Bendix Commercial Vehicle Systems AD-SP SYSTEM PURGE AIR DRYER 10/04 - Manual (Page 4)

4
DESICCANT
OIL
SEPARATOR
DELIVERY
CHECK
DELIVERY
PORT
CONTROL
PORT
COMPRESSOR
PURGE /
RELIEF
VALVE
FRONT
RESERVOIR
SC-PR ™
VALVE
REAR
RESERVOIR
ENGINE
TURBO
PURGE
ORIFICE
EXHAUST
PORT
TURBO
CUTOFF
GOVERNOR
FIGURE 5 - AD-SP ™ AIR DRYER PURGE CYCLE
PURGE CYCLE (refer to Figure 5)
When air brake system pressure reaches the cutout setting
of the governor, typically 120 psi, the compressor unloads
(air compression is stopped) and the purge cycle of the air
dryer begins. When the governor unloads the compressor,
it pressurizes the compressor unloader mechanism and
the line connecting the governor unloader port to the control
port of the AD-SP
™ air dryer body. Air entering the control
port is simultaneously directed to the turbo cut-off valve
and the purge valve control piston.
The initial purge cycle consists of two simultaneous
occurrences; the closing of the turbo cut-off valve and the
opening of the purge valve. Each is discussed below under
a separate subheading.
TURBO CUT-OFF VALVE (refer to Figure 5)
The primary function of the turbo cut-off valve is to prevent
loss of engine turbocharger air pressure through the
AD-SP
™ air dryer in systems where the compressor intake
is connected to the engine turbocharger. The turbo cut-off
valve also eliminates the “puffing” of air out the open air
dryer exhaust when a naturally aspirated, single cylinder
compressor is equipped with an inlet check valve.
Governor unloader pressure causes the turbo cut-off valve
piston to move and close. With the turbo cut-off valve piston
seated (closed position), air in the discharge line and
AD-SP
™ air dryer inlet port is prevented from entering the
air dryer.
PURGE PISTON
The AD-SP ™ air dryer purge piston also moves in response
to governor unloader pressure, causing the purge valve to
open to atmosphere. Contaminants in the body sump are
instantly expelled when the purge valve opens. Air which
was flowing through the desiccant cartridge immediately
changes direction and begins to flow back toward the open
purge valve. Oil and solid contaminants collected by the oil
separator are removed by air flowing from the desiccant
drying bed to the open purge valve.
DESICCANT RE-ACTIVATION (DRYING THE
DESICCANT)
The initial decompression of the air dryer assembly lasts
only a few seconds and is evidenced by an audible burst of
air at the AD-SP
™ air dryer exhaust. The initial
decompression of the air dryer assembly is “sensed” by
the purge control mechanism and is used by the purge
mechanism as a signal that desiccant re-activation should
begin. The purge control mechanism “remembers” the
highest pressure attained in the air system, the cutout
pressure of the governor.
The actual re-activation of the desiccant drying bed begins
as the purge mechanism allows dry air to flow from the
delivery port (air brake system) back into the AD-SP
™ air
dryer body and through the desiccant cartridge. The delivery
check valve assembly prevents air pressure in the brake
system from returning to the air dryer and flowing out the
open purge valve. The delivery check valve forces system
PURGE
PISTON
SUPPLY
RESERVOIR