**Units: **f and K are unit-less. D and L_{eq} have the same units. For
instance: If D is in meters, then L_{eq} is in meters. If D is in inches, then L_{eq}
is in inches. If D is in ft, then L_{eq} is in ft.

**Equivalent length equation: ** L_{eq} = K D / f

If rectangular duct, compute D from: D = 4 A / P

where A=Area of duct and P=Perimeter of duct. Equations from references listed at Discussion
and References for Closed Conduit Flow.

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