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Convert Flat Oval Ducts to Round Sizes Formula and Calculator

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Hydraulic and Pneumatic Engineering
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Convert Flat Oval Ducts to Round Sizes, Formula and Calculator

To convert round ducts to flat oval sizes, use Equation (1) (Heyt and Diaz 1975), the circular equivalent of a flat oval duct for equal airflow and resistance.

To determine the round duct size, enter oval duct minor and major axis sizes to calculate round duct diameter D_e.

Preview: Convert Flat Oval Ducts to Round Sizes Calculator

Eq. 1
D_e = ( 1.55 A_R^0.325 ) / P^0.250

where AR is the cross-sectional area of flat oval duct defined as

Eq. 2
A_R = ( π a² / 4 ) + a ( A - a )

and the perimeter P is calculated by

Eq. 3
P = π a + 2 ( A - a )

Where:

D_e = circular duct diameter for equal length, fluid resistance, and airflow, in.
a = length one side of duct, in.
P = perimeter of flat oval duct, in.
A = major axis of flat oval duct, in.
a = minor axis of flat oval duct, in.
A_R = cross-sectional area of flat oval duct, in²

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Reference:

• ASME Code for Pressure Piping, B31.5, Refrigeration Piping and Heat Transfer Components