CFM = Sensible Heat Load (Btu/hr) / (1.08 * ΔT)
Store a friction loss vs. CFM vs. diameter table. Use INDEX/MATCH or linear interpolation to find the nearest duct diameter for a given CFM at your target friction rate. hvac design calculations excel
Flow Rate (GPM) = 24 * Q (Tons of Cooling) / ΔT (°F) CFM = Sensible Heat Load (Btu/hr) / (1