Bendix Commercial Vehicle Systems TROUBLE SHOOTING COMPRESSOR - Manual (Page 2)

2
Introduction to the Air Brake Charging System
Powered by the vehicle engine, the air compressor
builds the air pressure for the air brake system. The air
compressor is typically cooled by the engine coolant
system and lubricated by the engine oil supply.
The compressor's unloader mechanism and
governor
(along with a synchro valve for the Bendix ® DuraFlo 596 ™
air compressor) control the brake system air pressure
between a preset maximum and minimum pressure
level by monitoring the pressure in the service (or
“supply”) reservoir. When the air pressure becomes
greater than that of the preset “cut-out”, the governor
controls the unloader mechanism of the compressor to
stop the compressor from building air and also causes
the air dryer to purge. As the service reservoir air
pressure drops to the “cut-in” setting of the governor,
the governor returns the compressor back to building
air and the air dryer to air drying mode.
As the atmospheric air is compressed, all the water
vapor originally in the air is carried along into the air
system, as well as a small amount of the lubricating
oil as vapor.
The
duty cycle is the ratio of time the compressor
spends building air to the total engine running time. Air
compressors are designed to build air (run “loaded”) up
to 25% of the time. Higher duty cycles cause conditions
that affect air brake charging system performance which
may require additional maintenance. Factors that add
to the duty cycle are: air suspension, additional air
accessories, use of an undersized compressor, frequent
stops, excessive leakage from fittings, connections,
lines, chambers or valves, etc.
The
discharge line allows the air, water-vapor and
oil-vapor mixture to cool between the compressor and
air dryer. The typical size of a vehicle's discharge
line, (see column 2 of Table A on page 3) assumes
a compressor with a normal (≤ 25%) duty cycle and
operating in a temperate climate. See Bendix and/
or other air dryer manufacturer guidelines as needed.
When the temperature of the compressed air that
enters the air dryer is within the normal range, the air
dryer can remove most of the charging system oil. If
the temperature of the compressed air is above the
normal range, oil — as oil-vapor — is able to pass
through the air dryer and into the air system. Larger
diameter discharge lines and/or longer discharge line
lengths can help reduce the temperature.
The discharge line must maintain a
constant slope
down from the compressor to the air dryer inlet fitting to
avoid low points where ice may form and block the flow.
If, instead, ice blockages occur at the air dryer inlet,
insulation may be added here, or if the inlet fitting is a
typical 90 degree fitting, it may be changed to a straight
or 45 degree fitting. For more information on how to
help prevent discharge line freeze-ups, see Bendix
Bulletins TCH-008-021 and TCH-008-022 (available
on www.bendix.com). Shorter discharge line lengths
or insulation may be required in cold climates.
The
air dryer contains a filter that collects oil droplets,
and a desiccant bed that removes almost all of the
remaining water vapor. The compressed air is then
passed to — and stored in — the air brake service
(supply) reservoir. The oil droplets and the water
collected are automatically purged when the governor
reaches its “cut-out” setting.
For vehicles with accessories that are sensitive to
small amounts of oil, we recommended installation of
a Bendix
®
PuraGuard
®
oil coalescing filter, designed to
further minimize the amount of oil present.
Air Dryer
Reservoir Drain
Service Reservoir
(Supply Reservoir)
Compressor
Governor
(Governor plus Synchro valve for the
Bendix ® DuraFlo 596 ™ Compressor)
Discharge
Line
Optional “Ping” Tank
The Air Brake Charging System
supplies the
compressed air for the braking system as well as other air
accessories for the vehicle. The system usually consists
of an air compressor, governor, discharge line, air dryer,
and service reservoir.