The formulas established for the calculation of pressure loss are
established for ducts with circular section. It is the form best
adapted, because it gives the greatest section for a given perimeter.
In practice such as for example for reasons of obstruction or esthetics,
one meets other forms of networks, used in particular to convey
air treated in the installations of distribution of airconditioning.
One has for that resort to the concept of hydraulic diameter corresponding
to the equivalent diameter or are really used in the case of a circular
conduit.
Chesy established about 1820 a theoretical relation for ducts of
an unspecified form:
with:
 D = Diameter hydraulic
 S = Section of the duct
 P = Perimeter of the duct
On the basis of the Chesy's formula, the hydraulic diameter for
a rectangular duct is calculated by means of the following expression:
With:
 a = Width of the duct
 b = Height of the duct
If in turbulent flow the mean velocity is practically homogeneous
in any point of the section of a conduit, in laminar flow that is
much less.
It is recommended to show much circumspection when one plans to
resort to the concept of diameter hydraulic in the case of a laminar
flow.
For this reason, it is useful to use other formulas of computation
based on the concept of equivalent diameter.
In the case of use of conduit rectangular, the equivalent diameter
can be obtained for example by the expression of the ASHRAE:
In the case of use of oblong ducts, the equivalent diameter is
obtained by the expression of the ASHRAE:
With:
 a = Width of the duct (in particular the large diameter for
an oblong duct)
 b = Height of the duct (in particular the small diameter for
an oblong duct)
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