TM 5-3805-260-24PURGE CYCLEFor the purge or regeneration cycle, the air flowthrough the desiccant bed of the air dryer goes inreverse.When the air pressure from the air compressor goesup to high pressure setting of the governor, it goesthrough the governor and into the area below theunloader pistons in the air compressor cylinder block.The unloader pistons go up and the unloader plungersmove up and hold the inlet valves off of their seats.With the inlet valves held off their seats, air goesthrough the unloader line (3) into the purge valveopening (14). The purge valve plunger (1 7) is moved offof its seat (16). The sudden opening of the purge valvepiston (15) and purge valve plunger (17) permits the airpressure that is in the dryer to exhaust the condensationand foreign material that has gathered in the sump (12)past the purge valve exhaust deflector (18) to theatmosphere. The sudden decompression (decrease inair pressure) of the drying bed of the desiccant cartridge(10) removes moisture from the pores (cavities) of themicro-crystalline particles that make up the drying bed.After the rapid decompression caused when the purgevalve 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) itexpands and goes through the drying bed in the reversedirection, and through the purge valve to exhaust toatmosphere.The air in the purge volume (2) was dried as it movedthrough the desiccant bed during the charge cycle. As itmoves back into the desiccant bed, it becomes "superdry" (extra dry) after expansion to atmosphere pressure,through purge orifice (8). This super dry air is veryefficient (very good) in removal of water in its reverseflow through the desiccant bed. If the purge cycle goeslonger than the time needed to completely drain thepurge volume to atmosphere, no more action takesplace. The air dryer check valve (6) will keep the air inthe air reservoirs from going into the purge volume (2).The electric heater (9) and thermostat (13) keepsmoisture in the sump (12) from freezing. The thermostatwill turn heater on at 500F (100C) and off at 850F(300°C).When the air pressure in the air system goes belowthe governor pressure setting, the governor will cut inand the unloader pistons and plunger will no longer holdthe inlet valves of the air compressor off their seats. Theair compressor will again begin sending compressed airto the air dryer through air supply line (1) and the chargecycle begins.2-136
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