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AIR DRYER AND RESERVOIR SYSTEM

OPERATION: GENERAL

The Bendix

AD-IS

air dryer and reservoir system

alternates between two modes or "cycles" during operation:

the Charge Cycle and the Purge Cycle. The following

describes these "cycles" of operation.

CHARGE CYCLE

(Refer to Figure 2.)

When the compressor is running loaded (compressing air),

compressed air ﬂ ows through the compressor discharge

line to the inlet (1/IN) port of the air dryer body. The

compressed air often includes contaminates such as oil,

oil vapor, water, and water vapor.

Traveling through the discharge line and into the air dryer,

the temperature of the compressed air falls, causing

some of the contaminants to condense and drop to the

bottom of the air dryer and purge valve

contaminants are ready to be expelled at the next purge

cycle. The air then ﬂ ows into the desiccant cartridge,

where it ﬂ ows through an oil separator—or coalescing ﬁ lter

if equipped with a PuraGuard

which removes water in liquid form as well as liquid oil and

solid contaminants.

Air then ﬂ ows into the desiccant drying bed and becomes

progressively more dry as water vapor adheres to the

desiccant material in a process known as adsorption.

Dry air exits the desiccant cartridge, through the center of

the base assembly, then ﬂ ows to the delivery check valve

and through an oriﬁ ce and into the purge reservoir. The

delivery check valve opens, supplying air to the pressure

protection valves (A) through (D) simultaneously, the

safety valve, and also to the reservoir port of the attached

governor. The purge reservoir ﬁ lls, storing air that will be

used to regenerate the desiccant during the purge cycle.

This air is available to supply downstream components

during the charge mode.

When the air pressure reaches approximately 106 psi, the

four pressure protection valves will open and air will be

supplied to the primary reservoir, secondary reservoir, and

accessories. If the pressure protection valves are preset

to different values the valves will open—in order of lowest

setting to highest setting—when charging a ﬂ at system.

The air dryer and purge reservoir will remain in the charge

cycle until the air brake system pressure builds to the

governor cut-out setting of approximately 130 psi.

PURGE CYCLE

(Refer to Figure 3.)

When air brake system pressure reaches the cut-out setting

of the governor, the governor unloads the compressor

activating the purge cycle of the air dryer and reservoir.

assembly.

These

oil coalescing cartridge—

The governor unloads the compressor by allowing air

pressure to ﬁ ll the line leading to the compressor unloader

mechanism. This suspends the delivery of compressed air

to the AD-IS

air dryer and reservoir system.

Similarly, the governor also supplies air pressure to the air

dryer. The pressure also moves the air dryer purge piston

down, opening the purge valve to atmosphere and closing

off the compressor air supply to the turbo cut-off valve

(covered in the Turbo Cut-off Feature section of this piece).

Water and contaminants captured are expelled immediately

when the purge valve opens. In addition, air—which

was ﬂ owing through the desiccant cartridge—changes

direction and begins to ﬂ ow toward the open purge valve.

Contaminants collected by the air dryer are removed by

air ﬂ owing from the purge reservoir through the desiccant

drying bed to the open purge valve.

The initial purge and desiccant cartridge decompression

lasts only a few seconds, evidenced by an audible burst

of air at the air dryer exhaust.

The actual regeneration of the desiccant drying bed begins

as dry air from the purge reservoir ﬂ ows through the purge

oriﬁ ce into the desiccant bed. Pressurized air from the

purge reservoir expands after passing through the purge

oriﬁ ce; its pressure lowers and its volume increases. The

ﬂ ow of dry air through the drying bed regenerates the

desiccant material by removing any water vapor adhering

to it. Approximately 30 seconds are required for the entire

contents of the purge reservoir of an AD-IS air dryer and

reservoir system to ﬂ ow through the desiccant drying bed.

The delivery check valve assembly prevents air pressure

in the brake system from returning to the air dryer during

the purge cycle. After the purge cycle is complete, the air

dryer and reservoir system is ready for the next charge

cycle to begin.

TURBO CUT-OFF FEATURE

The primary function of the turbo cut-off valve is to prevent

loss of engine turbocharger air pressure through the

AD-IS air dryer when the dryer is in the purge mode.

At the onset of the purge cycle, the downward travel of the

purge piston is stopped when the turbo cut-off valve (the

tapered portion of purge piston) contacts its mating metal

seat in the purge valve housing. With the turbo cut-off

valve seated (in the closed position), air in the compressor

discharge line—as well as the AD-IS air dryer inlet port—

cannot enter the air dryer. By completing these actions,

the turbo cut-off effectively maintains turbo charger boost

pressure to the engine.

(Refer to Figure 3.)