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PNEUMATIC - ENGINE BLEED AIR - BAR, 450F THERMOSTAT, AND PRSOV - FUNCTIONAL DESCRIPTION
PNEUMATIC - ENGINE BLEED AIR - BAR, 450F THERMOSTAT, AND PRSOV - FUNCTIONAL DESCRIPTION
EFFECTIVITY
GUN ALL 36-11-00
Page 19
D633A101-GUN Jun 10/2003
BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
36-11-00-212
Purpose
The 490F (254C) overtemperature switch protects the pneumatic
manifold from overheat conditions.
Physical Description
The 490F (254C) overtemperature switch has these parts:
* Wrench flats
* Electric connector.
Location
The 490F (254C) overtemperature switch is on the engine bleed
air strut duct, downstream of the precooler.
Functional Description
The 490F (254C) overtemperature switch operates by thermal
expansion. As the temperature of the sensor increases, internal
parts expand. If the temperature is more than 490F (254C), the
expansion of the internal parts closes switch contacts. When the
switch cools, the contacts open.
The 490F (254C) overtemperature switch monitors the
temperature of the engine bleed air downstream of the
precooler. Normally the precooler system cools engine bleed
air to 390-440F (199-227C). At 450F (232C), the 450F (232C)
thermostat reduces engine bleed air flow to reduce the
precooler load. If the temperature downstream of the
precooler is more than 490F (254C), the 490F (254C)
overtemperature switch operates. The switch supplies a
ground to an overheat relay in the air conditioning accessory
unit (ACAU). The contacts of the energized relay cause these
operations:
* Bleed air regulator solenoid valve closes
* Air conditioning/bleed air controls panel BLEED TRIP OFF
light comes on
* P7 MASTER CAUTION and AIR COND annunciator lights
come on
* Latching circuit connects through the air conditioning panel
TRIP RESET push-button switch.
Training Information Point
Replace the old switch packing ring with a new ring when you
replace the switch.
Apply a thin layer of antiseize compound to the mounting
threads of the switch before you install it. The combination of
mounting thread forces and high duct temperatures can cause
uncoated valve mounting threads to seize in their mounting
boss.
PNEUMATIC - ENGINE BLEED AIR - 490F OVERTEMPERATURE SWITCH
EFFECTIVITY
GUN ALL 36-11-00
Page 20
D633A101-GUN Jun 10/2003
BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
36-11-00-238
PNEUMATIC - ENGINE BLEED AIR - 490F OVERTEMPERATURE SWITCH
PNEUMATIC - ENGINE BLEED AIR - 490F OVERTEMPERATURE SWITCH
EFFECTIVITY
GUN ALL 36-11-00
Page 21
D633A101-GUN Jun 10/2003
BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
36-11-00-238
THIS PAGE IS INTENTIONALLY LEFT BLANK
PNEUMATIC - ENGINE BLEED AIR - 490F OVERTEMPERATURE SWITCH
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Page 22
D633A101-GUN Jun 10/2003
BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
General
The engine bleed air control and indication circuits use 28v dc.
The engine fire switch has the override authority in the control
circuit. If the handle is up (fire condition), the bleed air control
signal closes the bleed air regulator solenoid valve. If the
engine fire switch is down (normal condition), it arms the engine
BLEED switch on the air conditioning/bleed air controls panel.
When the engine BLEED switch is OFF, the control signal closes
the bleed air regulator solenoid valve. When the engine BLEED
switch is ON, the control signal goes through relaxed relays
(normal conditions) in the air conditioning accessory unit
(ACAU). This opens the bleed air regulator solenoid valve.
The relays in the ACAU protect the bleed air system during
these conditions:
* Engine start operations
* Engine bleed air interstage duct overpressure
* Engine bleed air overtemperature.
The start relay energizes when the engine start valve opens.
The energized relay contacts send a signal to close the bleed
air regulator solenoid valve.
The overheat relay energizes when a bleed trip off condition
exists. These conditions cause a bleed trip off:
GUN 018
* 180 psi overpressure switch operation
GUN 001-017, 019-099, 101-118, 201-999
* 220 psi overpressure switch operation
GUN ALL
* 490F (254C) overtemperature switch operation.
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