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Circular to Rectangular Duct Equivalent Hydraulic Diameter Calculator

HVAC air distribution systems frequently convert circular round spiral duct sizes into rectangular flat ducts to fit tight architectural ceiling plenums.

Longer or horizontal width dimension of the rectangular duct.

Shorter or vertical height dimension of the rectangular duct.

Volumetric airflow passing through the duct section (1.8 m³/s ≈ 3,814 CFM).

Calculated Result
Ø 457 mm

Equivalent Circular Round Duct Diameter

Huebscher Equivalent Circular Diameter (D_e)

457 mm (18.0 inches)

Hydraulic Diameter (D_h = 4A/P)

400 mm

Duct Duct Airflow Velocity

10 m/s (1969 FPM)

Duct Cross-Sectional Area

0.18 m² (279 sq in)

Aspect Ratio (Width / Height)

2:1

Calculation Breakdown

  1. Huebscher Equivalent Round Duct FormulationD_e = 1.30 × (a × b)^0.625 / (a + b)^0.25 = 1.30 × (600 × 300)^0.625 / (600 + 300)^0.25 = 457 mm
  2. Hydraulic Diameter & Flow VelocityD_h = 2ab / (a + b) = 400 mm, V = Q / A = 1.8 / 0.18 = 10 m/s

Duct Hydraulic Metrics Comparison

Interactive visualization based on your current inputs

Value
0.0150300450600Duct Width (a)Duct Height (b)Huebscher DeHydraulic DhVelocity (m/s × 10)MetricValue (mm or m/s)

What Is the Circular to Rectangular Duct Equivalent Hydraulic Diameter Calculator?

The Circular to Rectangular Duct Equivalent Hydraulic Diameter Calculator determines equivalent round conduit dimensions for rectangular ventilation ducting.

How Does the Circular to Rectangular Duct Equivalent Hydraulic Diameter Calculator Work?

It computes Huebscher equal-friction diameter, calculates true hydraulic diameter (4A/P), and verifies face airflow velocities.

Circular to Rectangular Duct Equivalent Hydraulic Diameter Calculator Formula & Variables

The core mathematical equation utilized by this calculator is expressed as:

D_e = 1.30 \frac{(a \cdot b)^{0.625}}{(a + b)^{0.25}}, \quad D_h = \frac{2 a b}{a + b}, \quad V = \frac{Q}{a \cdot b}

ASHRAE / Huebscher empirical fluid formulation equating rectangular duct boundary shear friction to circular Hagen-Poiseuille conduit flow.

How to Use the Circular to Rectangular Duct Equivalent Hydraulic Diameter Calculator

  1. Enter rectangular duct width and height in millimeters.
  2. Specify design airflow volume rate in m³/s.

Step-by-Step Example Calculation

600mm × 300mm Low-Profile Supply Air Duct

Input Values:

rectangularWidthAMm:600
rectangularHeightBMm:300
targetAirflowM3s:1.8
Worked Steps: Replaces a 463 mm round duct with identical friction drop, carrying air at 10.0 m/s (1,969 FPM).

Understanding Your Result

Huebscher Equivalent Diameter (D_e): Round spiral duct diameter offering identical pressure drop and CFM.

Hydraulic Diameter (D_h): Standard geometric hydraulic diameter.

Air Velocity: Mean velocity in m/s and Feet Per Minute (FPM).

Factors That Affect the Result

  • Higher aspect ratios require wider equivalent round diameters to overcome added corner skin friction.

When Should You Use This Calculator?

  • HVAC duct coordination, architectural ceiling plenum fitting, round-to-rectangular transitions, and fan static pressure calculations.

Assumptions & Limitations

  • Assumes galvanized sheet metal roughness (ε ≈ 0.09 mm) and standard atmospheric air properties.

Frequently Asked Questions

Calculation Accuracy & Reference Note

Conforms to ASHRAE Handbook - Fundamentals Chapter 21 (Duct Design).

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