I did a search and found some information but I am looking for more specific info such as what size, CFM wise, should an exhaust fan be for a given area. I am looking for more info on the technical side for air flows, cross draft, down draft, etc.

Anyone have any info?

Thanks

multiply width of room by height and x 100 I think. may be 150. length of room is not a factor. assuming fan is on end wall.

The exhaust must be slightly less CFM than the make-up air system to keep a positive pressure inside the booth and to eliminate infiltration. Having said that, the next step is sizing the system. How many air changes per hour do you need? Multiply the height times the width times the length to get total cubic feet of area inside the booth. How many times you want to change that air will dictate the size of the make-up air unit. For instance: If the booth was 15' wide, 24' long and 10' high you would have 3600 cubic feet of space in the booth. If you want to replace that amount of air every six minutes, you would need 600 CFM of make-up air. If you want to replace it every three minutes, you will need 1,200 CFM of make-up air. Size the exhaust about 10% less than the make-up air unit. I recommend at least 20 air changes per hour or once every three minutes. E-mail me for more info on how the air gets inside and back out of the booth (thekeens01@comcast.net)

If it's a homemade booth, you don't want a lot of air flow, where a professional booth like a downdraft handles the air different. All you need is to get the fumes out, but a lot of air movement only adds to dust problems. You can barely see the painter but he can see whats he is doing, takes a while for the fumes to get out, but the paint jobs are really nice, theres enough air going through that the rest of the shop you don't need a respirator, just another idea, works good for me.
Rob

I believe you have to pull air 110-150 feet per minute thru the booth to meet OSHA specs.

Its a little more involved than the replies I've seen posted- you have to know what type booth you are dealing with, plus static pressures. A crossdraft booth, for instance, is air in/air out. There is no positive or negative pressure if the fan is sized right. Downdraft booths are different, they can be positive or negative pressure booth. Both have pluses and minuses.

Easiest way is to determine your static pressure, your required CFM's, and then use calculations to determine fan size, fan hp, and fan RPM. You can also vary fan speed and output by pulley size.

Formulas can be found using Google search engine. Usually "Fan Laws" will bring up your formulas for you.

Its not as simple as doubling the CFM or figuring CFM alone as most fans are sized and move air at optimum rates and velocities based on blade size, static pressure, and hp.

My goal for a homemade booth would be 1cfm of fan per cubit foot volume on a positive pressure booth. Because you will have fan losses due to the nature of the positive pressure. In reality I probably got only 50% fan efficiency on my small 10'x10'x9' conduit and plastic positive pressure booth when I used a single 1500cfm squirrel cage furnace fan. Yes, there were undesireable air currents until I rigged up a diffuser baffle. However, there was relative little paint dust clinging to the plastic walls, but there was signficant paint dust on the floor near the exit duct.

I believe it is also important to have fresh air breathing apparatus not only for you health, but also to allow yourself time to stay in the booth an extra few minutes. In a plastic homemade booth, you need to let paint flash off and paint dust to settle, before you open the door and allow dust to stir up or dislodge from flexing plastic panels and embed in your fresh paint.

On a booth large enough for a car, I would use at least two carpet blower fans (2x1500cfm=3,000cfm) forcing air through some diffuser ducting above head level.
On a commercial booth, you are probably looking at 3-phase 5hp+ 10,000cfm fans where the fumes are gone within a fraction of a minute.

merch120 and bonecrusher67conv. That is some pretty good information, but I'm from the old school of trial and error, which is the hard way of doing things, but once I get it to working right, then, the old adage if it works don't monkey with it. I have had several inquires on different forums about my booth, ventilation and air supply. I might be able to get the pictures and write it up tonight or tomorrow. Theres a lot, or the majority of us are do it yourselfers and looking at pictures of how did he do that works pretty good.
Thats how I learned, looking at how he did it, plus being 64 years old you have a lot of years to see a lot of stuff.
The setup I have here will turn out a beautifull paint job, as good as any professional booth, but then again it's not for professional bodymen to use on a daily basis. It's the old home grown setup.
Somebody mentioned supplied air, I will show that setup to, pump is in the lower level of the hose, 20' underground to the shop.
There isn't anything professional about any of my setups, but they work, and there user friendly.
I don't mean to sound like I did this and I did that, like blowing your own horn, I just have a lot of ideas that I would like to share, and I really get into this kind of stuff, just an old retired guy having a whole lot of fun.

Rob

http://webpages.charter.net/2manitowoc

Nothing wrong with the trial and error method. Already been down that road- I built a metal stud/drywall booth about 10 years ago that worked really well. Then, I invested in the money to buy a booth kit, spent the money on a SATA air/oil seperator/charcoal breathing unit (2100.00) and its like night and day. The only advantage I have with my new booth over my home built one is airflow. But that airflow makes a big difference when you are in and out of a booth painting. Dirt doesn't accumulate, and the paint jobs themselves stay much cleaner.

I will be glad to give you any measurements off my booth if you need them. I have a semi-downdraft booth.

I use my booth mainly for this stuff:



http://img211.imageshack.us/img211/6792/bike1er7cw.jpg

Wow, that is beautifull, I wouldn't know where to start on something like that, but that is outstanding work. It just kind of pops out at you.

I am only into 69 super sports, my goal is to make the nicest restored non matching numbered chevelle out there, might be just an old man dreaming, but I intend to make the body nicer then the chassis. I'm setup to do all the mechanical and bodywork, used to do stuff for a lot of guys, but now it's just the ss, hows that go,. a man on a mission.

If it looks half as good as your bike, I will consider myself successfull. Thanks for sharing the picture with us. Have a good one

Rob

You can't just slap up a make-up air unit and exhaust fan and expect it to work properly....even if they are sized correctly. Duct sizes are critical. The external static pressure of the make-up air unit is critical. Exhaust fans are usually rated in free air CFM (no static). Air inlets and diffusers make a difference. Another thing to consider: If the make-up air unit is mounted on the roof of the building, the internal booth temperature will be the same as the outside air unless it is a heated unit and that's a whole other ballgame.

Toby
You stated you can't just slap up a air unit and expect it to work, sometimes that is true, and then again some of us home boys can make things work with a lot less money, usually because we don't have the cash, we have to improvise a little. Below is an example of what just came out of a slapped together, cobbled up, hodge podge assortment of whatever.
http://webpages.charter.net/2manitowoc

1969SS,
You're exactly correct. With some work and ingenuity, nice work can be turned out in a home made booth. I was just trying to keep the guy out of trouble with the law. They're real tough these days. It would really be a bummer if someone built a booth and the man came buy and told him he couldn't use it. Just trying to help.

You're right Toby, and that makes good sense
Rob

I thought I would add a comment about blowers because I'm sure some folks get used cage blowers out of furnaces for this use. Those blowers are designed to work into the backpressure generated by the house duct system. If you run them into free air, they will draw too much current and burn the motor out. I've seen a three amp motor draw ten amps when it was run without backpressure.

If you have a direct drive blower, which is one with the motor connected directly to the cage, you need to restrict the output. Hook up a multimeter set to measure AC current into the motor. Adjust a baffle at the output of the fan until the amperage draw is lower than the nameplate rating for the motor.

For a belt driven blower, you simply adjust the pulley ratios for correct draw. Older furnaces used these and the pulley set was adjustable to set things up.

Also, keep in mind these fans are filthy. They need to be thoroughly cleaned before using them for a paint booth.