Geldart classification (A/B/C/D) - powder behaviour map for bulk-solids handling

The Geldart classification groups powders by particle size and density difference between solid and gas to predict fluidisation behaviour. It is widely used when selecting aeration, fluidisation, hopper discharge and pneumatic handling approaches for industrial powders.

The four groups

Group	Typical behaviour	Industrial examples
A	Aerates and expands before bubbling	FCC catalyst, some fine fly ash
B	Bubbles readily with modest expansion	Sand-like particles, many granular solids
C	Cohesive and difficult to fluidise	Very fine powders, cohesive ash, some pigments
D	Large or dense particles requiring high gas velocity	Pellets, coarse sand, large granules

Why it matters

A flow aid that works on Group A powder can fail on Group C material. Fine cohesive powders may bridge, rat-hole or channel air rather than fluidise uniformly. Coarse dense particles may need high air velocity that becomes erosive or impractical. Geldart classification is therefore an early screening tool, not a substitute for flow testing.

Design implications

For hoppers and silos, the classification helps decide whether aeration pads, air slides, bin activators, vibrators, air cannons or acoustic cleaners are plausible. It also informs outlet size, wall angle, liner material and whether mass flow is required. Moisture, temperature, electrostatic charge and particle shape can shift real behaviour away from the ideal map.

Acoustic cleaning relevance

Sonic horns are often attractive where material is too cohesive for simple aeration but still responsive to vibration of the gas volume. For very cohesive Group C powders, horns may need to be combined with mass-flow hopper design, controlled aeration or mechanical discharge equipment. Classification helps avoid treating all ash or dust as if it behaves the same.

Testing notes

The Geldart group should be treated as a screening result. Real hoppers also depend on wall friction, moisture, consolidation pressure, storage time, temperature and electrostatic effects. A powder that behaves like Group A in a small dry test may bridge like Group C after sitting warm and humid in a full-scale hopper. For critical systems, shear testing and pilot discharge trials are better than classification alone. Acoustic cleaning proposals should ask for actual material behaviour, not only particle size.

Design use

The Geldart groups help engineers predict how powders behave when air or gas flows upward through them. Group A powders fluidise easily and expand before bubbling. Group B powders bubble readily. Group C powders are cohesive and difficult to fluidise. Group D particles are large or dense and need high gas velocity. Real plant solids can sit between groups or change behaviour with moisture, temperature, electrostatic charge or particle-size segregation.

This classification appears in fluidised-bed boilers, catalyst regenerators, pneumatic conveying, silo design, hopper aeration and ash handling. It helps decide whether a material is likely to flow through a cone, respond to aeration pads, form rat holes or require mechanical assistance. It also warns against copying a design from one powder to another just because the bulk density looks similar.

Operating and cleaning implications

Failure modes often follow the Geldart behaviour. Cohesive Group C dust can bridge in hoppers, blind filters and resist simple aeration. Coarse Group D solids can erode bends, settle in ducts and demand larger conveying velocities. Fine powders may fluidise well in a test rig but compact in a real hopper after vibration, humidity or long residence time.

Acoustic cleaning is most credible for deposits that can be broken by pressure fluctuation and then discharged by gravity or gas flow. Very cohesive powders may need aeration, liners, vibration or mechanical extraction as well. Geldart classification is therefore a useful first screen, not a replacement for testing the actual ash, catalyst, lime or product under plant temperature and moisture conditions.

Testing context

For design work, Geldart classification should be paired with shear testing, moisture testing and trial discharge where possible. Bulk solids from real plants are rarely uniform: fly ash may contain coarse economiser grit, fine ESP dust and unburned carbon in the same system. Segregation can make the first material discharged easy flowing while the remaining inventory becomes cohesive and difficult to aerate.

Geldart classification