| Section | Shape | Air flow, m³/h | Selected velocity, m/s | Required area, m² | Section length, m | Required size, mm | Selected size, mm | Equivalent diameter, mm | Actual velocity, m/s | Losses per 1 m, Pa/m | Friction losses, Pa | Local elements | Total Σζ | Dynamic pressure Pd, Pa | Local losses, Pa | Total losses, Pa |
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About Ductwork Design Calculation
This calculator performs aerodynamic calculations for ventilation ductwork design sections. It selects a duct size based on airflow rate and a chosen air velocity, then calculates pressure losses due to friction and local resistances. The result helps you estimate total losses for each section and compare alternative sizes.
Benchmarks and recommendations
Standards and reference basis
EN 16798-3 is commonly used as a basis for designing building ventilation systems and selecting design airflows, as well as general network calculation principles.
EN 12237 and EN 1507 are used for requirements for circular and rectangular ducts, including tolerances and air-tightness classes. This matters when interpreting results because leakage and installation quality affect actual airflow and pressure losses.
ISO 5801 is used for fan performance testing and for comparing the available fan pressure with the calculated system losses.
Air velocity and how the selected velocity is set
Typical velocity ranges are practical benchmarks. For natural ventilation, 1-2 m/s is often used. For mechanical ventilation, common ranges are: residential 2-4 m/s, offices 3-6 m/s, industrial spaces 6-12 m/s.
Selected velocity v is taken as the midpoint of the chosen range when velocity is not entered manually. This value is then used to calculate the required duct area and the initial duct size.
Required area and duct geometry
Cross-sectional area A is calculated from the airflow rate Q and the selected velocity v.
A = (Q / 3600) / v
Here Q is in m3/h, v in m/s, and A in m2. Division by 3600 converts m3/h to m3/s.
Circular duct diameter is calculated from the area.
d = sqrt(4A / π)
Rectangular duct is selected from the area and the chosen aspect ratio. A common aspect ratio is h/b within 1-4. With h·b = A, the sides are obtained from the selected ratio and the required area.
Square duct side is calculated as sqrt(A).
Equivalent diameter and actual velocity
Equivalent diameter deq is used to calculate friction losses in non-circular ducts. For a circular duct, deq equals the actual diameter. For a rectangle or square, the hydraulic diameter is used.
deq = 2ab / (a + b)
Here a and b are in mm. In the calculations, deq is then converted to meters.
Actual velocity va is calculated using the selected nominal size, because the real area after rounding differs from the required area.
va = (Q / 3600) / Aa
Friction losses
Air properties are taken as constants: density ρ = 1.2041 kg/m³, kinematic viscosity ν = 0.000015 m²/s.
Reynolds number Re defines the flow regime and affects the friction factor.
Re = va · deq / ν
Friction factor λ is calculated using an approximation that accounts for roughness ε and Re.
λ = 0.1 · ( (ε / deq) + (100 / Re) )0.25
Here ε is in mm and deq in mm, so ε/deq is dimensionless.
Friction loss per meter R′ is calculated from the Darcy-Weisbach equation in pressure form.
R′ = (λ / deq) · (ρ · va2 / 2)
Here deq is in meters and R′ is in Pa/m. Friction loss for a section of length L is R = R′ · L in Pa.
Roughness correction b in this calculator is typically 1.0 for the common ε values from the material list. Total friction loss for the section is taken as R = R′ · L · b.
Local resistances and how the final loss is determined
Dynamic pressure Pd is calculated from the actual velocity.
Pd = ρ · va2 / 2
Total local loss coefficient Σζ equals the sum of ζ for all local elements in the section, including the quantity of each element.
Σζ = ζ1·n1 + ζ2·n2 + …
Local pressure loss Z is calculated as follows.
Z = Pd · Σζ
Total pressure loss ΔP for the section is the sum of friction and local losses.
ΔP = R + Z
Selection principle is straightforward. If the chosen nominal size results in a higher actual velocity, both R and Z increase. When comparing options, it is common to see how changing the size affects va, then ΔP per section, and finally sum losses across the whole network.
FAQs
Why do pressure losses drop significantly when the duct size increases
With a larger cross-section, the actual velocity va decreases. Both friction losses and local losses depend on va2, so even a small decrease in velocity can lead to a noticeable reduction in pressure loss.
Which matters more, friction losses or local resistances
It depends on the layout. In long straight runs, friction losses often dominate. In networks with many elbows, tees, grilles, and dampers, the Σζ contribution can be comparable or even dominant, especially on short sections.
What does Σζ mean and where do ζ coefficients come from
Σζ is the sum of local loss coefficients for all elements in a section. ζ values are taken from local resistance reference tables and manufacturers’ data. It is important to use ζ for the correct fitting geometry and operating condition, because some devices have ζ that changes strongly with position and flow.
Why is airflow in m³/h while pressure loss is in Pa
m³/h is convenient for ventilation sizing and practical design work. Pressure loss in pascals is the standard unit for network pressure demand and for matching it with fan performance. Inside the calculation, airflow is converted to m³/s by dividing by 3600.
Can I select a fan directly from these results
The calculation shows pressure losses per section and helps estimate total system loss. For fan selection, you also account for losses in equipment, filters, silencers, and a fouling allowance. The operating point is then determined by total system loss and required airflow, and compared to the fan curve as per ISO 5801.