
moisture is stripped from the air by a desiccant bed. The dried process air is then
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
When a desiccant bed is
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 |
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Dryer Flow Schematic |
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| Dryer Flow Schematic |
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Right Desiccant in | Regeneration | Left Desiccant in | Regeneration | ||
Regeneration | Blower |
| Regeneration | ||
| Blower |
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Dryer Valve | Regeneration |
| Dryer Valve | Regeneration |
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| Filter | Insulated | Insulated | ||
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| Filter | |||
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| Drying |
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| Drying | |
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| Hopper |
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| Hopper | |
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Regeneration |
| Regeneration |
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Heaters | Process | Heaters | Process |
| Heater |
| Heater |
Regeneration | Process | Regeneration | Process |
Exhaust | Filter | Exhaust | Filter |
Left Desiccant | Right Desiccant | Left Desiccant | Right Desiccant |
| Process |
| Process |
| Blower |
| Blower |
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
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.
Large Dehumidifying Dryers | Functional Description | 13 |