1. Classification zone (around turbo wheel blades) — primary control zone
Optimal air velocity: 15–25 m/s for tire-grade rCB (D90 <10 μm, D97 ≤10–12 μm)
- 15–20 m/s: Sharper cut point, lower coarse tail leakage, narrower PSD, slightly lower throughput. Best for strict D97 tire-grade rCB.
- 20–25 m/s: Higher drag, higher throughput, but borderline coarse agglomerates are easier to pull through → D97 rises, PSD becomes wider.
Rule: Air velocity must be matched to turbo wheel rotational speed. Changing turbo speed requires adjusting airflow/velocity together. If you raise turbo speed but keep velocity too high, coarse particles bypass the wheel.
2. Upstream duct / transport velocity (piping leading to classifier)
- 1.5–2.5 m/s for powder conveying ducts.
- Below 1.5 m/s: rCB drops out, accumulates in dead zones, creates caking and cross-contamination.
- Above 3 m/s: excessive turbulence, particle clustering, misclassification, high pressure loss.
3. Jet mill grinding nozzle velocity (for fluidized bed jet mill only)
This is separate from classification air velocity:
- Supersonic 300–500 m/s at grinding nozzles, used for particle‑on‑particle collision to break agglomerates.
- The classifier section after jet mill still uses 15–25 m/s classification zone velocity.
How velocity affects rCB classification
- Too low air velocity (<15 m/s at turbo wheel)
Drag force is insufficient. Fine rCB cannot pass through the blade gaps; fines are recycled back, yield drops, over-grinding creates excess ultra-fines, energy consumption rises. - Too high air velocity (>25 m/s at turbo wheel)
Excessive drag force overcomes the turbo’s centrifugal rejection. Borderline large agglomerates pass through the wheel, D97 jumps, coarse tail leaks, PSD becomes broad. This is one of the most common causes for poor dispersion in rubber compounds.
Practical operating tips for rCB
- Use volumetric airflow (Nm³/h) as your main setpoint, not just velocity. Velocity is derived from airflow and cross-sectional area of the classifier inlet.
- Maintain very dry process air (dew point ≤-40°C). Moisture causes agglomerates to clump; clumps behave as large particles and upset the force balance, even if velocity is set correctly.
- When you increase turbo speed to get finer D97, you must slightly reduce airflow / classification zone velocity to preserve sharp cut point.
- Keep negative pressure stable inside the mill/classifier system; pressure fluctuations change actual air velocity.
Typical operating example for tire-grade rCB
Target: D90 <10 μm, D97 ≤10 μm
- Turbo classifier zone air velocity: 16–20 m/s
- Duct transport velocity: 1.8–2.2 m/s
- Adjust turbo speed first for fineness, then fine-tune airflow to hold classification zone velocity within range.
For turbo air classification of recovered carbon black, the optimal air velocity in the classification zone around the turbo wheel is 15–25 m/s. For high-quality tire-grade rCB with sharp D97 control, operate in the 16–20 m/s range, and match airflow to turbo wheel speed. Duct transport velocity is kept at 1.5–2.5 m/s to avoid powder deposition.
