I am currently installing 3 large southern pride smokers, xlr-1400 for a commercial smoking operation. I have run into an issue with sizing my fans and am looking for some help. The city is looking for calculations to justify the size of fan I am using for the duct work. Here are the specs:

Each Smoker has the following duct work and its own fan:

Duct = 12 inch x 12 inch
Length = 18 feet straight shot
Filter = 1, increasing static pressure by .2

I am using a two speed fan (3/4 hp) designed for 1 inch static pressure that runs at 2323 cfm on the low speed and 2705 cfm on the high speed. I am pretty sure this is adequate but I need to back it up with some calculations. Can anybody point me in the direction of where to find a way to do this? Or better yet, is it a simple equation that someone on here knows?

Thanks for you help!