MCi Crusader II 1989 Maintenance Manual page 166

Date 1-1-89
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MC-9 MAINTENANCE MANUAL
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Figure 4-43. Air Dryer Installed.
Figure 4-44. Air Dryer Cross-Section.
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OPERATION
The operation of the AD-2 dryer can be best described by
separating the operation into two cycles: the charge cycle and
the purge cycle.
CHARGE CYCLE - With the compressor in its "loaded" or
compressing cycle, air from the compressor enters the air dryer
through the discharge line. When the air, along with the water
and other contaminants, enter the air dryer, the velocity of the
air reduces substantially and much of the entrained liquid drops
to the bottom of the air dryer. The initial air flow is toward the
bottom of the dryer, but air flow direction changes 180° at the
bottom of the air dryer. dropping some water and oil.
The air now passes through the oil filter which removes
some oil and foreign material but does not remove water vapor.
At this point, the air remains saturated with water.
The filtered air and vapors penetrate the desiccant drying
bed and the absorption process begins. Water vapor is removed
from the air by the desiccant
The unsaturated "dry air" passes through the ball check
valve and purge orifice into the purge volume. From the purge
volume air flows through a check valve and into the air
reservoir.
PURGE CYCLE - When desired air system pressure is
reached, the governor cuts out (115-118 psi 1793-814 kPa).
pressurizing the unloader cavity of the compressor which_
unloads the compressor. The line connecting the governor
unloader port to the end cover purge valve port (bottom of the
air dryer) is also pressurized. opening the exhaust of the purge
valve to atmosphere. With the exhaust of the purge valve open.
contaminants in the discharge line and dryer sump are purged,
or forced past the open exhaust out to atmosphere.
The reverse air flow across the desiccant starts the removal
process of moisture from the desiccant surface.- Dry air flowing
from the purge volume through the purge orifice and across the
drying bed further dries the desiccant
The combination of these reverse flows strips the water
vapor from the desiccant (drying bed). This normally takes
12-15 seconds.
The desiccant becomes re-activated from this cycle and is
now ready for another charge cycle, which occurs when the
compressor returns to the-compressing cycle.
MAINTENANCE
The desiccant cartridge should be replaced or rebuilt when
the desiccant is contaminated and does not have adequate
water adsorption capacity. However. the following checks
should be made before replacing the desiccant cartridge to
ascertain that the water accumulation is not related to the below
listed conditions.
A.
An outside air source has been used to charge the sys-
tem. This air did not pass through the drying bed.
B. Air usage is exceptionally high and not normal for the
coach. This may be due to accessory air demands or some
unusual air requirement that does not allow the compressor to
load and unload (compressing and non-compressing cycle) in
a normal fashion. Check for high air system leakage.
C. The air dryer has been installed in a system that has been
previously used without an air dryer. This sytem will be satu-
rated with moisture and several weeks of operation may be
required to dry it out