MCi Crusader II 1989 Maintenance Manual page 172

Date
1-1-89
Page ____
4_-_42 _ _
MC-9 MAINTENANCE MANUAL
AD-4 AIR DRYER
Effective With Unit 41089
The air dryer is mounted on the coach ahead of the front
axle on the spare tire compartment bulkhead. The bottom
portion of the dryer is protected by a sheet metal shroud.
Refer to figure 4-56.
The function of the AD-4 Air Dryer is to collect and
remove air system contaminants in solid, liquid and vapor
form before they enter the brake system. It provides clean,
dry air to the components of the brake system which
increases the life of the system and reduces maintenance
costs. Daily manual draining of the reservoirs is eliminated.
The AD-4 consists of a die cast aluminum end cover
secured to a cylindrical steel outer shell with eight cap
screws and nuts. The end cover contains a purge valve
mechanism. a safety valve and three threaded air connec-
tions.
. <lioiii.
. SHROUD
~ - -
Figure 4-56. A0-4 Air Dryer Installed.
The three air connections on the AD-4 are identified with
embossed numbers and lettering. The identification and
function of each is as follows:
4 - CON. - Control port (purge valve control)
1 -
SUP - Supply port (air in)
2 - DEL - Delivery port (air out)
A cast-tn heater element and replaceable thermostat with
an external electric terminal are provided.
The voltage and wattage requirements of the heater and
thermostat are shown with embossed numbers and letters 1n
the recess adjacent to the control port marked CON.
OPERATION
Operation of the air dryer can most easily be understood if
1t is separated into two c:ycles - the CHARGE cycle and the
PURGE cycle.
Figure 4-57. Charge Cycle Diagram.
Charge Cycle - Refer to figure 4-57. When the air com-
pressor is loaded (compressing air) compressed air, along
with oil, oil vapor, water and water vapor flows through the
compressor discharge line to the supply port in the air dryer
end cover. The flow velocity, or the speed at which the air
and its contaminants travel down the discharge line, is
reduced substantially as it enters the air dryer end cover
battle system behind the supply port. As air travels through
the baffle system. its direction of flow changes by 180°
several times. causing contaminants to condense and drop
to the bottom (sump) of the air dryer end cover.
After exiting the end cover baffle system, the air flows into
the desiccant cartridge. Once in the desiccant cartridge the
air first flows through an oil separatorwhich removes oil, oil
vapor and solid contaminants.
Air. still
100%
saturated with water vapor, exits the oil
separator and enters the desiccant drying bed. Air flowing
through the column of desiccant becomes progressively
dryer as water vapor adheres to the desiccant material in a
process known as "adsorption." The desiccant cartridge
using the adsorption process typically removes 95% of the
water vapor from the air.
The ma1ority of dry air exits the desiccant cartridge
through its integral smgle check valve to fill the hollow purge
• volume between the desiccant cartridge and outer shell.
Some air will also exit the desiccant cartridge through the
purge orifice adjacent to the check valve.
Dry air flows out of the purge volume through the single
check valve and out the delivery port to the first (supply)
reservoir of the air system.
The air dryer w,11 rem am
in
the charge cycle until air brake
system pressure builds to the governor cutout setting