I have a 400 watt hps light on its way to my house. Will 2 inline duct fans; one intake, one exaust, both rated at 85 cfm be a good ventilation system for a 5' by 3' by 2' grow tent through out an entire grow?
Doubtful. I have a 600 w air cooled HPS in a 4x4x6 tent in a cool. Assent and I need a 435 cfm 6" fan running full blast to keep me temps and humidity in check while in flower. Better too much ventilation than not enough....buy a fan bigger than you need and put it on a speed controller.....this way you can control temps and humidity across a create range, and can use the fan if you expand to a bigger light/space.
as mentioned, that's a pretty weak fan. you really only need one fan to exhaust. use it to push pull, air through the hood and out of the room. my fans are about 460 cfm and work well.
Well I have 4 more 6'' duct ports. If each 6'' inline duct fan is rated at 250 cfm how many should I get?
This is a very Inefficient way to do it.....you'll draw more power, have more cords, and a bunch of ducting to connect all the fans, it'll be a mess. I recommend doing it right and keeping it simple....get one good 400 plus cfm 6" fan and a speed controller and you are good to go.
If your looking at booster fans, they really only push around 50% of the rated cfm once hooked to ducting. A single, inline high velocity fan is the way to go. I run a 424cfm cut down to medium in a 4 x 4. Worked great with my 400w.