Why is the designed air volume
CFM The model and the actual work site are not as designed?
🌪️ What are System Effects?
System Effects are the effects of flow direction, smoothness, and equipment near the fan.
That causes the air volume (CFM) and air pressure (Static Pressure) of the fan to decrease from the values specified by the manufacturer on the Fan Curve.
Simply put:
Fans in catalogs often perform better than fans that are actually installed.
Because the work site has curved pipes, square pipes, elbows, Y joints, 90° joints, reducers, etc.
which causes increased turbulence and static loss.
🚫 Why do System Effects occur?
✔ 1) Incorrect connection of the pipe from the fan.
such as
• Pipe exits Fan and immediately inserts 90° Elbow.
• There is no straight duct before and after the Fan.
• The pipe is connected at neck or in the wrong direction.
These things make
Airflow is not stable → Turbulence occurs → Air flow is less than calculated.
❗ Are System Effects important in design? — Very important!
✦ Because it directly affects “Actual air volume”
Even if the fan is selected correctly according to calculation, if system effect occurs
→ The air volume will easily decrease 10–30%.
✦ Because it affects “Static Pressure / ESP”
The ESP used in the calculation may not be sufficient. If there is additional resistance from system effects
The result is:
• AHU/FCU air volume does not reach.
• Negative Pressure Birth Room
• Lack of Fresh Air
• Cold does not reach the destination area.
• The fan noise is louder because the flow is not consistent.
🧩 Examples of System Effects that are commonly encountered in design/installation work.
❌ Wrong example:
• Elbow attached to Fan outlet
• There is no Duct Straight before connecting to the fan.
• Connect isthmus pipe immediately after AHU/Fan.
• Use the transition too quickly (steep angle).
✔ Cheap example:
• There is a straight duct 3–5 equal to the hydraulic diameter.
• Use radius elbow.
• Gradual Transition (≤15°)
• Use Turning Vanes in elbows.
📐 How should System Effects be thought about in design?
✔ 1) in case Static Pressure increases 10–20%
For fans with complicated duct connections, such as large exhaust or supply systems.
✔ 2) Add Duct Straight Section before and after Fan.
• Fan inlet → 3–5 times the pipe diameter.
• Fan outlet → 1.5–3 times the pipe diameter.
To make the flow calm before entering the fan and spread smoothly after the fan.
✔ 3) Avoid putting the Elbow 90° near the Fan.
If necessary, must use
✓ Radius Elbow
✓ Turning Vane
✓ Taper transition at steep angle not exceeding 15°
✔ 4) Use the Fan Curve along with the actual System Curve of the work.
Don't use "purely calculated" ESP values.
Always add system effect before selecting a fan.
🧠 Summary of the importance of the technician/designer
Topic: Truth
System Effects are the effects of connecting the duct around the fan causing the actual airflow to decrease.
What effect does it have? CFM is not reached, ESP is lacking, loud noise, Fan is working hard.
Is it important? Very important, especially for large AHU, Exhaust, and FCU systems.
Solution: Add a straight duct, choose good elbows, and add ESP.
