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### Hydraulic Diameter of Rectangular Ducts Calculator

HVAC Design and Engineering

Hydraulic and Pneumatic Design and Engineering

Hydraulic Diameter of Rectangular and Non-Circular Ducts Calculator

The characteristic length for internal flow in pipes and tubes is
the inside diameter. For noncircular tubes or ducts, the hydraulic
diameter *D _{h}* is used to compute the Reynolds and Nusselt numbers.

Inserting expressions for cross-sectional area and wetted perimeter of common cross sections shows that the hydraulic diameter is equal to

- The diameter of a round pipe
- Twice the gap between two parallel plates
- The difference in diameters for an annulus
- The length of the side for square tubes or ducts

For noncircular ducts Hydraulic Diameter :

Eq. 1

*D _{h} = 4 A / P*

or

Eq. 2

*D _{h} = 4 (h w) / (2 h +2 w) *

where

*A* = Cross-sectional area for flow (in^{2}, mm^{2})

*P* = Total wetted perimeter (in, mm)

h = height of duct (in, mm)

w = width of duct
(in, mm)

References

ASHRAE Fundamentals Handbook, 2021

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