So I'm creating what I believe to be a downdraft system. Boxing in the fan and going to have it draw air out from the bottom and have filters on the top of the front doors. @Russstang and I have been discussing this and maybe this is not the correct definition. Any input from you guys?
Also, the fan is a 4' fan and the opening is going to be a 2' x 5' (not sure how much I'm going to leave open at the bottom. Any one have a formula for figuring out what speed I need to run the fan so I don't create to much vacuum (a Russ question)?
Man, just a casual read on this subject brings a dizzying array of "stuff" to know. Downdraft, updraft, semi-downdraft, side draft. If it's a side draft (air comes in front wall and exits back wall for example) you need cfm (multiply face of exit wall and then multiply by 100.....so a 10' wide, 8' high exit wall x 100 would give you 8000 cfm air movement needed). Seems there could be some variation on cfm for up or down draft design, but the 100 multiplier seems to be the average. Looking at some industrial belt drive 48" fans I see some move around 21,000 cfm on high, so I'd say you will have to slow that big boy down.
Just for comparison, I have a Canarm S16-E1 16" exhaust fan in my 24'x30', 9" ceiling garage. I can tell you it will move a lot of air. If I start it up and open the man door without having a window open the force as you open the door will hit you and will shock you. It's actually moves too much air for a garage my size. I think your exhaust fan would be good if you were painting the car in the open shop. OR if you put a bunch of filters in both long walls to allow air in and out of the booth and just run the fan on the right. My exhaust fan moves between 2000 and 2200 CFM's
So total air flow is 15 * 24 * 100 (it's says to use 50-100 on downdraft systems) = 36,000 CFM total flow. I divide this number by 2880 (36000/2880) to give how many times the air needs to be exchanged per minute (12.5).
Ok, yay! I've got some numbers. Now how do I calculate what my fan would pull at a certain rpm?
I believe you're over-thinking it. Most home booths are just WAG cut-and-try. You have a better situation than most, already having lots of space and 2 fans. I would plan the ducting so you can run only one, or both fans. The more important part will be air inlet/outlet plenums and filter area/placement that gives you even air distribution, without introducing drafts that interfere with paint application. You're already way ahead of the "bank-of-box-fans" !