Centrifugal Fan Airflow Fluctuation: Causes and Solutions

When the airflow of a centrifugal fan suddenly increases and decreases, it is commonly referred to in the industry as “surge” or “pulsation.” This not only affects the production process but can also damage bearings and impellers within minutes in severe cases.

To solve this problem, troubleshooting should follow the principle of “check external factors first, then internal components.” Below are four common causes and corresponding solutions:

1. Power Supply and Electrical Issues (Most Easily Overlooked)

Symptoms:
The airflow fluctuation frequency is synchronized with the power frequency (for example, several times per second), accompanied by the motor sound becoming louder and softer alternately.

Causes:

• Unstable power supply voltage.
• Contactor contacts burned, causing phase loss.
• Variable frequency drive (VFD) frequency affected by electromagnetic interference, such as nearby high-power equipment starting or stopping.

Solutions:

• Use a multimeter to measure the incoming voltage. The fluctuation range should remain within ±10% of the rated voltage.
• For VFD-controlled fans, check whether the output current is stable.
  • If the current fluctuates significantly but the frequency remains stable, the problem may be with the motor.
  • If the frequency itself fluctuates, check the PID feedback signal (such as the pressure transmitter) for moisture, loose wiring, or signal interference.

2. Mechanical Transmission Failure (For Belt-Driven Fans)

Symptoms:
The airflow changes periodically, accompanied by sharp friction sounds or a burning smell.

Causes:
A slipping V-belt is one of the most common mechanical causes of unstable airflow. When the belt is worn or the tension is insufficient, the pulley may slip under higher load conditions (airflow decreases). When the load decreases, the belt regains grip (airflow increases), creating a repeated cycle.

Solutions:

• Stop the fan and press the middle point of the belt. The normal deflection should be approximately 1.5%–2% of the pulley center distance.
• If the belt is too loose, adjust the tension.
• If the inner surface of the belt becomes shiny and hardened, replace the belts as a complete set. (Mixing old and new belts may cause uneven loading.)

3. Rapid Changes in System Resistance (Main Process Cause)

Symptoms:
Severe airflow fluctuation, a dull “breathing” sound from the fan, and strong vibration of the unit.

Causes:
This is a typical fan surge condition. When downstream equipment (such as dust collector filter bags or heat exchangers) becomes clogged, or the inlet damper is opened too little, the fan operates in the surge area of its performance curve. Airflow separates from the impeller, causing periodic reverse flow.

Solutions:

• Emergency action: Slightly open the vent valve or bypass valve to increase the fan outlet airflow. This can usually eliminate surge immediately.
• Permanent solution:
  • Check the filter pressure drop. If it exceeds 1.5 times the initial resistance, clean or replace the filter.
  • If the control damper opening is too small, adjust it to 70%–85% opening to avoid operating under restricted conditions.

4. Impeller Damage and Loss of Dynamic Balance

Symptoms:
Airflow fluctuation accompanied by high-frequency vibration and abnormal bearing temperature rise.

Causes:

• Heavy dust accumulation on the impeller, causing uneven material shedding and imbalance.
• Impeller blade wear, deformation, or damage.

These problems can significantly reduce fan efficiency and disturb airflow stability.

Solutions:

• Stop the fan and clean the deposits on the impeller surface.
• After cleaning, perform dynamic balancing correction; otherwise, vibration may become worse.
• Check whether the clearance between the impeller and inlet cone (collector) is even. Excessive clearance can cause air recirculation and airflow pulsation.

Emergency Troubleshooting Flow (Recommended Sequence)

1. Listen to the sound:
If the fan produces a dull, periodic “hiccup-like” sound → immediately check Solution 3 and open the bypass valve.

2. Check the current meter:
If the current pointer fluctuates sharply → focus on Solution 1 (power supply/VFD issues).

3. Measure temperature:
If the bearing temperature rises rapidly → stop the fan immediately and inspect Solution 4 (impeller condition) and bearing housing bolts.

4. Check pressure difference:
If the inlet/outlet pressure difference exceeds the rated value → clean filters or remove debris from duct bends.

Final Reminder:

Never continue operating a centrifugal fan in a surge condition for more than 1 minute. Continuous operation under surge conditions can easily cause serious failures such as main shaft fracture.

If the above troubleshooting methods cannot solve the problem, the issue may be related to insufficient dynamic balancing during fan manufacturing. In this case, contact the manufacturer for professional inspection and testing.