ACM has two independent rotors: the grinding rotor and the classifier rotor. Their speeds are controlled by separate frequency converters, and each impacts particle size in distinct ways. Changing one rotor speed will alter particle breakage or classification cut point, while the other can be kept constant for fine-tuning.
1. Grinding rotor speed: controls particle breakage energy
The grinding rotor delivers impact, shear and particle collision forces to the incoming material. Its peripheral tip speed determines how much comminution energy is transferred to particles.
- Higher grinding rotor speed: Higher tip speed generates stronger impact and collision energy. Raw material is shattered into more fine fragments. The incoming particle mixture fed to the classifier contains more fines, which lowers the potential D50 of finished powder. More particle-particle collisions also create extra ultrafine fractions.
- Lower grinding rotor speed: Reduced tip speed delivers weaker impact force. Less material is fully broken, so the stream entering classification contains more coarse particles. The final product tends to be coarser, and ultrafine fines generation is suppressed.
Important note: Even if the grinding rotor produces plenty of fine fragments, the classifier still decides which particles exit as finished product. Raising grinding speed alone cannot guarantee finer product if the classifier cut point remains unchanged.
2. Classifier rotor speed: sets the particle cut point (primary size control)
The classifier wheel is the main tuning element for final particle size. It creates centrifugal force opposing air drag on particles.
- Higher classifier rotor speed: Greater centrifugal force rejects more particles back to the grinding chamber. Only very small particles can be carried by airflow through the classifier. The finished powder becomes finer (lower D50). Recirculation load rises, and particles undergo more grinding cycles.
- Lower classifier rotor speed: Centrifugal force weakens. Larger particles can overcome centrifugal resistance and pass through the classifier wheel with airflow. The finished powder becomes coarser (higher D50), recirculation load drops, and throughput typically increases.
3. Combined effect and process trade-offs
Operators tune both speeds together to hit target PSD:
- To produce finer powder: Increase grinding rotor speed to generate more fine fragments, and increase classifier speed to reject coarser particles. Trade-off: higher power consumption, lower throughput, more ultrafine fines.
- To produce coarser powder: Reduce grinding rotor speed to limit over-grinding, and reduce classifier speed to allow larger particles to pass. Trade-off: less fines generation, higher throughput.
If settings are mismatched:
- High grinding speed + low classifier speed: high fines generation, but coarse particles leak out, leading to broad PSD.
- Low grinding speed + high classifier speed: insufficient fine particles are created; heavy recirculation load, low throughput and unstable particle size.
4. Effect on PSD steepness
- A properly matched speed combination can maintain a steep PSD.
- Excessively high grinding speed creates excessive ultrafines, stretching the fine tail and broadening PSD.
- Too low grinding speed leaves many semi-ground particles; even with classifier rejection, some irregular coarse particles may remain, broadening the coarse tail.
Grinding rotor speed controls how much breakage energy is applied to create fine particles. Classifier rotor speed sets the aerodynamic cut point to select which particles become finished product. Increasing classifier speed is the primary method to reduce D50, while increasing grinding rotor speed supplies enough fine fragments for classification. Adjusting both rotors in coordination achieves the target particle size and steep PSD, while balancing throughput and power consumption.
