HVAC -SH Group Air Grill Design and Selection
HVAC Air Grille Design & Selection Guide
1. Define the Function (First Priority)
Identify the grille application:
| Function | Description | Typical Requirement |
|---|---|---|
| Supply Air Grille | Delivers conditioned air to space | Adjustable blades, airflow control |
| Return Air Grille | Returns room air to AHU/FCU | Large free area, low pressure drop |
| Exhaust Air Grille | Removes air from space | Backdraft protection if required |
⚠️ Return air grilles should not be supply grilles installed in reverse.
2. Determine Airflow Rate (Q)
If airflow is known
- Use m³/h or CFM
If airflow is not known
Common estimation methods:
- Air Changes Method
- Office / Commercial: 8–12 ACH
- Cooling Load Method
Q = Cooling Load (kW) × 360 / (ρ × Cp × ΔT)
3. Select Face Velocity (Critical Parameter)
Recommended design values:
| Grille Type | Face Velocity |
|---|---|
| Supply air grille | 2.0 – 3.5 m/s |
| Return air grille | 1.5 – 2.5 m/s |
| Exhaust air grille | 2.5 – 4.0 m/s |
| Noise-sensitive areas | ≤ 2.5 m/s |
Lower velocity = lower noise, larger grille size
4. Calculate Required Free Area
Formula
A = Q / (3600 × V)
Where:
- A = required free area (m²)
- Q = airflow (m³/h)
- V = face velocity (m/s)
Example
- Return air: Q = 1200 m³/h
- Face velocity: V = 2.0 m/s
A = 1200 / (3600 × 2.0) = 0.167 m²
5. Convert to Nominal Grille Size
Grilles do not have 100% free area.
Typical Free Area Ratios
| Grille Type | Free Area |
|---|---|
| Single / Double deflection | 60–70% |
| Fixed bar grille | 70–80% |
| Eggcrate grille | 85–90% |
Nominal Area Calculation
Nominal Area = A / Free Area Ratio
Example (70% free area):
0.167 / 0.7 ≈ 0.24 m²
Possible sizes:
- 600 × 400 mm
- 800 × 300 mm
- Linear grille (continuous)
6. Select Grille Type
Supply Air
- Double Deflection Grille
- With Opposed Blade Damper (OBD)
Return Air
- Fixed Bar Grille
- Eggcrate Grille for large airflow / low noise
Exhaust Air
- Fixed grille + backdraft damper
7. Installation Location
| Location | Notes |
|---|---|
| Wall-mounted | Most common |
| Ceiling-mounted | Often used with ducted systems |
| Floor grille | Raised floor / displacement ventilation |
8. Accessories & Engineering Details
| Item | Application |
|---|---|
| Opposed blade damper | Supply air (mandatory) |
| Filter | Return air |
| Insect screen | Exhaust / outdoor |
| Weather louver | Outdoor installation |
| Removable core | Maintenance access |
9. Pressure Drop & Noise Criteria
Typical design limits:
- Pressure drop: ≤ 15–25 Pa
- Noise level:
- Office: NC ≤ 35
- Hotel / Residential: NC ≤ 25
10. Information Required for Final Selection
To finalize grille sizing and specification, please provide:
- Room type / application
- Grille function (supply / return / exhaust)
- Design airflow (m³/h or CFM)
- Installation location (wall / ceiling / floor)
- Noise requirement
- Dimensional constraints (if any)
You can also say:
“Select a return air grille for an office room, airflow 1200 m³/h.”
Our engineers can then provide exact grille size, type, and specification wording for your HVAC drawings or BOQ.