TM 5-3805-260-24
PURGE CYCLE
For the purge or regeneration cycle, the air flow
through the desiccant bed of the air dryer goes in
reverse.
When the air pressure from the air compressor goes
up to high pressure setting of the governor, it goes
through the governor and into the area below the
unloader pistons in the air compressor cylinder block.
The unloader pistons go up and the unloader plungers
move up and hold the inlet valves off of their seats.
With the inlet valves held off their seats, air goes
through the unloader line (3) into the purge valve
opening (14). The purge valve plunger (1 7) is moved off
of its seat (16). The sudden opening of the purge valve
piston (15) and purge valve plunger (17) permits the air
pressure that is in the dryer to exhaust the condensation
and foreign material that has gathered in the sump (12)
past the purge valve exhaust deflector (18) to the
atmosphere. The sudden decompression (decrease in
air pressure) of the drying bed of the desiccant cartridge
(10) removes moisture from the pores (cavities) of the
micro-crystalline particles that make up the drying bed.
After the rapid decompression caused when the purge
valve is opened, air goes from the purge volume (2)
through the purge orifice (8) into the desiccant cartridge
(10). As the air passes through the purge orifice (8) it
expands and goes through the drying bed in the reverse
direction, and through the purge valve to exhaust to
atmosphere.
The air in the purge volume (2) was dried as it moved
through the desiccant bed during the charge cycle. As it
moves back into the desiccant bed, it becomes "super
dry" (extra dry) after expansion to atmosphere pressure,
through purge orifice (8). This super dry air is very
efficient (very good) in removal of water in its reverse
flow through the desiccant bed. If the purge cycle goes
longer than the time needed to completely drain the
purge volume to atmosphere, no more action takes
place. The air dryer check valve (6) will keep the air in
the air reservoirs from going into the purge volume (2).
The electric heater (9) and thermostat (13) keeps
moisture in the sump (12) from freezing. The thermostat
will turn heater on at 500F (100C) and off at 850F
(300°C).
When the air pressure in the air system goes below
the governor pressure setting, the governor will cut in
and the unloader pistons and plunger will no longer hold
the inlet valves of the air compressor off their seats. The
air compressor will again begin sending compressed air
to the air dryer through air supply line (1) and the charge
cycle begins.
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