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TECHNICAL SELECTION GUIDE
Air Terminal Selection: Airflow, Velocity, Throw and Noise
A grille, diffuser or air valve should not be selected from duct size alone. Design airflow, effective velocity, throw, drop, pressure loss, sound level and room geometry must be checked at the same operating point.
Direct answer
A grille, diffuser or air valve should not be selected from duct size alone. Design airflow, effective velocity, throw, drop, pressure loss, sound level and room geometry must be checked at the same operating point.
Technical review: MDES Engineering Technical TeamUpdated: 2026-10-25
Airflow and effective velocity
Required airflow comes from the room calculation. A larger terminal may reduce face velocity and noise at the same airflow, but nominal size alone is insufficient because blade geometry and effective area differ. Use manufacturer performance data.
Throw and drop
Supply air must reach the occupied zone without creating objectionable draught. Ceiling height, temperature difference, Coanda effect, terminal position and adjacent air streams influence throw and drop. Heating and cooling cases should be checked separately.
Noise and pressure loss
The terminal, damper, plenum box and flexible connection contribute to pressure loss and noise. Verify the selection against the room acoustic target and actual duct pressure, not only a catalogue NC/NR value.
Information required for quotation
Provide product type, neck or overall size, quantity, airflow per terminal, mounting position, ceiling height, allowable sound level, colour, damper and plenum requirements and duct connection size.
Core selection values
| Value | Influence |
|---|---|
| Airflow (m³/h or L/s) | Required capacity |
| Effective velocity (m/s) | Pressure loss and noise |
| Throw/drop | Air delivery to occupied zone |
| NC/NR or dB(A) | Acoustic comfort |
| Pressure loss (Pa) | Fan and duct calculation |
Key points
- State airflow per terminal.
- Check heating and cooling throw separately.
- Include damper and plenum losses.
- Confirm size with manufacturer performance data.
Frequently asked technical questions
Can a grille be selected only from duct size?
No. Different blade arrangements create different effective areas, pressure losses, throws and sound levels at the same nominal size.
Is a larger terminal always better?
No. It may reduce velocity and noise but may also provide insufficient throw; distribution pattern and architecture must be checked.
Does a fully open damper cause pressure loss?
Yes. Dampers, plenums and connection pieces add resistance.
What is the most important quotation input?
Design airflow and allowable sound level for each terminal, together with size, mounting and accessory requirements.
Technical references
- Manufacturer air-terminal performance data
- Project airflow, pressure-loss and acoustic calculations
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