Engineering: One System, Multiple Design Variables. An AHU is not just a box with a fan and cooling coil.

 Engineering: One System, Multiple Design Variables

An AHU is not just a box with a fan and cooling coil.
A small design mistake can increase:
✗ Fan power
✗ Pressure drop
✗ Noise
✗ Operating cost
✗ Maintenance frequency
1. Typical AHU Air Path
Fresh Air → Louver → pre-Filter → Heat Recovery → Cooling/Heating Coil → Fan → Silencer → Supply Air
Return/exhaust air paths must also be considered, especially where heat recovery, pressure control and IAQ are important.
2. Airflow Sizing
The basic relationship is:
Q = A × V
Where:
• Q = Airflow (m³/s)
• A = Face area (m²)
• V = Air velocity (m/s)
Example:
20,000 m³/h = 5.56 m³/s
At 2.5 m/s face velocity:
Required area ≈ 2.22 m²
Lower face velocity generally means a larger AHU footprint but can reduce component pressure drop and fan energy. ASHRAE notes that designers should not automatically default to 2.5 m/s; lower coil/filter velocities can provide energy benefits. (ASHRAE Handbook)
3. Pressure Drop = Hidden Energy Cost
Typical AHU resistance comes from:
• Louvers
• Dampers
• Filters
• Heat recovery
• Cooling coil
• Heating coil
• Silencer
• Fan inlet/outlet
• Internal transitions
Higher ΔP → Higher fan duty → Higher energy consumption
And don't forget:
Dirty filters can significantly increase system resistance and reduce airflow. (ASHRAE Handbook)
4. Fan Power
A useful preliminary calculation:
Fan Power (kW) = Q × ΔP / (η × 1000)
Where:
Q = m³/s
ΔP = Pa
η = total fan/motor efficiency
Example:
Q = 5.56 m³/s
ΔP = 900 Pa
η = 0.65
Fan power ≈ 7.69 kW
So, selecting the motor is not simply about airflow, system resistance and efficiency matter.
5. SFP – Specific Fan Power
SFP = Fan Power / Airflow
For the above example:
SFP ≈ 1.38 kW/(m³/s)
Lower SFP generally means better fan-system energy performance, provided the required airflow and system performance are maintained. (Eurovent)
6. Cooling Coil Selection
Don't select a cooling coil only from the room load.
Consider:
✓ Sensible + latent load
✓ Fresh-air load
✓ Entering/leaving air conditions
✓ CHW temperatures
✓ Coil face velocity
✓ Pressure drop
✓ Condensate management
✓ Frost/freezing risk where applicable
✓ Control stability
Coil capacity can be higher than the room load because ventilation and safety/control margins also contribute.
7. Filters Are Not "Just Filters"
Filter selection affects:
IAQ + pressure drop + fan energy + coil cleanliness + maintenance cost
ASHRAE highlights the importance of filter accessibility and notes that higher-efficiency filtration can increase resistance, making proper selection and maintenance critical. (ASHRAE Handbook)
8. 5 Things I Always Look at in AHU Design
1 Pressure Drop
2 Air Velocity
3 Coil Face Velocity
4 Unit Size
5 Serviceability
The best AHU is not necessarily the smallest AHU.
It is the AHU that delivers the required performance with the right balance of:
CAPEX + OPEX + Energy + IAQ + Reliability + Maintainability