Duct Segments
The building blocks of your duct system.
What is a Segment?
A segment represents one straight run of duct. It's a single piece with constant cross-section, carrying a known airflow. Every loss in the system belongs to a segment: the friction loss of the straight run itself, plus any fittings (elbows, tees, etc.) that are part of that run.
Segment Inputs
Airflow
Enter the airflow through this segment in CFM (cubic feet per minute). This is the total flow in the duct at this point, not the supply to each downstream branch.
Length
The straight-run length of the duct in feet (or meters). Do not include equivalent lengths for fittings. DuctStatic calculates fitting losses directly from loss coefficients, not equivalent lengths.
Duct Shape
Round: Enter the inside diameter in inches (or mm).
Rectangular: Enter width and height in inches (or mm). The calculator uses hydraulic diameter internally: Dh = 2WH / (W+H)
Material
Select the duct material. This determines the absolute roughness used in the friction factor calculation.
- Galvanized steel (spiral/seam). Roughness of approximately 0.09 mm. The default for most commercial HVAC.
- Galvanized steel (longitudinal seam). Slightly higher roughness.
- Flexible duct. Significantly higher roughness. Expect considerably more friction loss per foot.
- Fiberglass duct board. Intermediate roughness.
- Smooth (aluminum, PVC). Lowest roughness.
How Friction Loss is Calculated
For each segment, DuctStatic computes:
- Air velocity from flow and cross-sectional area
- Reynolds number from velocity, hydraulic diameter, air density, and viscosity
- Darcy friction factor via the Colebrook-White equation (iterative Newton-Raphson; laminar flow uses
f = 64/Re) - Friction loss:
ΔP = f × (L/Dh) × (ρV²/2)
See Calculation Method for the full explanation.