An air slide is a gently inclined conveying trough divided by a porous fabric. Low-pressure air enters below the fabric and passes upward through the bulk solid. If the air distribution is suitable, the material becomes aerated, its apparent resistance to shear falls, and gravity drives it along the upper chamber.
The air does not normally carry the solids pneumatically. Its job is to condition the material so gravity can move it. That distinction explains why a material can be easy to transport in a pneumatic line yet unsuitable for an air slide—or the reverse.
What makes a material suitable?
Fine enough to aerate, permeable enough to distribute air
Coarse, free-draining particles may allow air to channel through without developing useful support. Extremely fine or cohesive powders may require excessive pressure, retain air, form stable lumps, or behave unpredictably after storage. A workable material sits between these extremes and must be judged by test.
Low and controlled moisture
Moisture can increase cohesion, blind the fabric, create deposits, and produce startup plugs. “Dry” is not a complete specification: define expected moisture range, temperature, condensation risk, and whether the material is hygroscopic.
Stable enough across the process envelope
Grinding fineness, temperature, additive content, storage time, and recycle streams can change aeration behaviour. Test representative worst cases—not only a convenient fresh sample.
| Question | Favourable indication | Warning sign |
|---|---|---|
| Does the powder aerate uniformly? | Even expansion and smooth surface movement. | Localized bubbling, channels, or dead zones. |
| Does flow begin at a practical slope? | Repeatable flow with margin above the test threshold. | Intermittent surging or a very steep required slope. |
| Does the fabric stay open? | Stable pressure loss during the test. | Rapid blinding or material leakage through the cloth. |
| Does the material de-aerate? | Predictable discharge into the receiving equipment. | Flooding, flushing, or excessive entrained air downstream. |
Design variables that belong together
Capacity, width, bed depth, and slope
Capacity is not determined by width alone. The selected slope must provide the driving force for the aerated material, while the bed depth must allow air to distribute without excessive pressure or unstable flow. Transitions and feed distribution often control useful width more than the nominal trough size.
Airflow and pressure
The blower must provide enough pressure to overcome the fabric and material-bed resistance at the required airflow, plus distribution and piping losses. More air is not always better: excessive aeration can increase dust carryover, fabric stress, downstream venting demand, and unstable discharge.
Ventilation and air balance
Every cubic metre of aeration air must leave somewhere. The receiving vessel and intermediate vents must handle this flow plus displaced air and process leakage. Poor venting raises backpressure and can stop conveying even when the blower is correctly sized.
Common reasons air slides fail
- Feed is not distributed: one overloaded lane forms while the rest of the fabric contributes little.
- Fabric selection is based only on pore size: strength, permeability, temperature, chemical compatibility, and blinding resistance are neglected.
- Startup is attempted under a packed bed: the available pressure cannot establish uniform aeration.
- Condensation occurs: cold surfaces or wet air turn a flowable powder into deposits.
- The discharge equipment throttles the slide: airlock, diverter, chute, or vessel pressure creates backpressure.
- No cleaning or access strategy exists: minor accumulation becomes a permanent restriction.
Laboratory testing is a design input
For an unfamiliar powder, a representative aeration and inclined-flow test is more valuable than a confident calculation built on assumed material behaviour.
A practical feasibility workflow
- Define capacity, route, lift restrictions, operating time, and all feed conditions.
- Collect particle-size distribution, moisture, temperature, bulk density, and flowability information.
- Test aeration response, minimum conveying slope, airflow, and pressure drop with representative samples.
- Select fabric and operating air velocity together; verify temperature and chemical limits.
- Size the conveying width and air zones, then check feed distribution and transitions.
- Complete the blower system resistance and receiving-vessel air balance.
- Define startup, shutdown, cleaning, blockage detection, and maintenance access.
Sources and engineering boundary
Bulk-solid behaviour should be characterized with suitable test methods and specialist interpretation. The official ASTM D6128 listing provides context for shear testing, while ISO 80000-4 provides quantity and unit context for mechanics.
This article is a feasibility guide. It does not replace material testing, vendor fabric data, explosion-risk assessment, structural design, or a guaranteed conveying test.