Introduction
One of the most common complaints I hear from powder processing plants is:
“Our air classifier is producing too many coarse particles.”
The problem often appears gradually.
At first, the particle size drifts slightly outside the target range. Then product consistency begins to suffer. Eventually, customers start receiving complaints from downstream users.
Many operators immediately assume that the classifier itself is failing.
However, from my experience, excessive coarse particles are often caused by operating conditions, maintenance issues, or changes in material properties rather than a major equipment failure.
The good news is that many of these problems can be identified and corrected relatively quickly.
In this article, I’ll discuss seven common reasons why an air classifier may produce too many coarse particles and how to troubleshoot them effectively.
1. How to Recognize the Problem
Before looking for causes, it’s important to confirm that classification performance has actually deteriorated.
Common symptoms include:
- Increased D97 particle size
- Wider particle size distribution (PSD)
- Reduced product consistency
- Higher percentage of oversized particles
- Customer complaints regarding product quality
For example, a product that previously achieved D97 = 20 μm may suddenly begin producing D97 = 35 μm or higher.
When this happens, classification efficiency should be investigated immediately.
2. Classifier Wheel Speed Is Too Low
This is often the first parameter that should be checked.
The classifier wheel determines the cut point of the classification process.
When wheel speed decreases:
- Centrifugal force decreases
- Larger particles can pass through the classifier
- Product becomes coarser
In some cases, operators may unknowingly modify inverter settings during routine maintenance.
Electrical issues can also affect actual wheel speed.
What to Check
- Inverter settings
- Actual motor speed
- Control system parameters
- Maintenance records
In many cases, restoring the correct classifier wheel speed immediately improves product fineness.
3. Insufficient Airflow
Airflow is just as important as classifier speed.
The classification process relies on the balance between:
- Centrifugal force
- Air drag force
When airflow decreases, coarse particles are more likely to enter the finished product stream.
Common causes include:
- Dust collector filter blockage
- Worn blower components
- Air leakage
- Pipe buildup
Warning Signs
- Reduced system pressure
- Increased dust accumulation
- Lower airflow readings
I’ve seen situations where replacing clogged filter bags restored classification performance without any changes to the classifier itself.
4. Feed Rate Is Too High
Many plants try to increase production by feeding more material into the system.
Unfortunately, every classifier has a design capacity.
When feed rates exceed that capacity:
- Particle loading increases
- Classification efficiency drops
- More coarse particles bypass separation
The result is often lower product quality despite higher throughput.
What to Check
Compare current feed rate with:
- Original design specifications
- Historical operating records
If possible, temporarily reduce feed rate and observe whether particle size improves.
5. Classifier Components Are Worn
Wear is a normal part of long-term operation.
Over time, abrasive materials can gradually damage:
- Classifier wheel blades
- Internal liners
- Air distribution components
As wear increases, airflow patterns change and classification accuracy decreases.
This issue is particularly common when processing:
- Quartz
- Silica
- Wollastonite
- Other hard minerals
What to Check
- Blade condition
- Internal wear surfaces
- Rotor balance
- Maintenance history
6. Material Properties Have Changed
Many operators focus entirely on equipment while overlooking the material itself.
Even small changes in raw material characteristics can affect classification performance.
Examples include:
- Increased moisture content
- Different supplier batches
- Particle agglomeration
- Variations in bulk density
These changes may alter how particles behave inside the classifier.
Questions to Ask
- Has the supplier changed?
- Has moisture content increased?
- Has storage condition changed?
- Has the feed material PSD changed?
Sometimes the equipment is operating perfectly, but the material is no longer the same.
7. Air Leakage in the System
Air classifiers are designed to operate under carefully controlled airflow conditions.
Unexpected air leakage can disturb the pressure balance and reduce classification efficiency.
Common leakage locations include:
- Flange connections
- Inspection doors
- Seals
- Expansion joints
- Duct connections
Symptoms
- Unstable pressure readings
- Fluctuating particle size
- Increased energy consumption
Even relatively small leaks can significantly affect classification performance over time.
8. The System Has Reached Its Capacity Limit
Sometimes the problem is not a malfunction at all.
The production requirements may simply have outgrown the original equipment design.
For example:
A system originally designed for:
- 2 t/h
may now be expected to process:
- 4–5 t/h
Under these conditions, even a well-maintained classifier may struggle to achieve the original particle size target.
Consider
- System upgrades
- Larger classifier wheels
- Additional classification stages
- Capacity expansion solutions
A Real Example from the Field
Several years ago, we assisted a customer who reported a sudden increase in coarse particles.
Initially, they suspected that the classifier wheel had been damaged.
However, after reviewing operating conditions, we discovered that the dust collector filter bags had become severely clogged.
The reduced airflow disrupted the classification process.
After replacing the filter bags, the particle size distribution returned to normal without any modifications to the classifier.
This experience reinforced an important lesson:
Classification performance depends on the entire system, not just the classifier itself.
A Simple Troubleshooting Checklist
When excessive coarse particles appear, I typically recommend checking the following in order:
Step 1
Verify classifier wheel speed.
Step 2
Measure system airflow.
Step 3
Review feed rate.
Step 4
Inspect wear components.
Step 5
Evaluate material changes.
Step 6
Check for air leakage.
Step 7
Review whether production demand exceeds system design capacity.
Conclusion
When an air classifier begins producing too many coarse particles, it does not automatically mean the equipment has failed.
In many cases, the root cause is related to:
- Airflow
- Wheel speed
- Feed rate
- Material properties
- System maintenance
By following a structured troubleshooting process, operators can often restore classification performance quickly and avoid unnecessary downtime.
Before replacing equipment, it is usually worth checking the operating conditions first.
If you are experiencing particle size control problems or would like assistance evaluating your classification system, feel free to contact us.
Mills Powder Engineering — Helping Customers Achieve Consistent Powder Classification Through Better Process Understanding.