In the attempt to improve the drying efficiency expressed in formula (17),
page 129, the air outlet temperature To was in the two-stage drying (page 131)
reduced to the limit where the powder with moisture contents of 5-7% became
sticky and started to adhere to the chamber walls.
However, further
improvements have been obtained by the introduction of a static fluid bed
integrated in the conical part of the drying chamber. The secondary drying air
is introduced into a plenum chamber below the perforated plate, through which
the drying air is distributed. This type of dryer can be operated, so that the
primary particles reach a moisture as high as 8-12%, corresponding to an air
outlet temperature of 65-70ºC. This extreme utilization of the drying air means
that for a given capacity the size of the plant can be reduced considerably.
Milk powders have always been regarded as difficult to fluidize.
However, a specially designed and patented perforated plate, see Fig. 82 and
page 131, provides an air/powder rotation in the same direction as that of the
primary drying air. By means of this plate, and the correct combination of bed
height and fluidization velocity, any milk-based product can now be fluidized
in a static fluid bed.
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