The technology inside the Air Classifier
Classifying · no grinding — how it works, the equations we size it with, and what can be adjusted.
Air Classifier
A classifier does not grind. A driven rotor cage spins inside an upward air stream, and every particle is sorted by whether drag or centrifugal force wins.
- 01Powder enters the classifying zone dispersed in an upward air stream.
- 02The rotor cage spins, applying centrifugal force outward on every particle.
- 03Fines, dominated by drag, pass between the blades and leave with the air.
- 04Coarse particles are flung outward and spiral down to the reject outlet.
One equation decides everything
A particle sits in equilibrium when the inward drag from the radial air flow exactly balances the outward centrifugal force from the cage. That equilibrium diameter is the cut size, and it is the only number a classifier really has.
Sharpness matters as much as the cut itself. We measure it as the ratio d₂₅/d₇₅ — a well-dispersed feed and a clean cage give a sharpness index above 0.6, which is what keeps oversize out of the fine product.
- Cut sized₅₀ = √( 18 · η · Q / (2π · Δρ · ω² · r · h) )
- d₅₀
- cut size (m)
- η
- air viscosity (Pa·s)
- Q
- volumetric air flow (m³/s)
- Δρ
- particle minus gas density (kg/m³)
- ω
- cage angular velocity (rad/s)
- r, h
- cage radius and height (m)
Cut size falls with the square of cage speed — the primary operating handle.
- Sharpness indexκ = d₂₅ / d₇₅
- κ
- sharpness — 1.0 is a perfect cut
- d₂₅ / d₇₅
- sizes at 25 % and 75 % separation efficiency
Above 0.6 is a well-dispersed feed; below it usually means agglomerates.
Tuned to your material
- AdjustCage speedA VFD moves the cut point live, from 3 to 200 µm on the same machine, with no shutdown.
- AdjustAir volumeDamper or fan control trims the radial velocity to sharpen the cut at a fixed size.
- AdjustStandalone or in-millRuns on its own powder or bolts onto any THOR mill to close the grinding circuit.
- UpgradeMulti-stage cascadeTwo or three classifiers in series split a powder into three or four defined fractions in one pass.
- UpgradeWear-protected cageCeramic or carbide-coated blades for abrasive minerals that would round off a steel cage.
- UpgradeInline particle sizingA laser sensor on the fines line reports D50 and D97 continuously into your plant system.
