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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 (in², mm²)
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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