Operation; Charge Cycle; Purge Cycle - BENDIX AD-4 AIR DRYER Manual

DESICCANT
CARTRIDGE
CHECK
VALVE
DELIVERY
PORT
HEATER
™
FIGURE 2 - AD-4 AIR DRYER CHARGE CYCLE
OPERATION OF THE AD-4
The AD-4 ™ air dryer alternates between two operational
modes or "cycles" during operation: the charge cycle and
the purge cycle. The following description of operation is
separated into these "cycles" of operation.

CHARGE CYCLE

(refer to Figure 2)
When the compressor is loaded (compressing air)
compressed air, along with oil, oil vapor, water and water
vapor flows through the compressor discharge line to the
supply port of the air dryer end cover. The flow velocity or the
speed at which the air and its contaminants travel down the
discharge line is reduced substantially as it enters the air
dryer end cover baffle system behind the supply port. As air
travels through the baffle system, its direction of flow changes
by 180 degrees several times, reducing the temperature,
causing contaminants to condense and drop to the bottom
or sump of the air dryer end cover.
After exiting the end cover baffle system, the air flows into
the desiccant cartridge. Once in the desiccant cartridge air
first flows through an oil separator which removes water in
liquid form as well as oil, oil vapor and solid contaminants.
2
CHECK
VALVE
EXHAUST
PORT
PURGE
VALVE
SUPPLY RESERVOIR
™ AIR DRYER
ORIFICE
PURGE VOLUME
DESICCANT
OIL SEPARATOR
CONTROL
GOVERNOR
Air exits the oil separator and enters the desiccant drying
bed. Air flowing through the column of desiccant becomes
progressively dryer as water vapor adheres to the desiccant
material in a process known as "adsorption". The desiccant
cartridge using the adsorption process typically removes
95% of the water vapor from the pressurized air.
The majority of dry air exits the desiccant cartridge through
its integral single check valve to fill the purge volume between
the desiccant cartridge and outer shell. Some air will also
exit the desiccant cartridge through the purge orifice adjacent
to the check valve.
Dry air flows out of the purge volume through the single
check valve assembly and out the delivery port to the first
(supply) reservoir of the air system.
The air dryer will remain in the charge cycle until air brake
system pressure builds to the governor cutout setting.

PURGE CYCLE

(refer to Figure 3)
When air brake system pressure reaches the cutout setting
of the governor, the compressor unloads (air compression
stopped) and the purge cycle of the air dryer begins. The
BED
COMPRESSOR