How it works
In an air classifier a gas stream carries the feed through a separation zone. Each particle feels two competing actions: aerodynamic drag from the gas, which tends to carry it along, and gravity or centrifugal force, which tends to settle it out. The deciding quantity is the particle’s settling velocity — large, dense particles settle faster than the gas can carry them and report to the coarse fraction, while fine, light particles are swept off to the fine fraction.
The boundary between the two is the cut velocity: particles whose settling velocity equals it have a 50/50 chance of going either way, which defines the cut size. How sharply the split happens around that cut — the separation sharpness — sets how much material is misplaced into the wrong fraction. A perfect classifier would be a step function at the cut size; real ones smear it.
In a static (gravity) classifier the cut is fixed by the gas flow and the geometry; in a dynamic classifier a rotating wheel adds a centrifugal field, so the cut size can be tuned on the fly through rotor speed.
The model
Dyssol Pro computes the split at the population level: for every particle size class it weighs the particle’s settling velocity against the gas cut velocity and assigns the matching fraction to the fine and coarse streams — mass-conserving, for every solid compound, at every time point of the dynamic flowsheet.
Each size class’s settling velocity follows from a standard drag correlation across the whole Reynolds range, so the separation responds to particle size, solid density, and gas properties together. The gas cut velocity is set by the volume flow drawn from the connected stream and the separation geometry — and in a dynamic unit by the classifying-wheel speed, which is what lets the cut be shifted finer or coarser on the fly — while a sharpness term fixes how steeply the separation curve rises around the cut.
On this basis Dyssol Pro covers the full range of dry gas classification — from static gravity and cross-flow sifters to high-speed impeller-wheel classifiers — including the throughput dependence and fine-powder agglomeration that blunt the cut at high loading, and even machine-specific behavior beyond the standard descriptions can be brought into the flowsheet.
Equipment this model can represent
Any dry size-classification duty where drag is played against gravity or a centrifugal field.
Gravity / cross-flow sifters
Static classifiers where a gas stream lifts fines against gravity; cut set by air velocity and geometry.
Zigzag (cascade) classifiers
Multiple gravity stages in series for a sharper cut.
Impeller-wheel (dynamic) classifiers
A rotating classifying wheel sets a centrifugal cut velocity, giving an adjustable, fine cut size.
Air-swept mill classifiers
A dynamic classifier integrated on a mill to return oversize for regrinding.
Typical engineering studies
What teams investigate with the classifier models.
Fine/coarse split prediction
Predict the fine/coarse split and cut size for a given classifier setting and feed.
Closed mill–classifier circuit
Couple with a crusher/mill and a recycle, and study recycle load and product fineness.
Operating-point sensitivity
Map how rotor speed, gas flow, and sharpness move the cut size.
Cut optimization
Optimize the operating point to hit a target fineness with minimum misplacement.
Partition-curve fitting
Calibrate the model’s cut and sharpness to a measured partition curve, then scale up.
Technical FAQ
How does an air classifier separate fine and coarse particles?
It plays drag against gravity or centrifugal force: particles whose settling velocity exceeds the gas cut velocity drop into the coarse fraction, while finer, lighter ones are carried out with the fines. Dyssol Pro reproduces exactly this — it computes each size class’s settling velocity, compares it to the cut velocity, and returns the size-resolved split for a gravity or impeller-wheel classifier.
How can I improve cut size accuracy in a classifier?
Cut size is set by the gas velocity and, in dynamic units, the rotor speed, so accuracy comes from stable flow and a sharp cut. In Dyssol Pro the cut velocity follows from volume flow and geometry (or rotor speed); calibrate the sharpness parameter to your data and run a sensitivity study to find the settings that land the cut where you want it.
Why is my classifier producing too many fines in the coarse fraction?
That is misplacement — usually a low separation sharpness, overloading, or bypass. In Dyssol Pro you study how the sharpness and cut velocity move that misplaced-fines fraction, and whether fine-powder agglomeration is blurring the cut.
What parameters affect classification efficiency?
Gas velocity, rotor speed, geometry, solids loading, and particle density. These map onto the model’s cut velocity and sharpness in Dyssol Pro, so you can vary each in a sensitivity study and see its effect on the grade curve before touching the machine.
How do I calculate the cut point of a particle classifier?
The cut point is the size where the grade efficiency is 0.5 — where a particle’s settling velocity equals the cut velocity. Dyssol Pro computes the cut velocity from flow, geometry, and rotor speed and returns the full grade curve, so you read the cut size directly off it.
How can I reduce bypass in an air classifier?
Bypass means coarse reporting to fines (or vice versa) regardless of size, often from poor flow distribution or fine re-agglomeration. Dyssol Pro can include an agglomeration term and a throughput dependence, so you study how loading and operating point drive the apparent bypass and find a better point.
What is the difference between static and dynamic classifiers?
A static classifier fixes the cut through geometry and gas flow; a dynamic one adds a rotating wheel so the cut size is set by rotor speed and can be tuned online. Dyssol Pro provides both static gravity and dynamic impeller-wheel classification, so you can compare the two for your duty.
How does particle density affect classifier performance?
Settling velocity rises with particle density, so at the same size a denser particle reports coarse. Dyssol Pro takes solid density from the materials database in the settling calculation, which means multi-density feeds separate by both size and density automatically.
How do I optimize airflow in a powder classifier?
Airflow sets the cut velocity: too high and you lose coarse to the fines, too low and the cut drifts coarse. In Dyssol Pro you vary the gas volume flow and read the cut and yield off the grade curve to find the airflow that meets the fineness target.
How can I sharpen the separation curve of a classifier?
Mechanically, sharpness comes from uniform flow and low turbulence; in the model it is a single sharpness term. In Dyssol Pro you fit the sharpness to a measured curve, then study how a sharper cut would change product quality and recycle load across the circuit.
Why is my classifier cut size drifting over time?
Drift usually traces to changing gas flow, rotor speed, or solids loading (or build-up). Dyssol Pro lets you impose those feed and flow variations dynamically and see how the cut moves, helping you separate a process cause from a mechanical one.
How does rotor speed affect air classifier performance?
Higher rotor speed raises the centrifugal cut velocity and shifts the cut finer. Dyssol Pro’s impeller-wheel model takes rotor speed directly (it sets the circumferential and cut velocities), so you can map cut size against speed in a sensitivity study.
How can I reduce coarse contamination in the fine product?
Either move the cut finer or sharpen it. In Dyssol Pro you study how cut velocity and the sharpness parameter trade off against the coarse-in-fine fraction, and pick the operating point that meets the product spec.
What causes poor classification of irregular particles?
Irregular shape changes drag and effective settling velocity, so shape spreads the cut. In Dyssol Pro this is captured through a cut and sharpness calibrated to your measured data, so the shape effect is carried automatically.
How do feed rate changes affect classifier efficiency?
Higher loading typically blunts the cut and shifts it. Dyssol Pro can carry a throughput dependence in the separation, so you can run a dynamic feed-rate study and see how efficiency and cut respond.
How can I select a classifier for micronized powders?
Fine cuts need the high centrifugal field of a dynamic impeller-wheel classifier, and fine-powder agglomeration becomes decisive. Dyssol Pro lets you compare static gravity versus dynamic impeller-wheel classification and use an agglomeration term to see how realistic a fine cut is for your powder.
How do I validate classifier performance experimentally?
You measure the partition (grade) curve by sampling both fractions. In Dyssol Pro you calibrate the model’s cut velocity and sharpness to that measured curve, giving a validated unit you can then use predictively in the flowsheet.
What is the influence of humidity on powder classification?
Humidity makes fines stick together, so they behave like coarser particles and the apparent cut shifts. In Dyssol Pro an agglomeration term captures that apparent-cut shift, and you model the upstream drying in the flowsheet to find a safe moisture window.
How can I reduce energy use in air classification?
Energy goes into the fan and rotor, so the lever is the minimum airflow and rotor speed that still hit the cut. Dyssol Pro computes the cut that each airflow and rotor speed produces, so you can find the lowest-airflow, lowest-speed operating point that still hits the target.