I am also not clear on this concept, I did google , here is the information and link.
http://www.engineeringtoolbox.com/hydrauli...eter-d_458.html
Note! The hydraulic diameter is not the same as the equivalent diameter. The equivalent diameter is the diameter of a circular duct or pipe that gives the same pressure loss as a rectangular duct or pipe.
The hydraulic diameter - dh - is used to calculate the dimensionless Reynolds Number to determine if a flow is turbulent or laminar. A flow is
•laminar if Re < 2300
•transient for 2300 < Re < 4000
•turbulent if Re > 4000
The hydraulic diameter is also used to calculate the pressure loss in a ducts or pipe.
The hydraulic diameter is not the same as the geometrical diameter in a non-circular duct or pipe and can be calculated with the generic equation
dh = 4 A / p (1)
where
dh = hydraulic diameter (m, ft)
A = area section of the duct (m2, ft2)
p = wetted perimeter of the duct (m, ft)
The equivalent diameter is the diameter of a circular duct or pipe that gives the same pressure loss as an equivalent rectangular duct or pipe.
The equivalent diameter of a rectangular tube or duct can be calculated as (Huebscher)
de = 1.30 x ((a x B)0.625) / (a + B)0.25) (1)
where
de = equivalent diameter (mm, inches)
a = length of major or minor side (mm, inches)
b = length of minor or major side (mm, inches)
