Glossary
Hoppers and silos
Geldart classification
Also known as Geldart A B C D, Geldart powder classification.
The Geldart classification (Derek Geldart, 1973) groups powders by particle size and density into four classes that predict fluidisation, bridging and discharge behaviour. It is the most widely used powder-behaviour map in industrial bulk-solids handling.
The four classes
| Class | Particle size / density | Behaviour | Example materials |
|---|---|---|---|
| A | Small (30–100 µm), low density | Fluidises well; expands before bubbling | Cracking catalyst, alumina fines |
| B | Medium (100–500 µm), medium density | Bubbles immediately on fluidisation | Sand, salt, larger cement particles |
| C | Very fine (< 30 µm), cohesive | Hard to fluidise; channels; cohesive arching | Cement, fly ash, flour, talc |
| D | Large (> 500 µm), dense | Spouts rather than fluidises | Coal, gravel, grain |
Why it matters for hopper design
- Class C powders are the most prone to bridging and rat-holing. Sonic horns, air cannons and aeration are routinely needed
- Class A powders flow well from properly-designed hoppers; problems usually trace to wet incoming material
- Class B powders are predictable and well-suited to standard hopper geometry
- Class D powders rarely bridge but are abrasive and shock-loading the hopper
Acoustic-cleaning fit
Sonic horns are most often deployed on Class C powders — fly ash, cement, lime, fine carbon black, food powders — because that is where cohesive flow problems concentrate.
Related terms
Related terms
- HopperA hopper is an inverted-pyramid or conical vessel for storing and discharging bulk solids. Bridging and rat-holing are the universal failure modes; sonic horns are a clean, low-maintenance remedy.
- Bridging (bulk-solids)Bridging (also arching) is the formation of a stable arch of bulk solids above the discharge outlet of a hopper or silo, stopping material flow. The universal failure mode of bulk-solids storage.
- Rat-holingRat-holing is a flow pattern in which material discharges through a narrow vertical channel above the outlet, while the surrounding material remains stagnant and consolidates.
- Fly-ash hopperA fly-ash hopper collects particulate ash from ESP, baghouse, economiser and air-heater equipment. Bridging and rat-holing of fly ash are persistent operational problems.