In order to improve the drying economy the drying is divided in two or more
steps. The first step is done in a spray drying chamber transforming the liquid
into powder particles and evaporating the main portion of the water. As it will
be discussed later, see page 123, the evaporation of moisture from a particle
will become more difficult and require more time, as the residual moisture
content approaches 0%.
The subsequent drying is done in a fluid bed,
see Fig. 63. The fluid bed drying technology has proved especially suited, as
the residence time in the fluid bed is so long that the moisture from the
centre of the particle can reach the surface from where the evaporation takes
place.
In a fluid bed the drying air is introduced to the powder
through a special perforated plate, the BUBBLE PLATE™, see Fig. 64. Special
features of the BUBBLE PLATE™ are:
- The air is directed downwards towards the plate surface.
- The bubble plate has few, but large holes.
- The unique manufacturing method prevents crevices.
- The bubble plate has shown a very good emptying effect.
The fluid bed can be vibrating, see page 133, or stationary, see page 141.
The fluid bed offers at the same time a very efficient and lenient tool for
cooling of fatcontaining and agglomerated products.