Ducts carry CFM at a velocity that balances noise, pressure, and size: about 600 FPM for quiet branches, 800 to 900 for supply trunks, up to 1,200 for returns. ACCA Manual D designs at roughly 0.08 inches of water column friction per 100 feet, counting every fitting as equivalent length.
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Ductwork Basics: CFM, Velocity, and Manual D
Velocity, friction rate, and Manual D in plain terms.