The dimensions of pipes or tubes of a hydraulic system or ventilation are generally calculated according to the desired flowrate. The diameters of the ducts are usually the input size . For non-circular ducts, the oblong or rectangular section, using the concepts of equivalent diameters, diameters iso kinetic or hydraulic diameters.This allows to calculate the pressure drops, flow, or fluid speed in rectangular ducts, as if they were circular.
The Section of the pipe may be circular, oblong or rectangular
A conversion window gives equivalent diameter to setting the friction pressure loss.
Conversion equivalent hydraulic diameters and iso kinetic software mecaflux
Sizing of fluid conduits (aerodynamics hydraulic)
aspiration air handling and ventilation