Do not mix diameter definitions
Rectangular Duct Diameter Definitions
Compare the ASHRAE equal-friction round equivalent with hydraulic diameter and equal-area diameter. These quantities answer different questions and are generally not interchangeable.
Geometric comparison
ASHRAE Handbook, Duct Design defines De as the circular equivalent of a rectangular duct for equal airflow, resistance and length. Its SI Table 3 gives 378 mm for 400 × 300 mm; the equation returns 377.7088 mm before rounding.
Hydraulic diameter uses flow area and wetted perimeter. A NASA duct-flow derivation gives Dh=4A/Pw and, for a rectangle, Dh=2ab/(a+b). It is the characteristic length used in Reynolds/friction correlations when that model is applicable; it is not the ASHRAE equal-friction round size.
DA is only the round diameter with the same cross-sectional area. Equal area does not imply equal resistance. De alone also does not calculate pressure drop: the selected duct method still needs flow, length, roughness, fluid properties and applicable fitting losses.
Removed unsupported claims: universal aspect-ratio recommendations and an accuracy percentage without an applicability range/source. Presets and prefilled dimensions were removed so the page opens without a hidden design assumption.
Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
