Round-to-Rectangular Duct Equivalent

Formula and source

ASHRAE equal-friction equivalent diameter D_e = 1.30*(a*b)^0.625/(a+b)^0.250. Round-to-rect solves the unknown side for a target D_e; aspect ratio = long side / short side.

The ASHRAE equal-friction circular equivalent of a rectangular duct, D_e = 1.30*(a*b)^0.625/(a+b)^0.250, per ASHRAE Fundamentals (duct design) and SMACNA, by name; first-principles.

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