SERVICE MANUALFig. 1 AD-1 Air DryerBENDIX AD1 & AD2 TYPESDESCRIPTIONThe air dryer function is to collect and removemoisture and contaminants before air reaches the firstreservoir, thus providing "super dry" air for the air brakesystem. Daily draining of the air system reservoirs is nolonger necessary with an air dryer and air brake systemmaintenance is considerably reduced when compared toan air brake system without an air dryer.The air dryer is installed in the discharge linebetween the air compressor and supply reservoir. Theunit is mounted vertically, usually on the frame of thevehicle.The housing assembly consists of two cylindricalsteel stampings welded together. The housing on theAD-2 Air Dryer incorporates two inlet ports, one reservoirport and one purge volume port. The side outlet port ofthe AD-2 has an integral single check valve with the topport used for the addition of purge volume.The AD-1 Air Dryer has two inlet and three outletports, but is not equipped with a single check valve inany of the three outlet ports.The safety valve protects against excessive pressurebuild-up within the housing.The desiccant sealing plate is located midway in thehousing assembly and houses a replaceable single typecheck ball and a purge orifice. The desiccant cartridgeand paper pleated oil filter are removable and com-Fig. 2 AD-2 Air Dryerprise a complete serviceable unit.The desiccant beads, referred to as the "drying bed,"are a drying substance which has the unique property ofexposing a tremendous surface area in proportion to itsbulk. Desiccant beads weighing .454 kg. (1 lb.) haveabout 186,000 m2 (two million square feet) of absorptivearea made up of a large number of submiscroscopiccavities in each bead. Each bead absorbs or extractsmoisture from the compressed air.The desiccant beads are held in place in thecartridge with steel perforated plates and filter cloths.The top plate is held in place by a spring, while thebottom plate rests on a shoulder about 3 mm (1/8") fromthe bottom of cartridge housing.The end cover assembly is retained by a lock(retaining) ring, capscrews and retainers, and houses thepurge valve and heater assembly if so equipped.The heater and thermostat assembly prevent freeze-up in the purge drain valve when the dryer is used insevere winter conditions. The heater and thermostatassembly is standard in the AD-2 and was optionalequipment for the AD-1. The 60 watt, 12 or 24 volt DCheater and thermostat assembly has an operating rangebetween 100 C and 320 C (500 F and 85 F).OPERATIONThe operation of the air dryer can best be describedby separating the operation into two cycles; the chargecycle and the purge cycle.CTS-4079 - CHAPTER XIV - Page 3
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