Do you have a version of the venerable “Trane Ductulator”? It’s okay, we know you use them, even though we are all supposed to be using static regain duct design programs by now. Look on the right-hand side in the red-orange area of the wheel, where “Air Volume – CFM” overlaps “Velocity – FPM”. Did you ever notice how there is a darker shading on velocities greater than 2000 FPM and a note to verify that your velocities are correct? That’s because rectangular ducts are not recommended to be used over 2000 FPM velocity.