Duct Sizing
1. Design Assumptions
Flowrate: 4.0 m³/s (4000 L/s)
Target velocity: 6.0 m/s (to limit air noise and pressure loss)
Duct material: Galvanised steel
Application: Supply air duct for a processing area
Retrieved: https://www.engineeringtoolbox.com/velocities-ventilation-ducts-d_211.html
Main Supply Ducts
Commercial and Industrial Applications: Up to 6–8 m/s (approximately 1,200–
1,600 ft/min)
2. Circular Duct Option
Using the EngineeringToolbox “Duct Velocity and Flow Rate” table, a circular duct diameter
of 922 mm corresponds to a velocity of approximately 6.0 m/s at 4.0 m³/s.
Selected Circular Size:
Diameter: 922 mm
Area: 0.667 m²
Velocity: 6.0 m/s
Retrieved: https://www.engineeringtoolbox.com/duct-velocity-d_928.html
3. Rectangular Duct Option
As an alternative, a rectangular duct has been considere:
Using a practical 2:1 aspect ratio for ease of fabrication and installation:
Width: 1150 mm ~1200mm
Height: 580 mm ~600mm
Area: 0.667 m²
This sizing achieves the same airflow and velocity performance as the circular option and
may be preferred in applications with vertical or spatial constraints.
4. Alternative Scenario: Higher Velocity
If space limitations require a reduced duct size, increasing the design velocity to 7.0 m/s
reduces the required duct area:
A = 0.571m²
This results in:
Circular duct: Ø ~854 mm
Rectangular duct: e.g., 1050 mm × 545 mm
However, this introduces trade-offs, including increased noise and static pressure,
and is therefore less favourable for processing zones sensitive to air turbulence and
sound.
Duct Sizing and Pressure Loss Calculation
System Inputs
Airflow rate: 4.0 m³/s
Design velocity: 6.0 m/s
Duct material: Galvanized steel (roughness = 0.15 mm)
Air density: 1.2 kg/m³
Kinematic viscosity: Approx. 1.5×10⁻⁵ m²/s
Sizing Result
Recommended duct diameter: 921 mm
This diameter allows 4.0 m³/s of airflow at 6.0 m/s velocity without exceeding noise or
pressure loss limits for a main duct.
Pressure Loss Summary
Component Loss (Pa)
Straight Duct – 28 m 10.3 Pa
Straight Duct – 19 m 7.0 Pa
90° Elbow 7.6 Pa
Total Pressure Loss 24.8 Pa
Additional Technical Details
Friction factor (Darcy-Weisbach): 0.01563
Reynolds number: 368,527 (fully turbulent flow regime)
This result confirms that a 921 mm diameter duct is suitable for HVAC system. The
calculated total pressure drop of ~25 Pa is well within acceptable limits for efficient fan
operation and acoustic performance.
System Flowrate and Design Velocity
Flowrate: 4.0 m³/s
Main duct velocity: 6.0 m/s
Inlet grille velocity: 2.0 m/s
Diffuser velocity (assumed): 4.0 m/s
Component Losses
Component Value Notes
Inlet Grille Area 0.72 m² 1200 × 600 mm
Grille Entry Loss 1.2 Pa Based on Kₑ = 0.5
Main Duct Losses 24.8 Pa From previous calc (28 m + 90° bend +
19 m)
Diffuser Terminal Area 2.16 m² 3 diffusers (1200 × 600 mm)
Diffuser Losses 9.6 Pa Based on K = 1.0
Total End Losses 10.8 Pa Entry + outlet
Total System Pressure Loss 36.0 Pa End-to-end
Add AHU Losses at filters (check budget quote) (clean, medium, dirty)
Formula Sheet:
1. EngineeringToolBox (HVAC Friction Loss Guidelines)
Low-pressure HVAC systems typically operate with total pressure losses in the
range of 100–250 Pa across the entire duct network (supply + return).
A segment pressure loss of <30 Pa for a long straight duct run is considered efficient
and low-resistance.
Source:
EngineeringToolBox – Ductwork pressure loss
2. ASHRAE Fundamentals Handbook
The ASHRAE Handbook recommends:
Target pressure losses per metre of duct:
o Main ducts: 0.8–1.2 Pa/m
o Branches: 0.5–1.0 Pa/m
Typical total allowable loss in a well-designed supply system: <250 Pa from fan
outlet to terminal.
Recommended Minimum Branch Lengths (From the Main Duct Takeoff):
🔹 For Rectangular or Round Ducts:
Straight branch run after takeoff (before any bends or terminals):
Minimum 4 to 6 times the branch diameter (4–6 × D)
Example: For a 200 mm diameter branch, the straight length before a bend,
elbow, or diffuser should be 800–1200 mm.
🔹 Before a branch takeoff (from the main duct):
Leave 3 to 5 times the branch diameter of straight duct upstream of the takeoff.
This allows uniform airflow approaching the branch and avoids eddies from
nearby fittings.
total required air supply (m³/s)
Stage 1: Determine Total Required Flow
1. Start with known loads:
o Your system must supply 4 m³/s of fresh air (likely to high-occupancy or
process areas).
o Now, determine how much additional air is needed for vestibules and any
other zones.
2. Typical flowrate recommendations for vestibules (to maintain overpressure):
o 0.3 to 0.5 m³/s per vestibule (depending on size, door usage, and adjacent
pressure zones)
o Example:
If you have 3 vestibules:
👉 3 × 0.4 m³/s = 1.2 m³/s
3. Total system supply:
o Main process area: 4 m³/s
o Vestibules: 1.2 m³/s
➤ Total: 5.2 m³/s
Stage 2: Duct Sizing Based on Total Flow
Your main duct must be sized for 5.2 m³/s.
Using a velocity of 6 m/s, you’d need:
A square duct of approximately 950 mm × 950 mm
Or a round duct with diameter ≈ 1,050 mm
🔹 Step 3: Flow Distribution to Branches
Assign flowrates to each branch based on:
Room air change requirements (ACH)
Room function & pressure requirement
Size (volume)
Distribute like this:
Zone Flowrate (m³/s) Notes
Portioning/Process 4.0 High occupancy or process load
Vestibule 1 0.4 To pressurize and block infiltration
Vestibule 2 0.4
Vestibule 3 0.4
Total 5.2
🔹 Step 4: Duct Branching
Main duct supplies 5.2 m³/s
Install dampers at each branch to regulate flow
Use manual or automatic volume control dampers (VCDs)
Balance system with commissioning airflow measurements
✅ Summary
Determine air demand per zone based on occupancy, purpose, and pressurization
goals.
Size your main duct for total required flow.
Distribute flow to branches by assigning realistic flowrates and install control
dampers for tuning.
To size ducts for 0.4 m³/s to each vestibule, follow this simple process:
🔹 Step 1: Choose Design Velocity
For vestibule supply ducts, use a moderate velocity to:
Prevent noise
Avoid excessive pressure drop
Recommended range:
4–5 m/s (industry standard for branch ducts to small rooms)
Let’s go with 4.5 m/s.
🔹 Step 2: Apply Duct Sizing Formula
A=QVA = \frac{Q}{V}A=VQ
Where:
AAA = area in m²
QQQ = airflow in m³/s (0.4 m³/s)
VVV = velocity in m/s (4.5 m/s)
A=0.44.5=0.0889 m2A = \frac{0.4}{4.5} = 0.0889 \text{ m}^2A=4.50.4=0.0889 m2
🔹 Step 3: Convert to Duct Dimensions
📦 Rectangular Duct:
Choose standard aspect ratio, say 1:2 (height × width).
Let height = 300 mm
Then width =
0.08890.3=0.2967 m= 300 mm\frac{0.0889}{0.3} = 0.2967 \text{ m} = \
text{~}300 mm0.30.0889=0.2967 m= 300 mm
✅ 300 mm × 300 mm duct (a square option that’s easy to fabricate)
🔵 Circular Duct:
A=πD24⇒D=4Aπ=4×0.0889π≈0.336 mA = \frac{\pi D^2}{4} \Rightarrow D = \sqrt{\
frac{4A}{\pi}} = \sqrt{\frac{4×0.0889}{\pi}} ≈ 0.336 mA=4πD2⇒D=π4A=π4×0.0889
≈0.336 m
✅ Use Ø350 mm circular duct
🔹 Summary Table
Flowrate Velocity Area Needed Rectangular Duct Circular Duct
0.4 m³/s 4.5 m/s 0.089 m² 300 × 300 mm Ø350 mm
For grille/diffuser sizing in your vestibule with an airflow of 0.4 m³/s, here’s how to select
the appropriate size:
🔹 Step 1: Target Face Velocity
To ensure quiet operation and proper throw in small areas like vestibules:
Recommended face velocity: 2.0 – 2.5 m/s
(per ASHRAE and typical SANS acoustic best practices)
Let’s use 2.2 m/s as our target.
🔹 Step 2: Calculate Grille Area
A=QV=0.42.2=0.182 m2A = \frac{Q}{V} = \frac{0.4}{2.2} = 0.182 \text{ m}^2A=VQ
=2.20.4=0.182 m2
🔹 Step 3: Select Standard Grille/Diffuser Size
Choose a standard rectangular supply grille size close to this area:
Size (mm) Area (m²)
600 × 300 0.18
625 × 300 0.187
600 × 400 0.24
✅ 600 × 300 mm grille = 0.18 m², perfect for 0.4 m³/s @ 2.2 m/s face velocity.
🔹 Optional – Circular Diffuser (if ceiling-mounted)
If using a ceiling diffuser:
D=4Aπ=4×0.182π≈0.48 mD = \sqrt{\frac{4A}{\pi}} = \sqrt{\frac{4×0.182}{\pi}} ≈ 0.48 \
text{ m}D=π4A=π4×0.182≈0.48 m
✅ Ø500 mm circular diffuser
🔹 Final Recommendation
Application Airflow (m³/s) Grille Type Size Velocity (m/s)
Vestibule 0.4 Supply grille 600 × 300 mm ~2.2
Vestibule 0.4 Diffuser (opt.) Ø500 mm ~2.2
