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When the engine fire switch is in the normal (down) position, it
arms the engine bleed switch. When the engine fire switch is in
the FIRE (up) position, it sends a signal to close the BAR
solenoid valve. This causes the PRSOV to close. This happens
regardless of the bleed switch position.
There is protection so that the engine bleed air manifold will not
have reverse flow during engine starts. The engine start valve
sends a signal to the ACAU during engine start. This causes the
ACAU to send a close signal to the BAR solenoid valve,
regardless of the bleed switch position.
GUN 018
There is protection for the pneumatic manifold from
overpressure or overtemperature conditions. Operation of
either the 180 psi overpressure switch or the 490F (254C)
overheat switch will cause a bleed trip off condition. A bleed trip
off condition sends a signal from the ACAU to close the BAR
solenoid valve regardless of bleed switch position.
PNEUMATIC - ENGINE BLEED AIR - FUNCTIONAL DESCRIPTION
EFFECTIVITY
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GUN 001-017, 019-099, 101-118, 201-999
There is protection for the pneumatic manifold from
overpressure or overtemperature conditions. Operation of
either the 220 psi overpressure switch or the 490F (254C)
overheat switch will cause a bleed trip off condition. A bleed trip
off condition sends a signal from the ACAU to close the BAR
solenoid valve regardless of bleed switch position.
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5th and 9th Stage Engine Bleeds
Bleed air comes from the 5th and 9th stages of the engine high
stage compressor. At low engine speed, 5th stage air is not
sufficient for the pneumatic system demands and the 9th stage
supplies bleed air. At high engine speed the high stage valve
closes, and the 5th stage supplies bleed air. The high stage
regulator controls the high stage valve. High stage valve
operation is automatic.
Engine Bleed Air Cooling Functional Description
A precooler system cools engine bleed air. The precooler
system is automatic. The precooler system keeps engine bleed
air temperatures between 390F (199C) and 440F (229C). The
precooler is a cross flow heat exchanger. It uses engine fan air
to cool the engine bleed air.
The precooler control valve controls fan air flow to the
precooler. The precooler control valve modulates in response
to these components:
* The precooler control valve sensor
* The wing thermal anti-ice (WTAI) solenoid valve.
The 450F (232C) thermostat bleeds control pressure from the
PRSOV if engine bleed air downstream of the precooler is 450F
(232C) or higher. This causes the PRSOV to modulate toward
closed. The reduced airflow through the PRSOV has these
effects:
* Prevents bleed trip off conditions
* A drop in pneumatic manifold pressure.
PNEUMATIC - ENGINE BLEED AIR - FUNCTIONAL DESCRIPTION
EFFECTIVITY
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D633A101-GUN Oct 10/2006
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
36-11-00-204
PNEUMATIC - ENGINE BLEED AIR - FUNCTIONAL DESCRIPTION
PNEUMATIC - ENGINE BLEED AIR - FUNCTIONAL DESCRIPTION
EFFECTIVITY
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36-11-00-204
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
Purpose
The bleed air precooler system controls the temperature of the
engine bleed air before it goes into the pneumatic manifold.
General Description
Each engine has a precooler system. The two precooler
systems are identical.
A precooler control valve controls the amount of fan air that
flows through the precooler. These two devices control the
precooler control valve:
* The precooler control valve sensor
* Wing thermal anti-ice (WTAI) solenoid valve.
When the pressure regulating and shutoff valve (PRSOV) is
open, bleed air goes through the precooler and to the
pneumatic manifold. While the bleed air goes through the
precooler, fan air removes the heat and discharges as fan
exhaust air. The fan exhaust air flows over the engine core.
Operation of the precooler systems is automatic.
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