11
PRINCIPLE OF OPERATION
SECTION 6
6.1-1 REPRESSURIZATION STAGE
Repressurization occurs in the regenerating tower.
Repressurization must occur before tower changeover. This
reduces shock to the desiccant and the possibility of
downstream pressure spikes.
When the inlet transfer valve changes position, the position indica-
tor will move and actuate the other pneumatic limit switch. This
will open the purge valve on the tower that was drying the process
air, causing the tower to depressurize.
The purge valve is closed and the purge air, which was vented
to atmosphere earlier in the cycle, is now used to repressurize
the regenerated tower.
The pilot valves SV1 & SV2 de-energize causing the purge valve
on the regenerating tower to close. The purge air, which was
vented to atmosphere earlier in the cycle, is now used
to repressurize the regenerated tower.
6.1-2 CHANGEOVER/DEPRESSURIZATION STAGE
Tower changeover occurs after the previous regenerating tower
is pressurized. The controller signals the inlet transfer valve
to switch. The tower that was on line and drying the process air
will begin to depressurize.
Pressure in the regenerating tower will approach full line pressure.
The pilot valves SV1 or SV2 will energize. Pilot air will be supplied
to one side of the inlet transfer valve. The inlet transfer valve shuttle
will move to the side that has pilot pressure.
6.1-3 DRYING AND REGENERATION STAGE
During the drying stage, one tower is pressurized with process air.
This tower is in the drying stage. The process air passes through the
desiccant bed, which adsorbs moisture from the air. The
regeneration stage of one tower occurs at the same time as the drying
stage in the opposite tower. During the regeneration stage, a
percentage of dry air is directed through the desiccant bed of the
offstream tower. The purge air is vented through the purge valve to
the atmosphere.
The pilot valves SV1 or SV2 are either energized or de-energized
depending on which tower is on stream (drying). If SV2 is energized,
the LEFT tower is drying and if SV1 is energized, the RIGHT tower
is drying.
6.2 MOISTURE INDICATOR
The moisture indicator is in the center of the gauge panel. A sample
of outlet air is directed through the indicator. The moisture indicator is
a clear plastic tube filled with moisture sensing crystals that will
change color from
PINK (indicating wet air)
to
BLUE (indicating
dry air)
as the dew point of the air changes from +20
O
F to -40
O
F.
FIGURE 6A PNEUMATIC SCHEMATIC
