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For example, it is used to calculate flow through circular and non-circular tubes in order to examine flow conditions (i.e., the Reynolds number). In those cases, the characteristic length is the diameter of the pipe or, in case of non-circular tubes, its hydraulic diameter D h {\displaystyle D_{h}} :
The hydraulic diameter, D H, is a commonly used term when handling flow in non-circular tubes and channels. Using this term, one can calculate many things in the same way as for a round tube. When the cross-section is uniform along the tube or channel length, it is defined as [1] [2] =, where
Thus, the outer diameter of a catheter in millimeters can be calculated by dividing the French size by 3. [2] For example, a catheter with a French size of 9 would have an outer diameter of approximately 3.07 mm. While the French scale aligns closely with the metric system, it introduces redundancy and the potential for rounding errors.
For a circular pipe, the hydraulic diameter is exactly equal to the inside pipe diameter: D H = D . {\displaystyle D_{\text{H}}=D.} For an annular duct, such as the outer channel in a tube-in-tube heat exchanger , the hydraulic diameter can be shown algebraically to reduce to
In this expression for Reynolds number, the characteristic length D is taken to be the hydraulic diameter of the pipe, which, for a cylindrical pipe flowing full, equals the inside diameter. In Figures 1 and 2 of friction factor versus Reynolds number, the regime Re < 2000 demonstrates laminar flow; the friction factor is well represented by ...
The duct flange normally has 3/4" bolts at 6" nominal; spacing. Duct flange angle thickness needs to be designed for duct plate tensile stresses, as flanges will bend. 5/16" or 3/8" angle thicknesses are common. See above photo of round duct elbows, transitions, and stiffeners. The duct elbow radius is from 1 1/2 to 2 times the duct diameter.