhausters are right on the money.

A couple of things

FKD a couple of considerations for you and your project.........

14"x 14"x14" is probably too small for the booth.  Like was mentioned it doesn't scale down. Your hands are the still the same size, your airbrush is the same size, you'll want some light in there so there needs to be room for that to get in there.  Some how there is always a ton of junk that winds up in there.... paint bottles, cleaning agents, etc. that eat up room. 

The 2" pipe is probably not going to do it - its a volume and pressure thing.Your 95 CFM computer fan probably doesn't have enough power (PSI) to push 95cfm of air through a 2" pipe. 

There is a ton of info on DYI spray booths on the net, especially from the military guys.  Some of if it is real good and some it is not so and you have to wade through it.

Be sure to caulk all the gaps in the corners, edges, fan area etc and tape the exhaust pipe gaps to get the maximum efficiency from your booth.  You would be amazed on how much air loss there can be.

A lot of the military guys are using the plastic tote tubs from stores like Walmart for the box of the spray booth.  Great idea and only about $5.00

I wouldn't us less than 150cfm for a small spray booth. You need that kind of volume to be effective

I wouldn't use kitchen and bath fans for the fan as they only produce about 45 to 90 cfm... not enough air movement to me.  You probably could double up the fans but you double the cost and that defeats the purpose.

Most fans these days are brushless so the the whole bomb thing is all but moot for the hobby sprayer. 

Passcahe sells their  motor fan combo for about $45 and it produces about 250cfm.  Their fan motor combo is brushless but not sealed and is intended to be used with hobby paints and solvents.

Something most hobbyist don't realize is just because paint is solvent free doesn't mean that it is chemical free. Use a spray booth and eye protection

You can buy a nice used booth sometimes if you look around - Craigslist etc

You can buy a nice new Paasche 22-26 booth from that auction site for $199 plus $20 s/h. It's a good price.  If you look around you can beat that price.

These are just my opinions, as they say your mileage may vary, and I am not looking to start a flame war here just sharing my opinion

Steve

DO NOT USE COMPUTER FANS...

Most computer case cooling fans have brushless motors, hence no sparks while spinning, so it shouldn't ignite solvent based paint fumes with sparking. Notice I used "shouldn't". Depending on the load on the motor, it can get pretty hot, though. Maybe even enough to ignite the fumes. My model airplane brushless motors are so hot after a run that you can't touch them comfortably for very long.

Anyway, seems to me that the cfm would be too low for proper venting. Put your hand in front of the vents on your computer while its running and you'll find very little air coming out. Almost like someone just breathing on your hand. These fans are only made to move air around and out of the pc, not really to replace air. That's why you should never run your computer with the covers off. It may overheat.

If you really want to use one (or 2 or 3), check the labeling on the fan for the specs. Should say somewhere on there brushless. If it doesn't say so, Google the part number on the label and that should lead you to some specs on the fan telling you if it is. If no labeling, then I wouldn't use it.

Smokey

venturi effect

  With all these worries about fans causing vapors to ignite, why not use a venturi effect to pull the air out of the spray booth?  True, you need to move a LOT more air than a simple fan in the pipe setup, but there would be zero risk of catching fire and you could use any type of cheap motor and fan to do it.  Basically you just build a box or tube big enough to fit around your exhaust duct and blow air through it at a high velocity.  The air will "drag" the air from the spray booth with it.  A 6" or 8" pipe around a 4" stovepipe with about a 1 foot "mixing" section should do it.

Toni

Centrifugal fans

Most pressure ventilation is carried out with centrifugal fans, although I have seen axial fans used in walk in spray booths. All these fans have external motors, to keep the motor out of the exhaust fumes. Axial fans are normally belt driven to achieve this, centrifugal fans normally have the motor on the fan shaft, really big ones are belt driven.

My booth was less than £150, it came with two sets of filters, and a turntable. It has a filter face size of 20" x 12", big enough for HO.

Smaller 14" booths are available in the UK for £90 ($145)

Mike Ruby

Centrifugal fan source

  Just wondering why no one has brought up using automotive blower fans for paint booths.  I know that running on 12 volts might scare off a few folks, but and old power pack should run one nicely and allow you to tune your airflow as well!

I have an old washing machine

I have an old washing machine motor waiting for a squirrel cage and a box.

Even commercial hobby booths

Even commercial hobby booths are not recommended for solvent paint. Read the paasche paint booth instructions.

Real Spray Booths

That is their way of skirting the requirements for a compliant booth which if they did make would be way, way too expensive for customers to buy and install. Actual spray booths (in the US) require: 1) certification; 2) a permit to install; 3) a permit to operate, and; 4) if the capacity to utilize a certain amount of material is present, reporting of VOC output. There is also a host of regulations concerning placement of a spray booth within a structure not to mention fire suppression and filter disposal. It is possible, although expensive as sin, to legally install a spray booth in a residential structure. However, once aware of what must be done, there isn't a customer on the planet that would be willing to do it. Unfortunately the regulations are not relaxed regardless of how small the booth is. All in all, a legal quagmire that Paasche and/or customers do not want to be involved in.