APV Dryer Handbook - Umbc

52
Figure 24 illustrates the very rapid reduction in air temperature that occurs due
to the high heat transfer rate obtained in the fluid bed.
OPERATING PARAMETERS
Inlet temperature of the drying air introduced into the chamber is dependent on the
particular characteristics of the product being dried but generally would be similar
to that used on a spray dryer for the same product.
Outlet temperature is selected by test work to provide the desired powder moisture
and is controlled by the speed of the feed screw. Since the Spin Flash dryer
produces a finer particle size than does a spray dryer, it has been found that a
slightly lower outlet temperature may be used to obtain the same powder moisture.
This provides an increase in thermal efficiency.
95°
105°
110°
125°
85°
Figure 24. Fluid bed provides rapid air
temperature reduction
Air velocity through the cross
section of the drying chamber
is an important design factor
and is determined, in part, by
the final particle size required.
A lower velocity will tend to
reduce the final dried particle
size carried out of the
chamber. The major factor,
however, is the stability of
the very complex bed which
must neither settle back into
the air distributor nor blow
out of the top of chamber.
Once the maximum velocity
has been determined by
test work for a given product,
the diameter of a drying
chamber can be selected
to provide the desired water
evaporation rate.