The classifier wheel (rotor speed) is the primary parameter to set the aerodynamic cut size of an air classifier. Higher wheel speed produces finer cut points; lower wheel speed gives coarser cut points, with airflow kept as a matched secondary variable.
Basic principle
The rotating classifier wheel creates a centrifugal force field. Coarse particles with higher mass are thrown outward and rejected to the coarse fraction. Fine particles with lower aerodynamic resistance follow the inner air stream and pass through the wheel into the fine product (protein fines).
Increase wheel speed → stronger centrifugal force → only smaller particles can penetrate the rotor → finer D90 of fines.
Decrease wheel speed → weaker centrifugal force → larger particles can pass through → coarser fines.
Practical adjustment workflow for dry fractionation of legume protein
Target: finer protein fines (smaller D90)
Raise classifier wheel speed. At the same time, slightly reduce the primary air flow rate or keep airflow stable.
Outcome: Less coarse material enters fines; PSD shifts smaller.
Side effect: Lower fine fraction yield; more material returns as middling/coarse recycle stream; higher power consumption. Excessively high speed can overload the wheel and increase temperature.
Target: coarser fines (larger D90)
Reduce classifier wheel speed. Airflow can be slightly increased to improve particle transport.
Outcome: More medium-sized particles are collected in fines; higher protein yield.
Side effect: Larger starch granules may enter protein fines, increasing residual starch content and reducing protein purity.
Critical pairing: match air flow with wheel speed
Wheel speed cannot be adjusted independently. Air volume determines drag force.
If wheel speed increases without airflow tuning: insufficient air drag reduces classification sharpness, leading to wide particle size distribution.
If airflow rises while wheel speed is fixed: higher air drag carries more coarse particles into fines, making product coarser.
Process constraints for plant protein dry fractionation
Do not jump to extreme speed changes. Make small incremental adjustments (5–10% rotor speed step), wait for system stabilisation, then test PSD, protein content and starch level.
Monitor powder temperature. Higher wheel speed increases shear and air friction, which may heat protein and risk denaturation.
Check hopper feeding stability: very high rotor speed often reduces fines throughput and may cause powder build-up on wheel blades.
Troubleshooting reference
Fines PSD is coarser than target: increase wheel speed or reduce air flow.
Fines are too fine and yield is too low: reduce wheel speed or increase air flow.
Poor cut sharpness (broad PSD): fine-tune the ratio of wheel speed to air volume; check wheel blade wear and air leakage.
Higher wheel speed delivers finer, purer protein fines but sacrifices yield and raises energy consumption. Lower wheel speed improves protein recovery at the cost of higher residual starch in the protein-rich fraction.
