PNEUMATIC一ENGINE BLEED AIR一FUNCTIONAL DESCRIPTION（BOEING）
<P>**** Hidden Message *****</P> 737-600/700/800/900一SDS HNA D633A101-HNARev 37一15 Oct 2008
Printed by Toolbox: 24 Nov, 2008, 20:02:2
DO NOT USE AFTER 29 Nov. 200
丁ASK36-11-00-204 PNEUMATIC一ENGINE BLEED AIR一FUNCTIONAL DESCRIPTION
EFFECTIVITY: HNA ALL
Issue Date: 15 Oct 200
Purpose
The engine bleed airdistribution system supplies temperature and pressure regulated airto the bleed air manifold.
The pressure regulator and shutoffvalve (PRSOV) controls the flow of engine bleed airto the pneumatic manifold.
The bleed air regulator (BAR) operates the PRSOV with control pressure. The source of pressure for the BAR is
the engine bleed interstage duct. The BAR gets electrical control from a 28v do signal. This signal comes from
the air conditioning accessory unit (ACAU). When the BAR solenoid valve opens electrically, it supplies control
pressure to open the PRSOV against spring force. When the BAR closes electrically, it bleeds the control pressure
from the PRSOV. Spring force closes the PRSOV.
The ACAU gets inputs from these controls and sensors:
The engine fire switch
Engine bleed switch
Engine start valve
The BAR 220 psi overpressure (OVPR) switch
The 490E (254C) overheat switch (engine strut duct).
Usually, the engine bleed switch controls the BAR. In the ON position, the switch supplies a signal through the
ACAU to open the solenoid valve in the BAR. In the OFF position, the switch supplies a signal to close the solenoid
valve in the BAR.
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.
There is protection for the pneumatic manifold from overpressure or overtemperature conditions. Operation of
either the 220 psi overpressure switch or the 490E (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.
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.
A precooler system cools engine bleed air. The precooler system is automatic. The precooler system keeps
engine bleed air temperatures between 390E (199C) and 440E (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 450E (232C) thermostat bleeds control pressure from the PRSOV if engine bleed air downstream of the
precooler is 450E (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一ENGINE BLEED AIR一FUNCTIONAL DESCRIPTION
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