Bendix Commercial Vehicle Systems TU-FLO 750 COMPRESSOR 6/08 - Manual (Page 17)

17
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 fi ttings, 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 18) assumes a
compressor with a normal (less than 25%) duty cycle,
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 fi tting
to avoid low points where ice may form and block the
fl ow. If, instead, ice blockages occur at the air dryer
inlet, insulation may be added here, or if the inlet fi tting
is a typical 90 degree fi tting, it may be changed to a
straight or 45 degree fi tting. For more information on
how to help prevent discharge line freeze-ups, see
Bendix Bulletins TCH-08-21 and TCH-08-22 (see pages
35-37). Shorter discharge line lengths or insulation may
be required in cold climates.
The air dryer contains a fi lter that collects oil droplets,
and a desiccant bed that removes almost all of the
remaining water vapor. The compressed air is then
passed to 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 ® system fi lter, designed to 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
Optional Bendix ® PuraGuard ®
System Filter or PuraGuard ® QC ™
Oil Coalescing Filter
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.