8
The purge valve will now open, depressurizing the tower that had been in drying service for
the previous 4 hours. At this point the heater is energized for the three hour heating period.
The purge air flow begins to carry heat into and through the wet desiccant bed. During the
4th hour of the regeneration cycle, the heater is turned off and the purge air is allowed to
continued flowing through the bed.
The now cool purge air cools the desiccant, carrying the heat out of the regenerating tower.
This cool down period is important in reducing both desiccant thermal shock (pulverization
of desiccant caused by excessive temperature differentials) and the exposure of
downstream equipment and processes to excessive temperatures at dryer tower switch
over.
C.TIMER OPERATION
The circuit board timer supplied will provide complete and reliable control for all dryer func-
tions. The timer is completely factory tested and calibrated. No field adjustments of settings
should be necessary. Refer to voltage sequence page for timer operation.
Upon startup the timer will initiate regeneration of the right tower. At the same time the left
tower will begin drying the process gas stream.
Regeneration begins with blower startup. The blower and heater will now remain in opera-
tion for approximately 3 hours.
After approximately 2 hours, the heater will de energize, allowing the regenerating bed to
cool down. The cool down period will last 50 minutes.
At this point the blower will shutoff and a 10 minute stabilization period will be initiated. After
the 10 minute stabilization period, tower shift will occur and regeneration will be initiated as
described above.
D.HEATER OPERATION
For long and efficient heater element life, Arrow externally mounted heaters are operated at
a low watt density. When supplied with proper purge flow, heaters are rated and sized not to
overheat.
CAUTION
HEAT REACTIVATED DRYERS MUST ALWAYS BE PRESSURIZED WHEN
ENERGIZED. HEATERS CAN OVERHEAT AND PREMATURELY AGE IF
PURGE AIRFLOW IS INTERRUPTED OR DISCONTINUED DURING HEATER
OPERATION.
Prior to tower switch over, a 10 minute time interval is allowed to bring both towers to full
line pressure before switch over (right and left tower switch drying/regenerating functions).
This is accomplished by closing both purge valves, and allowing the purge air flow to bring
the off-line tower up to full line pressure. After the pressure in the regenerated tower
reaches line pressure, the 4-way valves shift. This shift should always occur at full line
pressure to prevent rapid pressurization which would agitate, and cause attrition of the
desiccant bed.
