Sterlco SDAP 90-225 CFM Series Drying and Conveying Systems - Manual (Page 11)

SDAP Series PD 2 to PD 4 Drying and Conveying Systems Page 11
1-7 What is Desiccant?
Desiccant is a material which attracts and holds (absorbs) water from the air. Sterling dryers use
a synthetic crystalline metal aluminosilicate desiccant blended with a clay binder and formed
beads.
Absorbed water is driven from saturated desiccant by heating it to a high temperature (reducing
desiccant capacity to hold water) and forcing air through it. This moisture removal process is
called regeneration.
1-8 The Process/Regeneration Cycle
Sterling 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, indicates that the bed is
dry. At bed breakthrough, the bleed air temperature peaks between 350°F (176ºC) and 400°F
(204ºC).
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). This ultra-low humidity level is more than
adequate to dry plastics to as little as .003% moisture, depending on the process and material.
1-9 Closed Loop Machine Conveying
A high efficiency centrifugal blower and special designed take-off compartment provides closed
loop dry air conveying of material from the drying hopper to a sight glass loader located at the
press.
The single-station control includes a low level sensor that sends a signal to the PLC and starts
the blower and conveys material for a set amount of time. An adjustable convey timer is located
on the control panel. Once the convey time is over, the blower shuts down and starts a dump
delay time to allow material to flow into the processing machine. If a demand is still present, the
PLC starts the convey cycle over.