How airslides work — and when they don't

An airslide has no moving parts along its length, uses a fraction of the power of any mechanical conveyor and keeps the powder sealed from end to end. In a cement plant or a power station it is often the cheapest thing on the flow sheet. It is also the most particular: it works on a narrow class of materials, and only downhill.

How does an airslide move powder?

Air is blown at low pressure into a plenum under a porous membrane and passes up into the bed of powder above at a velocity just high enough to support the particles. The bed fluidises: the particles are separated by a film of air, the friction between them collapses and the bed behaves like a liquid. A liquid on a slope flows downhill, and that is the whole machine: tilt the channel a few degrees, feed powder in at the top, and it runs to the bottom while the air leaves through a vent at the discharge.

Which powders will fluidise?

Dry, fine, air-retentive powders — those that fluidise smoothly at low air velocity and take a while to deaerate when the air stops. Cement, raw meal, fly ash, alumina, hydrated lime, gypsum and many fine chemicals are the classic list; the fly ash handling note covers one of the commonest. Powders that are moist, coarse or fibrous, or that deaerate the moment the air stops, do not qualify however fine they look; the material properties note covers the measurements that tell them apart.

What are the parts?

• Channel — a rectangular steel duct in bolted sections, split horizontally into a conveying section above and a plenum below, with the membrane clamped between the flanges.
• Membrane — usually a woven multi-layer polyester fabric; ceramic or sintered materials for hot duties.
• Plenum — the air chamber under the membrane, sectioned along the length so each part can be supplied and balanced.
• Blower — a low-pressure fan or blower delivering dry, oil-free, filtered air; oil or moisture blinds the membrane from below.
• Cover and inspection ports — the sealed top of the conveying section, with ports for looking at the bed.
• Vent and dust filter — at the discharge end and along long runs, so the air can leave without carrying powder or pressurising the channel.
• Diverters and gates — flap boxes to send powder to one of several outlets, and shut-off gates; each is a place for powder to settle.

How much slope, and how much air?

Slope is commonly quoted in the mid-single-digit degrees — less for very fluid powders such as cement, more for those that fluidise reluctantly — and it is found by test on a fluidisation rig. The channel must fall along its entire length; a level or rising section is a dam. Air volume is small, commonly in the region of one to two cubic metres per minute per square metre of membrane, at a fraction of a bar. It must be drier than the coldest surface it meets, or it condenses on the membrane and the powder cakes there. Capacity comes from channel width, slope and bed depth; a longer slide carries no less.

What are the symptoms telling you?

When don't they work?

• Moisture — a little surface moisture makes a powder cohesive and unfluidisable; hygroscopic powders and damp air are the usual route.
• Coarse or widely graded material — the coarse fraction is not supported by the air and settles onto the membrane.
• Fibrous or flaky material — it mats rather than fluidises.
• Non-air-retentive granules — sand, granular fertiliser or pellets deaerate the instant the air stops.
• Uphill or level runs — the slope is the drive. Every metre of height an airslide consumes has to be recovered by an elevator, which is why the two are usually found together.

Where do airslides fit in a plant?

Wherever a fluidisable powder needs to travel a modest distance downhill in a sealed duct: from a silo outlet to a bucket elevator or a packing plant, around a mill circuit, from a silo to a loading spout, and along a row of silos with a diverter at each. Aerated silo bottoms use the same membrane and air to keep the cone live; the silo flow note explains what they can and cannot do. Where the powder will not fluidise, or the route climbs, the comparison of conveyor types points elsewhere. An airslide is also straight: a change of direction is a turning box with its own fluidised floor, and every box is a settling point.

Airslides are among the fine, dusty material equipment we engineer within our conveying systems work, usually alongside the elevator that gives the height back.