Air-assisted gravity conveyors

The air-assisted gravity conveyor or air slide is generally constructed from modular components giving a reasonably wide flexibility of design. For example, it is relatively easy to construct a system with multiple inlets and outlets, to divert oversize material and curve the chute around comers to deliver to all parts of the plant. Of course, the declination must be maintained over the length or else the flow will halt. 

There are a number of ways to feed an air-assisted gravity conveyor. The most common feed method is simply a flood feed from a mass flow hopper. A non-mass flow hopper may cause problems with inconsistent feed which may result in blockages. That is, if too much material is fed onto the conveyor, then there will be insufficient air to fluidise the material, and if too little material, or none at all, then conveying may halt due to a maldistribution of air from the plenum chamber. While an air-assisted gravity conveyor does not work independently as a feeder, if the bulk solid is metred onto it by the use of a screw feeder or rotary valve, it will then convey this material at a constant rate. Another method of controlling the flow of material is to use a gate or baffle at either end of the conveying duct. 

When selecting an air-assisted gravity conveyor, the following four areas are key considerations ... 

In terms of the air requirement for an air-assisted gravity conveyor, the two parameters to be specified are the volumetric flow rate and the plenum pressure. Experience has shown that the total plenum pressure is typically in the range of 250-500 mm H2O or 24.5 9 mbar and fluidisation velocities must be determined by bench scale tests of the proposed material together with conveyor dimensions (length and width). The slope of the channel and the solids mass flow rate need to be also considered. For free-flowing powders 2-3 times the minimum fluidisation velocity is usually sufficient, but for finer more cohesive particles, the optimum value can be up to 10 times the minimum fluidisation velocity (Woodcock Mason 1987). 

While the air-assisted gravity conveyor has a relatively straightforward construction, and a limited number of moving parts to cause problems, there are issues with its maintenance... 

Air slide (air-assisted gravity conveyor) Transfer/convey Powders (Geldart A/B) Few m to 100 m near horizontal declination special design - few degrees incline Up to few 1000 tonnes/h... 

Haugland, L. (September 1996) Air assisted gravity conveyors. The POSTEC Newsletter No. 15. 

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