Airflow and ducts
Rectangular duct area and air velocity
- where
- cross-sectional area (m²)
- internal duct width (mm)
- internal duct height (mm)
- mean air velocity (m/s)
- air volume flow rate (m³/s)
Explain this equation
Works out the area and mean air speed in a rectangular duct, and its aspect ratio. Use it to check a proposed size against velocity limits.
a and b are the internal width and height in millimetres; their product divided by 1 000 000 gives area in m². V̇ divided by that area is the mean velocity v. The aspect ratio is the long side divided by the short side.
Flow equals area times velocity. Two ducts with the same area but different shapes carry air at the same velocity, but the flatter one has more perimeter, so more friction.
Sense check: 0.5 m³/s through 400 × 250 mm (0.1 m²) gives 5 m/s.
Watch out: aspect ratios above about 4:1 raise friction, noise and the need for stiffening; use the equivalent diameter for pressure-drop work.
Air velocity in a rectangular duct from the flow and the internal dimensions.
Enter your values to see the result.
Good to know
The 1 000 000 converts mm² to m². Keep the aspect ratio low (ideally under 3:1, not much over 4:1) for lower pressure drop, less noise and less material per unit of airflow. BESA DW/144 covers construction and stiffening for each pressure class.
Common mistake
Using external dimensions of insulated or lined ductwork.
Limits
Mean velocity only. For pressure drop use the equivalent diameter and the duct pressure drop calculator.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the equipment manufacturer’s data, CIBSE guidance, BS EN 378, the F-gas Regulations and your company procedures. Refrigerant work needs the correct F-gas certification.