| Section | Airflow, m³/h | Section length, m | Required size, mm | Selected size, mm | Actual velocity, m/s | Loss per 1 m, Pa/m | Friction loss, Pa | Local elements, pcs. | Local loss, Pa | Total loss, Pa | ||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
About Ductwork Design Calculation
This ductwork design calculator calculates airflow-related duct parameters for individual sections of a ventilation system: it determines the required duct cross-section from the airflow rate, selects a round or rectangular duct size, calculates the actual air velocity, and estimates pressure drops. Friction losses and selected local losses are calculated separately for each section and then combined into the total pressure drop.
The calculation can be used for preliminary duct sizing and for comparing ventilation system options by air velocity and airflow resistance.
Guidelines and recommendations
Design air velocity and required cross-section
Design air velocity determines the initial required duct area. The calculator uses fixed values: natural ventilation — 1.5 m/s, mechanical ventilation in residential spaces — 3 m/s, offices — 4.5 m/s, and industrial spaces — 9 m/s.
Required cross-sectional area is calculated from the airflow rate Q in m3/h and the design velocity vdesign in m/s. The airflow rate is first converted from m3/h to m3/s by dividing by 3600.
A = (Q / 3600) / vdesign
The resulting area A in m2 is the minimum calculated cross-section used to determine the required duct size.
Round and rectangular duct sizing
Round duct sizing is based on the area of a circle. The required internal diameter d is determined in millimetres.
d = 1000 · sqrt(4A / π)
The calculator then selects the first diameter from the size series that is not smaller than the calculated value: 63, 80, 100, 125, 150, 160, 200, 250, 300, 315, 355, 400, 450, 500, 560, 630, 710, 800, 900, 1000, 1120 or 1250 mm.
Rectangular duct sizing is performed with either the height h or width b specified by the user. The second side is calculated from the required area.
b = A · 106 / h
h = A · 106 / b
The calculated side is rounded up to the first suitable value from the series 100, 150, 200, 250, 300, 400, 500, 600, 800, 1000, 1200, 1400, 1600, 1800 or 2000 mm. The side specified by the user is not changed during automatic sizing.
Selected duct size is used for all subsequent calculations. If the user manually changes the selected diameter or the sides of a rectangular duct, air velocity and pressure losses are recalculated using the entered dimensions.
Actual air velocity and hydraulic diameter
Actual air velocity is calculated from the selected duct cross-section rather than the theoretical required area. For this reason, after the duct size is rounded, the actual velocity usually differs from the initial design velocity.
v = (Q / 3600) / Aactual
Here Aactual is the actual cross-sectional area of the selected duct in m2.
Hydraulic diameter for a round duct is equal to its diameter. For a rectangular duct with sides a and b, it is calculated as:
dh = 2ab / (a + b)
The values a, b and dh must use the same units. For pressure-loss calculations, the hydraulic diameter is converted from millimetres to metres.
Duct material roughness
Absolute roughness ε affects the friction factor. The calculator uses the following values in millimetres:
- galvanized steel — 0.09 mm;
- stainless steel — 0.015 mm;
- aluminium — 0.05 mm;
- PVC, PP or PE plastic — 0.0015 mm;
- FRP or GRP — 0.005 mm;
- plaster — 0.3 mm;
- smooth concrete — 0.6 mm;
- ordinary concrete — 1.0 mm;
- brickwork — 3.0 mm.
The calculation uses the relative roughness ε / dh. Therefore, the same duct material can have a different effect on ducts of different sizes.
Flow regime and friction factor
Air properties are assumed to be constant: density ρ = 1.2041 kg/m3 and kinematic viscosity ν = 0.000015 m2/s.
Reynolds number is calculated from the actual air velocity and hydraulic diameter.
Re = v · (dh / 1000) / ν
Darcy friction factor λ depends on the airflow regime. When Re < 2300, the laminar-flow relationship is used:
λ = 64 / Re
When Re ≥ 2300, λ is determined by iteratively solving the Colebrook-White equation using Reynolds number and relative roughness:
1 / sqrt(λ) = -2 · log10(ε / (3.7dh) + 2.51 / (Re · sqrt(λ)))
This means that duct resistance depends simultaneously on air velocity, duct size and internal surface roughness.
Friction pressure loss
Pressure loss per metre is calculated using the Darcy-Weisbach equation. The actual air velocity after the final duct size has been selected is used in the calculation.
R′ = (λ / dh) · (ρ · v2 / 2)
Here dh is expressed in metres, and the result R′ is obtained in Pa/m.
Friction pressure loss for a duct section is proportional to its length L in metres:
R = R′ · L
Therefore, two sections with the same duct size and airflow rate have the same pressure loss per metre but different total friction losses if their lengths are different.
Local pressure losses
Dynamic pressure is calculated from the actual air velocity:
Pd = ρ · v2 / 2
Local loss coefficients ζ are predefined in the calculator for common ductwork elements:
- sharp-edged duct inlet — 0.50;
- rounded duct inlet — 0.05;
- free duct outlet — 1.00;
- smooth 45° elbow — 0.10;
- smooth 90° elbow with R/D = 1.5 — 0.15;
- 90° elbow with turning vanes — 0.20;
- mitered 90° elbow without turning vanes — 1.20;
- tee or cross, flow through the main duct — 0.10;
- tee or cross, flow into the branch — 1.00;
- 45° branch connection, flow into the branch — 0.35;
- air damper in the fully open position — 0.30.
For an element with a different resistance, the ζ coefficient can be entered manually. If several identical elements are present, the number of elements is multiplied by the coefficient of one element.
Total local loss coefficient is obtained by adding the resistance coefficients of all elements in the duct section while accounting for their quantities.
Σζ = ζ1 · n1 + ζ2 · n2 + ...
Local pressure loss is calculated by multiplying dynamic pressure by the total local loss coefficient:
Z = Pd · Σζ
Total pressure drop
Total pressure drop for a duct section is the sum of friction losses and local losses:
ΔP = R + Z
When several duct sections are added, the total pressure drop is calculated as the sum of the ΔP values for all added sections.
Final duct size selection follows a simple principle: the minimum required cross-section is first calculated from the specified airflow rate and design velocity, and then the size is increased to the nearest available value. All aerodynamic duct parameters are then recalculated using the selected size rather than the theoretical required cross-section.
Related European standards
EN 16798-3 “Energy performance of buildings. Ventilation for buildings. Part 3: For non-residential buildings. Performance requirements for ventilation and room-conditioning systems” provides general requirements for the performance of ventilation and air-conditioning systems.
EN 1506 “Ventilation for buildings. Sheet metal air ducts and fittings with circular cross-section. Dimensions” applies to the dimensional system of circular sheet-metal ducts and fittings.
EN 1505 “Ventilation for buildings. Sheet metal air ducts and fittings with rectangular cross-section. Dimensions” applies to the dimensions of rectangular sheet-metal ducts and fittings.
EN 12237 “Ventilation for buildings. Ductwork. Strength and leakage of circular sheet metal ducts” and EN 1507 “Ventilation for buildings. Rectangular sheet metal air ducts. Requirements for strength and leakage” are used for assessing structural duct characteristics. Pressure-drop calculations in this calculator use the aerodynamic relationships described above.
FAQs
Why is the selected duct size larger than the required size
The required size is calculated directly from airflow and design air velocity and can have any numerical value. For automatic duct sizing, the calculator selects the next larger value from the available size series so that the actual cross-section is not smaller than the calculated requirement.
Why does pressure drop decrease when the duct size increases
At the same airflow rate, increasing the duct cross-section reduces the actual air velocity. Dynamic pressure depends on the square of velocity, so both friction losses and local pressure losses decrease as the duct becomes larger.
Why can local pressure losses be greater than friction losses
In a short duct section, resistance from the straight duct itself may be small, while elbows, tees, branch connections and other fittings can add significant losses. The higher the air velocity and the total Σζ value, the greater the contribution of local resistance to the total pressure drop.
How does duct material affect ductwork design calculations
Duct material is taken into account through the absolute roughness of the internal surface. Greater roughness generally increases the friction factor, so pressure loss from friction becomes higher when airflow rate, duct size and duct length remain the same.
Why is actual air velocity different from the selected design velocity
Design velocity is used to determine the required duct cross-sectional area. After the nearest available duct size is selected, the actual area changes, so the calculator recalculates air velocity from the selected duct dimensions and uses that value for all pressure-drop calculations.