APV Dryer Handbook - Umbc

68
Condensation deposits can frequently be seen as straight lines inside the dryer
chamber, tracing the wall and roof stiffening steel. Due to the heavier mass of this
steel, these points take longer to warm up than the adjacent sheet metal skin of
the chamber.
If there is too much cooling air being drawn through the ring, condensation
deposits can sometimes be seen on the inner surfaces of the cooling ring around
the hot air inlet to the drying chamber.
Good, dry, insulation and careful design of the chamber stiffeners, combined with
an adequate warm-up time, will generally eliminate condensation deposits.
THERMOPLASTICITY
This is the name given to a property of materials which become soft at higher
temperatures, but harden again as the temperature is reduced. Many flavor
products containing sugars or fats exhibit this property. As the powder temperature
rises, the particles first soften then become sticky. This latter temperature is known
as the “sticking temperature.”
If the wall temperature is higher than the “sticking temperature” of the powder, the
powder particles soften on contact with the wall and tend to stick. The sticking
temperature of a powder is difficult to measure in the laboratory, as it is affected
by the moisture content of the powder. The simplest method is to use a graduated
hot bench, but this tends to dry the powder during the test. The powder is sprinkled
along the surface of a polished metal
plate, which is electrically heated so that
one end is considerably higher in
temperature than the other end. The
powder is then swept off the surface
using a soft brush. The point on the
plate’s surface where the powder ceases
to be swept clean is noted – and the
temperature at that point is measured
using crystals of a known sharp
Figure 33. Air broom
melting point.