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Compressors

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 compressor

lubricating oil) as vapor.

The Duty Cycle is the ratio of time the compressor

spends building air, relative 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 (such as higher compressor head

temperatures) that may adversely affect air brake

charging system performance. These conditions may

require additional maintenance due to a higher amount

of oil vapor droplets being passed along into the air

brake system. 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. See page 9 for compressor maintenance

and usage guidelines. Use the Bendix

BASIC test

(p/n 5013711) where the amount of oil present in the

air brake system is suspected to be above normal.

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 table on page 9) assumes a compressor with

a normal (less than 25%) duty cycle, operating in a

temperate climate. See Bendix and/or vehicle or air

dryer manufacturer guidelines as needed.

Air dryer inlet temperatures play a key role in air

system cleanliness and air dryer performance. 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 to prevent moisture

build-up and freezing. For more information on how

to help prevent discharge line freeze-ups, see Bendix

Bulletins TCH-08-21 and TCH-08-22. Conversely,

shorter discharge line lengths or insulation may be

required in extreme cold climates to maintain proper

dryer operation.

The air dryer contains a ﬁ lter that collects oil droplets,

and a desiccant bed that removes almost all of the

remaining water vapor before the compressed air is

passed to the air brake service (supply) reservoir. The

oil droplets and the water collected are automatically

purged at the dryer when the governor reaches its

“cut-out” setting.

For vehicles with accessories that are sensitive to

small amounts of oil, we recommend installation,

downstream of the air dryer, of a Bendix

PuraGuard

QC oil coalescing ﬁ lter to further minimize the amount

of oil present.

See the Bendix Advanced Compressor Trouble-

shooting Guide (BW1971) or the compressor’s Service

Data sheet, available online at www.bendix.com for

more information.

Governor

Air Dryer

Safety

Valve

Air

Compressor

Supply Reservoir

Safety

Valve

Discharge Line

Purge

Valve

Drain Valve

Optional Bendix

PuraGuard

QC Oil

Coalescing Filter

1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 69 70

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