Fluidisation

Fluidisation or activation of finely divided materials by the use of diffused air has been extensively developed during the past 50 years, particularly for the handling of powders in bulk.

Fluidisation is the process of moving, transporting and treating powders and other granular materials by injection of air to make the powder act as a fluid. This means that air passes through a porous filter material where it is finely distributed and then creates a cushion or film to considerably reduce the friction between material and base.

Also, the air is mixed with the material in such a way that the friction between the particles is reduced, allowing the material to flow like water.

Experience has shown that porous ceramic tiles, used as the permeable membrane through which air is injected and which also supports the powder bed, provide the most efficient means of ensuring the even dispersion of gas, which is essential to the success of an operation.

Porous Ceramic Tiles

The main product supplied by Fairey for fluidisation applications is the Porous Ceramic Tile. These are arranged either in a bed for fluid fire treatment or in series as a fluidised conveyor for transporting powder from one location to another. Another application is to mount the tiles on the side of powder silos to aid flow and prevent solidification.

Porous ceramic tiles are subject to close control during manufacture. Individual tiles are matched one against the other to ensure even overall permeability and the grades used present sufficient resistance to spread the air evenly over the whole area.

Air or gas fed into a compartment or plennum chamber beneath the tiles can only escape by way of the ceramic tiles into the powder above. When velocities are high enough this induces fluidisation. In most cases no more than local fluidisation is necessary to bring about movement in the powder. Relatively small quantities of air at low pressures are needed with velocities of between 0.6 and 1.2 metres per minute (3.4-6.8 metres3/ hour for a standard tile of 305x305mm) being adequate for most purposes and with pressures seldom exceeding 0.3 or 0.4 even in the largest of storage bins.

Typical Applications

The Fluidisation principle is used on many powders:

• Coal Dust
• Alumina
• Sand
• Fly Ash
• Salt
• Gypsum
• Zircon
• Flour
• Sugar
• Molochite
• Cement
• Soda Ash

In Various Process:

• Silo/Hopper Discharge
• Powder Mixing
• Fluidised Burners
• Fluidising Conveyors
• Fluidisation Tanks
• Fluid Bed Dryers