Flat-Oval Duct Equivalent Round Diameter

Flat-oval duct (a rectangle capped by two semicircular ends) is the shape used where a round won't fit the plenum depth and rectangular would leak, so a tech needs its equal-friction round size to enter into a friction-loss, velocity, or fan calculation. ASHRAE equal-friction: area A = (pi/4) b^2 + b (a - b), perimeter P = pi b + 2 (a - b), equivalent round De = 1.55 A^0.625 / P^0.25 (a = major axis, b = minor axis). A 20 x 10 in flat oval carries air like a 14.8 in round, a little smaller than the 15.1 in equal-AREA round because the flatter shape has more rubbing surface; at a = b it becomes a round of diameter b. Equal-friction, not equal-velocity. A design aid; the duct-design method and the fabrication drawing govern.

A = (pi/4) b^2 + b (a - b); P = pi b + 2 (a - b); De = 1.55 A^0.625 / P^0.25 (a = major axis, b = minor axis). Equal-friction equivalent round diameter.

The ASHRAE equal-friction equivalent round diameter of a flat-oval duct, De = 1.55 A^0.625 / P^0.25, per ASHRAE Fundamentals (duct design) and SMACNA, by name; first-principles.

First-principles equal-friction equivalence (not equal-velocity); the flat-oval area and perimeter are standard geometry. The fabrication drawing governs.

Estimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.

Field names used by the API: major_axis_in, minor_axis_in, equivalent_diameter_in, area_in2, perimeter_in

• Equivalence basis equal friction, not equal velocity; De = 1.55 A^0.625 / P^0.25ASHRAE Fundamentals
• Flat-oval geometry A = (pi/4) b^2 + b (a - b); P = pi b + 2 (a - b), a = major, b = minorflat-oval geometry