Fig. 5 AD-2 Operational Charge Cycle
The unsaturated "dry air" passes through the ball
check valve and purge orifice into the purge volume.
From the purge volume air flows through a check valve
and into the first reservoir.
The AD-1 Air Dryer does not incorporate an integral
check valve. The single check valve in an AD-1
installation would be located in the line between the
AD-1 outlet port and the first reservoir.
Purge Cycle (Fig. 6)
When desired system pressure is reached the
governor cuts out, pressurizing the unloader cavity of the
compressing cycle). The line connecting the governor
unloader port to the end cover purge valve port (bottom
of the air dryer) is also pressurized, opening the exhaust
of the purge valve to atmosphere. With the exhaust of
the purge valve open, contaminants in the discharge line
and dryer sump are purged or forced past the open
exhaust out to atmosphere.
The reverse air flows across the desiccant and starts
the removal process of moisture from the desiccant
surface. Dry air flowing from the purge volume through
the purge orifice and across the drying bed further dries
The combination of these reverse flows strips the
water vapor from the desiccant (drying bed). This
normally takes between 12-15 seconds.
Fig. 6 Operational Purge Cycle
The desiccant becomes activated from this cycle and
is now ready for another charge cycle, which occurs
when the compressor returns to the compressing cycle.
It is for this reason the air dryer must be purged for 20
seconds after receiving moisture saturated air for a
maximum of 60 seconds from a 1.2 m2 (12 CFM)
MAINTENANCE & CHECKING SERVICEABILITY
Every 37,000 km (23,000 miles) or every three
Inspect for moisture in the air system by opening
reservoirs, drain cocks or valves and checking
for presence of water. In areas where more than
approximately a 170 C (300 F) range of
temperature is common, small amounts of water
can accumulate in the air brake system due to
condensation. The presence of small amounts of
water due to condensation is normal and should
not be considered as an indication that the dryer
is not performing properly.
The desiccant cartridge should be replaced or
rebuilt when it has been determined that the
desiccant is contaminated and does not have
adequate water adsorption capacity. However,
the following checks should be made before
replacing the desiccant cartridge to ascertain
that the water accumulation is not related to the
following listed items:
An outside air source has been used to
charge the system. This air did not pass
through the drying bed.
CTS-4079 - CHAPTER XIV - Page 5