APV Dryer Handbook - Umbc

temperature will NOT be reduced. This can be seen if the centrifugal atomizer
speed is reduced to the point that a “Mud Ring” forms around the upper
chamber wall. When this occurs, the outlet temperature will rise, more feed
is automatically introduced to try to reduce the temperature and the system
will soon trip on high outlet temperature, leaving a very messy chamber to
be cleaned.
POWDER MOISTURE
The residual moisture content in the powder is largely controlled by the dryer
outlet temperature. A lower outlet temperature causes the powder moisture
to increase.
Inlet temperature also affects the powder moisture by changing the humidity in
the outlet air. If the inlet temperature is increased to get a higher production rate,
the outlet temperature will also need to be raised slightly to maintain the same
powder moisture. As a rule of thumb, for every 100 degrees rise in inlet
temperature, the outlet temperature should be raised by 12 degrees. This
will retain the same relative humidity in the outlet air and hence, the same
powder moisture.
CONDENSATION
When humid air is cooled, the moisture vapor condenses and forms water
droplets on the nearest surfaces. The “dew” seen on the ground on some
mornings is caused by this effect. The “dew point” of a humid air mass is
dependent on the humidity or water content of the air, but is higher in
temperature as the humidity increases.
If any parts of the inside surface of the dryer are at a temperature below the
dew point, there will be a localized condensation at those points. Powder
particles stick to the condensed water droplets, and in turn, become sticky
and adhere to the hard surface.
When the spray dryer is started, the warm-up time is being used to heat the
inner surfaces of the chamber to a temperature above the dew point.
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