The ACM mill runs under negative‑pressure airflow driven by the downstream induced‑draft fan. Its air circulation contains two distinct flow patterns: internal mill‑body local circulation (inside the machine housing for coarse‑particle recirculation) and main system through‑flow (air carrying fine powder out to collection equipment). Air does not fully circulate back inside the mill for finished fines; only rejected oversize particles form local internal recirculation by gravity plus airflow guidance.
Overall airflow architecture
Total airflow path:
Induced‑draft fan → negative pressure inside ACM mill → upward air‑particle flow from grinding zone → classification zone → two split paths:
- Fine‑particle stream: passes through classifier wheel → mill outlet → cyclone separator → pulse dust collector → fan exhaust (main through‑flow, leaves the mill)
- Oversize‑particle stream: rejected by classifier wheel → drops down along baffle ring back into grinding chamber (local internal circulation inside mill housing)
Important clarification: The whole ACM is not a fully gas‑closed gas‑loop. Most air flows through the mill and exits to collection. Only coarse material circulates inside the mill body by gravity and local secondary airflow, while air itself keeps moving downstream.
Step‑by‑step air flow inside mill housing
1. Air enters the grinding chamber
Make‑up air enters the grinding chamber via feed inlet and secondary air ports. The grinding disc / rotor also acts as an impeller, boosting local air movement. The entire chamber is maintained at slight negative pressure by the rear fan, preventing powder leakage outward.
2. Upward main air‑particle transport flow
Air picks up mixed‑size ground particles above the grinding rotor. Air‑particle suspension flows vertically upward along the annular gap between grinding‑chamber liner and classifier shroud toward the classifier wheel assembly.
This upward flow transports newly ground particles to classification, removes grinding heat, and prevents fine‑particle deposition at the mill bottom.
3. Flow splitting at classifier wheel zone
At the classifier wheel, airflow separates into two streams:
- Through‑flow (product stream): Air drag pulls fine particles through gaps between classifier‑wheel blades. This airflow carries qualified fines out of the mill towards cyclone and dust collector. This air stream leaves the mill body and does not return.
- Local recirculation flow for oversize: Coarse particles are thrown outward by centrifugal force of classifier wheel. They lose velocity, slide down the inner surface of the stationary baffle ring under gravity and weak downward local air currents, falling straight back into the lower grinding zone.
Air itself does not loop back down; only solid coarse particles recycle. Air near the baffle ring turns and rejoins the upward main airflow again, forming local vortex circulation around the classifier shroud area.
4. Mixing with fresh feed inside grinding zone
Returned coarse particles mix with fresh incoming feed inside the grinding chamber. They are impacted and comminuted again by grinding rotor. Once crushed, particles are entrained once more by upward airflow to the classifier zone. This creates the continuous “grind‑classify‑re‑grind” particle‑recirculation cycle.
Local vortex features inside ACM
- Annular upward vortex between liner and classifier shroud: Carries particle suspension upward toward classifier.
- Down‑flow zone along classifier baffle ring: For rejected coarse‑particle fall‑back; low air velocity here so coarse particles can settle downward instead of being swept upward again.
- Turbulent flow inside grinding rotor zone: Rotor rotation creates strong turbulence, enhancing particle‑particle collision and comminution.
The mill housing geometry, shroud and baffle ring are specially shaped to create these flow zones; poor chamber geometry will produce dead zones, particle buildup and unstable classification.
How operating parameters change circulation pattern
- Fan air volume:
- Higher air volume: Up‑flow velocity rises. More particles are lifted; throughput increases. Risk: coarse particles may not drop back properly and get dragged through classifier wheel.
- Lower air volume: Up‑flow slows. Fines yield drops; particles may deposit inside grinding cavity.
- Classifier‑wheel rotating speed:
The spinning classifier wheel distorts local airflow field around blade gaps. Higher wheel speed strengthens centrifugal effect and modifies local vortex distribution, changing cut‑point. - Secondary air intake:
Adjusting secondary air inlet damper modifies air proportion entering grinding chamber, optimizes air‑particle mixing and reduces material sticking on chamber walls.
Special modified circulation: inert‑gas closed‑loop ACM
For explosive or oxidation‑sensitive battery materials, ACM can adopt fully closed‑gas circulation pattern. Air is replaced by nitrogen or argon. After dust collector, most gas is circulated back to mill inlet, only small purge gas is discharged. This eliminates oxygen exposure, different from standard open‑air ACM mode.
Key misunderstandings corrected
- ❌ The whole gas circulates inside ACM mill.
✅ Only coarse solid particles recycle inside the mill. Most air flows through and exits to the collection system. - ❌ Classifier wheel pushes air downward to send coarse particles back.
✅ Coarse particles return mainly by gravity; only weak local secondary airflow assists downward movement. Main airflow direction is always upward toward classifier wheel.
Inside a standard ACM mill under negative‑pressure driven by induced‑draft fan:
- Main airflow moves upward from grinding zone to classifier wheel; fine‑particle‑laden airflow passes through classifier and exits the mill to collection equipment.
- Rejected coarse particles fall by gravity along the baffle ring back into grinding chamber, forming solid‑particle closed‑loop recirculation inside the mill housing, supported by local vortex airflow.
- Air itself does not circulate back; only solid oversize material repeats grinding‑classification cycles.
