System effect of the fan (Fan)

Article details

System effect of a fan refers to the effect on the duct system.

It has to do with the amount of air and pressure that a real fan can deliver when working with real pipes.

It is not a value measured from the fan curve alone.

🔹 Principles
 • Fans are tested at the factory under standard conditions assuming “ideal” duct connections.
 • But when actually installed, there are often imperfections such as
 • There is no straight pipe before entering the fan (Inlet Condition).
 • There is no straight pipe behind the fan (Outlet Condition).
 • There is an obstructing device such as Elbow, Damper, Transition too close to the fan.

This will cause a System Effect Factor (SEF) = additional pressure loss not covered by the Fan Curve.

🔹 Impact
 1. Extra Static Pressure Loss
→ The fan must work harder. But the actual air came out less than designed.
 2. Reduced Airflow, CFM.
→ Because the Fan Curve will “move down” from the tested value.
 3. Noise & Vibration (Noise & Vibration)
→ Because the air flow is not smooth and turbulence occurs near the fan.

🔹 How to prevent
 • Install Straight Duct before and after the fan according to standards ( / recommended)
 • Inlet: Length ≥ 2–3 times Hydraulic Diameter.
 • Outlet: Length ≥ 3–5 times Hydraulic Diameter.
 • Use Bellmouth, Guide vane to help adjust flow.
 • Avoid placing Elbow, Damper near the fan.
 • In the calculation, the SEF value must be added to the External Static Pressure (ESP) of the fan.

🔹 Short summary

System Effect = “extra” pressure loss caused by incorrect fan installation → causing CFM

Less true than what the Fan Curve says → Must be considered in design.