In the Vibro-Fluidizer the whole fluid bed is vibrating. The perforations
of the plate are made so that the drying air is directed with the powder flow.
To avoid that the perforated plate will vibrate with its own frequency,
supporting is necessary. See Fig. 78.
In the spray dryer the outlet
temperature is decreased resulting in an increase of the moisture content and a
decrease of the particle temperature. The moist powder enters by gravity from
the drying chamber into the Vibro-Fluidizer.
There is, however, a limit
as to how far it is operationally possible to go, as the powder will become
sticky with the increased moisture content despite the lower temperature, and
the powder will form lumps and deposits will occur in the chamber.
Usually a reduction in the outlet temperature of 10-15ºC may be
achieved. This results in a much gentler drying especially during the critical
drying stage (30% to 10% moisture), the shrinkage will continue, see Fig. 79,
not interrupted by any case hardening thus reaching conditions close to the
optimal drying example. The lower droplet temperature is achieved partly due to
the lower surrounding temperature, but also due to the higher moisture content
in the particles thus being closer to the wet bulb temperature. This has
naturally a positive influence on the solubility properties of the final
powder.
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