Wait, wait, wait, wait, wait...
We've been assuming I'll need to run the fans at 340 cfm! That is their MAX rating at 0 static pressure!
We all know that they won't produce those numbers, but they won't have to.Air Exchange Formula:
q = V x n / 60
n = air changes per hour: My basis of design = 6/hr
V = volume of the room: My room = 1638 ft³ (not taking into account the soffits, room treatments, seating, people nor speakers)
q = fresh air flow through the room (cfm)
Entering my data:
q = 1638 x 6 / 60 = 163.8 cfm
Therefore rounding up to 165 is my required cfm needed for my space to exchange the air in the room 6 times per hour.
Now, let's figure out the cross sectional area of duct I will need stay below the maximum 250 fpm velocity.
I used this website to calculate my required CSA: http://www.borino.com/GYC/airflowcalculator.htm
Which gave me a CSA of 0.66 ft². That equates to a 5.5" radius duct (11" diameter). Now because I want to split my supply into two separate ducts, I can go with two 8" ducts given that their CSA in total is 0.7 ft².Two 8" ducts pushing 165 cfm will put me at 235.71 fpm.