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Large Dehumidifying Dryers Functional Description 13
moisture is stripped from the air by a desiccant bed. The dried process air is then re-heated
and delivered back into the drying hopper to dry material again.
This system is a "closed loop", because ambient (outside) air is never introduced into the
process air. The closed loop system is used by the manufacturer because the process air is
typically much drier than ambient air, even after carrying moisture out of the plastic resin.
Recycling process air maintains drying efficiency at a consistently high level.
The Process/Regeneration Cycle
There are two desiccant beds in a dryer up to 1500 cfm. Larger dryers have four beds (two
left and two right beds). While one desiccant bed is on-line in the process air loop, the other
is off-line being regenerated (see Figure 1).
When a desiccant bed is on-line, it absorbs moisture from the process air. The desiccant will
become saturated with moisture and need to be regenerated. The dryer automatically redirects
the process airflow to the second bed and starts regenerating the first bed.
During regeneration, air is heated to approximately 550°F and forced through the saturated
desiccant. The moisture driven off the desiccant is bled to the atmosphere. The regeneration
heater is then turned off while the regeneration blower continues to blow air through the
desiccant bed until it is cooled.
Figure 1: Regeneration Cycle
Regeneration
Filter
Regeneration
Blower
Process
Blower
Regeneration
Exhaust
Process
Heater
Process
Filter
Insulated
Drying
Hopper
Right Desiccant
Regeneration
Heaters
Dryer Valve
Left Desiccant
AD600-AD1500
Dryer Flow Schematic
Right Desiccant in
Regeneration
Regeneration
Filter
Regeneration
Blower
Process
Blower
Regeneration
Exhaust
Process
Heater
Process
Filter
Insulated
Drying
Hopper
Right Desiccant
Regeneration
Heaters
Dryer Valve
Left Desiccant
AD600-AD1500
Dryer Flow Schematic
Left Desiccant in
Regeneration
Sequence of Operation
When the dryer is started, the main air valve rotates 360º and then moves to place the last
successfully regenerated desiccant bed online. If the dryer was powered off using the main
power disconnect or E-stopped, then the regenerating bed is placed back into regeneration,
and the regeneration cycle is started over. The main air valve is allowed a period of time to
move from one position to the other. If the main air valve does not complete positioning
within the allotted time, then a “Main Air Valve Fail” alarm is generated and the dryer stops
the process air blower and disables the drying hoppers and regeneration circuit.
If the closed loop regeneration option is enabled, the regeneration closed loop valve moves to
the open loop position. The system always begins the cooling phase for a regeneration cycle
using ambient (open loop) air. The closed loop regeneration valve is allowed a period of time
to move from the closed to the open loop position and a “Closed Loop Valve” alarm is
generated if the valve fails to reach the commanded position within this time. This alarm will
disable the regeneration circuit but does not stop the process air system or drying hoppers.