I think a cross draft booth would be the most effective and simple to set up. It is the most efficient because you would only need one fan and one duct going straight through the roof. Every time air moves through a filter, through a duct, or through a bend in a duct, it suffers pressure loss. You have to compensate for these pressure losses by using bigger duct and/or a more powerful fan; both of which add cost fast.
I too am learning how to set up a booth, and what I present below kind of paraphrases the things I have read. I'm assuming your booth uses an exhaust fan that removes air from the booth downstream of a paint arrestor filter bank.
Mad57,
You want laminar flow to quickly remove overspray, resulting in a good quality paint job. There are also minimum velocity requirements to remove VOC's for safety reasons. To achieve laminar flow you need to have the size of your exhaust/paint arrestor bank be similar to the cross section of your booth. Likewise the area of your intake filters should be the same size or bigger than the area of your paint arrestors. Since you probably won't be placing parts in the corners of your booth, you don't have to have the paint arrestor bank be the same size as the back wall of your booth. Making it this large would require a much bigger fan to achieve the minimum air velocity. As long as the paint arrestor bank is pretty close to square and is a significant portion of the back wall of the booth, you should get good results.
Since the back wall of your booth is 14' wide, I think you should have a paint arrestor bank of about 8 to 10 feet wide. How high is your ceiling? If you're in a shop with a really high ceiling, you should build a lower ceiling for your booth. Make it as low as possible and still be able to manipulate the gun on the car effectively. Having a low ceiling will reduce your arrestor bank and fan size requirements. If your arrestor bank is significantly shorter than the height of your ceiling, then you will have turbulence in the top of your booth which will suspend overspray.
I'm guessing here, but you should get good results with an 8' x 8' arrestor bank. Center it on the back wall of your booth, and that leaves 3' on either side of the bank where turbulence will develop near the corners (see attached sketch).
The math part comes in when you need to figure out what size fan you need. OSHA used to require 100 ft/min flow, but now you can design for slower flow if you know that your solvent concentration in air justifies a slower flow. Most people don't know and design for 100 ft/min. I have also read that 50 ft/min works pretty well, so you should shoot for something in that range. Keep in mind also that since your arrestor and filter banks are smaller than the cross section of your booth, the flow will actually be slower than what this simple calculation gives. Buy the biggest fan you can afford.
So for your 8' x 8' filter, you need a 6400 cfm fan to achieve 100 ft/min flow velocity. Realistically, you should probably double that requirement to compensate for the pressure drops through the filters and paint arrestors, ducts, and the fact that you're filter area is smaller than your booth cross sectional area.
Wow that was long, but feel free to ask for clarification on certain points. Any experts' input would be appreciated, too.
thks ahead of time.