Blow gas up through a bed of particles and, past a threshold velocity, the solids behave like a boiling liquid. The same parametric plate-and-twin pipeline as Plate 1 — pointed at a completely different physics.
| Group | Character | Typical |
|---|---|---|
| C | Cohesive — hard to fluidize, channels | flour, <~30 µm |
| A | Aeratable — smooth, expands before bubbling | FCC catalyst |
| B | Bubbling — bubbles at umf, sand-like | sand, ~100–800 µm |
| D | Spoutable — large/dense, large bubbles | grain, gravel |
All quantities recompute from particle and gas properties through the standard correlations — named in the prose so you can check them.
| Symbol | Quantity | SI value | U.S. customary |
|---|
A fixed bed of particles resists gas flow, and the pressure needed to push gas through rises with velocity. At one particular velocity the drag on the particles exactly equals their submerged weight — the bed is on the verge of lifting itself. That is the minimum fluidization velocity, and above it the solids unlock and circulate like a simmering fluid.
The threshold is found by balancing the Ergun pressure gradient against the bed weight. The dimensionless form, due to Wen and Yu, gives the particle Reynolds number at minimum fluidization directly from the Archimedes number:
At the other end, a single particle is blown clear when the gas reaches its terminal velocity. The Haider–Levenspiel correlation gives it without iteration, and the gap between u_mf and u_t is the whole usable operating window of a bubbling bed:
Once fluidized, the bed pressure drop stops rising — it simply equals the bed weight per unit area, which is why Δp is a flat plateau and a reliable measurement of inventory:
Heating the gas, as in a solar particle receiver or a combustor, thins it and raises its viscosity slightly, pushing u_mf up — so a hot bed needs more blower velocity to stay fluidized than a cold one at the same flow. The twin shows this the moment you drag the temperature control.
| FLUIDIZED BED COLUMN FREEBOARD = 1.5 × H₀ (rule) | |
| Drawing No.FBR-DWG-001 | RevA |
| ScaleNTS | Unitsmm |
| DrawnTWIN | Date— |
| NotesAll dims mm. Geometry schematic; callouts track the twin. Freeboard sized to disengage entrained fines. | |
Same drafting pattern as Plate 1: live dimension callouts (⌀D, H₀, freeboard) driven by the controls. DXF/G-code export transfers here too — say the word and I'll wire it.