Low Noise Duct Design

Low Noise Duct Design

It's a matter of controlling air speed + pressure + vibration + equipment selection.

From the origin to the dispenser The main principles are as follows.

1. Limit wind speed (Air Velocity) ⭐ Most important.
The faster the wind, the louder the sound.
Position recommended speed
Main Duct 5–7 m/s
Branch Duct 3–5 m/s
Near the dispenser 2–3 m/s
Return Duct 3–5 m/s
If the Main Duct is designed at 9–10 m/s, there will be higher air noise and pressure drop.

2. Choose the appropriate air supply head.
* Do not choose a dispenser that is too small.
* Neck Velocity should not be higher than approximately 2.5–3.0 m/s.
* Check NC (Noise Criteria) values from Catalog
Example
* Meeting room: NC 25–30
* Hotel bedroom: NC 25–30
* Office : NC 30–35
* Factory : NC 40–50

3. Reduce speed in front of AHU.
The Face Velocity of the Cooling Coil should be approximately
* 2.0–2.5 m/s
If it is too high, both wind noise and pressure will increase.

4. Use a Duct Silencer installed behind the fan or AHU to help reduce noise by approx.
* 10–30 dB is suitable for
* Meeting room
* Hotel
* Hospital
* Studio

5. Install Flexible Connection between AHU ↔ Air pipe to protect.
* Vibration
* Sound from the motor
* Sound from the fan

6. Insulate air ducts.
* Fiberglass insulation 25–50 mm
* Reduces noise and prevents condensation

7. Avoid sudden changes in direction. 90° elbows should not be used immediately at sharp angles.
* Long Radius Elbow* Turning Vanes to reduce both noise and pressure drop

8. Select the fan to operate near the Best Efficiency Point (BEP). If the fan operates outside of its maximum efficiency range, it will
* Propeller sound
* Vibration
* Consumes more electricity

9. Set the ESP appropriately. Do not allow too much ESP, for example, the design requires 450 Pa.
But choosing the 900 Pa fan ended up having to dim the damper, causing the wind noise in the damper area to be very high.

10. Use the Damper properly. The Damper should not be closed more than 50% if it must be closed a lot. It shows that the system design is not balanced.
You should adjust the size of the air pipe or make a TAB (Testing, Adjusting & Balancing) system with recommended NC values.
NC area
* Bedroom 25–30
* 25–30 Hotel
* Meeting room 25–30
* 30–35 Office
* Classroom 30–35
* Restaurant 35–40
* 40–50 Factory

Summary of design principles
1. Limit the wind speed appropriately.
2. Choose a dispenser with a low NC value.
3. Use a suitable size pipe to reduce speed.
4. Use Long Radius Elbow and Turning Vanes.
5. Install Flexible Connection
6. Use Duct Silencer when necessary.
7. Insulate air ducts.
8. Choose a fan to work near BEP.
9. Design ESP to fit the system.
10. Make TAB after installation to adjust the wind balance.
For office buildings, hotels, and hospitals, if both 10 can be controlled, the noise level can be kept within the NC 25–35 criteria, which is a level that most users feel comfortable with.