The reason why Centrifugal Fan "wind does not reach as designed"

There are many main reasons that cause the Centrifugal Fan purchased to have "not enough airflow as designed" even though the model matches the catalog.
Below is a pure engineering answer with the most complete reasoning in the field of HVAC.

🔥 Why does the air volume of the Centrifugal Fan "not reach" as designed?

✅ 1) External Static Pressure (ESP) of the actual system is higher than in the model.

This is the reason for the 1 ranking in every air duct work project.
• Fan selected from the catalog may set ESP = 1.0 in.wg.
• But the real system has real ESP = 1.4–1.8 in.wg
This causes the Fan to go up the Performance Curve and the wind immediately drops.

Causes of increased ESP:
• The air duct is smaller than designed.
• Longer pipe distance.
• Elbow 90° more than
• Use a long Flexible Duct.
• High resistance Grille / Diffuser.
• The filter is dirty.
• No air adjustment. This causes some pipes to consume a lot of air, and some pipes to consume a little.

✅ 2) The gear teeth / motor does not rotate until one revolution (RPM is lower than specified)

Fan Performance = depends directly on RPM

If RPM is lower than 10% → the air volume falls approximately 10% but
Static Pressure will fall to 20%.
Brake Horsepower will fall approximately 27%.

Reasons for low RPM:
• It is a model with a V-belt, but the belt is slack.
• Pulley Ratio is not as specified.
• Low power motor / Phase Loss / reverse rotation
• The motor is not as efficient as designed.

✅ 3) Bought a Fan model like the one in Cat, but the seller gave me an incorrect Wheel Diameter model.

In Centrifugal Fan, there are many models. They look the same, but the propellers (Wheel) are different sizes.
For example, models 355 mm and 400 mm, but the frame is the same.

If the model has a smaller wheel than in the specs, the air volume will decrease 15–30%.

✅ 4) The air intake vent (Inlet) is squeezed or there is back pressure from the system.
• Attached too close to the wall → Inlet Loss.
• No Inlet Bell → Efficiency reduced.
• Suction pipe is too small → Fan Starvation
• There is negative pressure reversed → the air conditioner does not flow.
This makes the Fan almost unable to supply air. Even if the rotation is correct

✅ 5) Many air duct leaks.
Especially for thin grade galvanized pipe work. or a technician who shoots screws far away
If 10–20% leaks → the air volume at the destination is significantly reduced.

✅ 6) Static Pressure used to select the machine is incorrect.
Many HVAC projects have design errors such as:
• Use a lower diffuser resistance value than actual.
• Wrong air pipe value from table was used (CFM vs m³/hr).
• Use too high air speed → Friction increases.
• Total Equivalent Length is calculated too low.
When the actual site was installed → ESP was more than the design → The wind dropped immediately.

✅ 7) The selected Fan Curve is not in the Stable Operating Region zone.
If you choose a work spot near Stall Point → Just a slight mistake and the wind will drop immediately.
The merchant doesn't tell you much. But it is a problem that is often encountered in cheap Centrifugal Fan work.

✅ 😎 Choose a Fan Type that is not suitable for the job.
Example:
• Forward Curve Fan → Air drops easily when encountering high pressure.
• Backward Inclined → More resistant to ESP.
• Airfoil → Best performance but expensive.

If you choose Forward Curve for long air duct systems or thick filters
The wind can't rise 100%

🔧 How to check to know for sure where the fault occurred.
1. Use anemometer to measure at every outlet.
2. Use a Manometer to measure the pressure on the Fan side (ESP).
3. compared to Fan Curve (from Cat)
4. If ESP is higher → Problems with the air duct system.
5. If RPM is low → Motor/belt problem.
6. If everything is correct but there is little wind → The model you purchased is wrong (Wheel is not correct).

🧩 Engineer's summary
The most common causes: Ranking Cause Effect
1 Actual ESP is higher than the instantaneous wind drop 20–50%
2 RPM lower than specs, wind drops 10–30%
3 chose the wrong Fan Curve, working near Stall.
4 Inlet/Outlet installation work is bad, no air comes out.
5, the fan model is distorted from the specs, the air doesn't reach it from the beginning.

Source catalog reference. Confirm site conditions and the manufacturer’s instructions before applying this information.

The reason why Centrifugal Fan "wind does not reach as designed"

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