What is equivalent duct diameter?

Round duct with same friction loss/length as the rectangular duct at same flow.

Equal area≠equal friction. Rectangular has more perimeter per area.

Recommended aspect ratio?

Keep a/b≤4:1. Above 6:1 use two ducts.

Very accurate for turbulent flow. Based on ASHRAE Fundamentals.

Different formula: d_eq≈1.55×A^0.625/P^0.25.

Free Engineering Tool — #162

Round equivalent of rectangular: d_eq=1.3×(a×b)^0.625/(a+b)^0.25.

d_eq FormulaHVAC Design

Equivalent Round Diameter

—

Rectangular Area

—

Equivalent Circle Area

—

Aspect Ratio

—