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

 

 

AD600-AD1500

 

 

AD600-AD1500

 

 

Dryer Flow Schematic

 

 

Dryer Flow Schematic

 

 

Right Desiccant in

Regeneration

Left Desiccant in

Regeneration

Regeneration

Blower

 

Regeneration

 

Blower

 

Dryer Valve

Regeneration

 

Dryer Valve

Regeneration

 

 

Filter

Insulated

Insulated

 

 

Filter

 

 

Drying

 

 

 

 

 

Drying

 

 

Hopper

 

 

 

 

 

 

Hopper

 

 

 

 

 

Regeneration

 

Regeneration

 

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 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.

Large Dehumidifying Dryers

Functional Description

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Sterling 882.00295.00 specifications Process/Regeneration Cycle, Sequence of Operation
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882.00295.00 specifications

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