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### Convert Flat Oval Ducts to Round Sizes Formula and Calculator

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 De.

Eq. 1
De = ( 1.55 AR0.325 ) / P0.250

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

Eq. 2
AR = ( π a2 / 4 ) + a ( A - a )

and the perimeter P is calculated by

Eq. 3
P = π a + 2 ( A - a )

Where:

De = 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.
AR = cross-sectional area of flat oval duct, in2

Related:

Reference:

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