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Absorbed water is driven from saturated desiccant by heating it to a high temperature (reducing
the desiccant's capacity to hold water) and forcing air through it. This moisture removal process is
called "regeneration".
The Process/Regeneration Cycle
Our dehumidifying dryers have two desiccant beds. While one bed is on-line in the process air
loop, the other is off-line being regenerated.
When a desiccant bed is on-line, it absorbs moisture from the process air. In time, the bed
becomes saturated with moisture and needs to be regenerated. The dryer automatically redirects
the process airflow to the second bed and starts the regeneration cycle on the first bed.
During regeneration, the dryer heats air to over 500°F (260ºC) and forces it through the desiccant
bed. Moisture driven off the bed bleeds to the atmosphere.
If you measure the temperature of the air bled to the atmosphere (bleed temperature), you should
observe a rise after a period of time. This condition, bed breakthrough or bed break, indicates that
the bed is dry. At bed breakthrough, the bleed air temperature peaks between 350°F (176ºC) and
400°F (204ºC). After bed breakthrough the desiccant is hot and needs to be cooled down to avoid
a temperature spike when the beds are switched.
The Dew Point meter measures the dew point of the process air. A properly regenerated bed
produces process air dew point of -40°F (-40ºC) or lower. This ultra-low humidity level is more
than adequate to dry plastics to as little as .003% moisture, depending on the process, material,
and ambient conditions.
Aftercoolers, Pre-Coolers, and Plasticizing Traps
These dehumidifying dryers force hot, dry air through the resin in the drying hopper, where the
High temperature applications (250°F to 400°F / 121°C to 204°C) require aftercoolers to cool
moist air returning to the dryer from the drying hopper. An aftercooler cools the return air by
100°F (38°C); this maintains dryer efficiency and condenses unwanted plasticizers from the air-
stream. Heat Exchangers are also required (called pre-coolers) when processing materials that
have low drying temperatures (typically 120 F to 150 F / 49 C to 66 C). Check your specific
dryer model technical specifications for actual requirements. Some models have a minimum low
end temperature range that will determine actual requirements.
Optional Plasticizer Trap:
• Plasticizer traps should be used when resins are processed in the family of
acetates, butyrates and/or propionates. They should also be used when any
resin (when dried) is known to produce residue, contaminates or volatiles. This
contaminate is trapped in the air stream and is carried back from the drying
process as the air leaves the drying hopper.
• The plasticizer trap will condense the contaminate that is in the air stream and
remove it before it enters the dryer unit. If this contaminant is not removed, it
can foul the desiccant beds and damage mechanical components in the dryer
and in the drying process
• Review your process and confirm with your resin supplier if this will be an
issue. If it is, a Plasticizer trap must be used or it will void the warranty on the
dryer.
General Information 15
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