F89676
APU Pressure Relief Valve Installation  501 
36-11-61 Page 402  Figure 401 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Aug 01/74 

ENGINE BLEED OVERHEAT SWITCH - REMOVAL/INSTALLATION
1. General
A.  The engine bleed overheat 4900F thermal switch is located on a pneumatic duct downstream of the engine bleed air valve. Access to the switch is through a wing upper surface access door.
2.  Prepare for Removal of Switch
A.  Open OVERHEAT circuit breaker on load control center circuit breaker panel P6.
B.  Open access door 6301 to gain access to overheat switch.
3.  Remove Switch (Fig. 401)
A.  Disconnect electrical connector from overheat switch.
B.  Remove lockwire.
C.  Unscrew switch and remove with packing ring from mounting boss on duct.
4.  Install Switch (Fig. 401)
A.  Replace packing ring on overheat switch.
B.  Screw switch into mounting boss on duct, tighten and lockwire.
C.  Connect electrical connector to 4900F overheat switch.
D.  Close access door.
533 
Aug 01/95  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  36-11-71 Page 401 

F89686
Engine Bleed Overheat Switch Installation  501 
36-11-71  Figure 401  Aug 01/74 
Page 402 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

ENGINE BLEED OVERHEAT SWITCH - ADJUSTMENT/TEST
1. General
A.  The engine bleed overheat 4900F thermal switch is located on a pneumatic duct downstream of the engine bleed air valve. A temperature calibrated heat source is required to operationally test the bleed overheat switch.
2.  Equipment and Materials
A.  Controlled Heat Source - Temp Cal probe heater (attachment to Jet Cal engine analyzer) or equivalent
3.  Prepare to Test Overheat Switch
A.  Open OVERHEAT circuit breaker on load control center circuit breaker panel P6.
B.  Open access door 6301 to gain access to overheat switch.
C.  Disconnect electrical connector from overheat switch.
D.  Remove lockwire.
E.  Unscrew switch and remove with packing ring from mounting boss on duct.
F.  Connect electrical connector to 4900F overheat switch.
4.  Test Overheat Switch Operation
A.  Connect external electrical power.
B.  Close OVERHEAT circuit breaker on load control center circuit breaker panel P6.
C.  Push-to-test BLEED TRIP OFF light for the 4900F overheat switch.
D.  Apply controlled heat of approximately 5000F to thermal switch probe and observe that the BLEED TRIP OFF light on forward overhead panel, and MASTER CAUTION and AIR COND annunciator lights on lightshield panel come on.
CAUTION:  DO NOT HEAT PROBE ABOVE 5500F. TEMPERATURE ABOVE 5500F MAY RUIN THE SWITCH.
NOTE: Duct overheat lights should illuminate at a probe temperature of approximately 4900F.
E.  Remove the heat source.
F.  Allow sufficient time for the switch probe to cool, then push the TRIP RESET button.
(1)  Check that BLEED TRIP OFF light and all MASTER CAUTION annunciator lights go out.
G.  If no longer required, remove electrical power from airplane.
H.  Install packing ring on overheat switch.
I.  Screw switch into mounting boss on duct, tighten and lockwire.
J.  Close access door.
Aug 01/95 36-11-71 Page 501 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

BLEED AIR PRECOOLER SYSTEM - DESCRIPTION AND OPERATION
1.  General
A.  The purpose of the bleed air precooler system is to reduce the temperature of the high pressure bleed air which is supplied to the manifold system to approximately 370oF. Two precooler systems are provided, one for engine No. 1 and one for engine No. 2. Each precooler system consists of an air-to-air heat exchanger, and a five-inch-diameter precooler valve. (See figure 1.)
2.  Heat Exchanger
A.  The air-to-air heat exchanger located below each engine is of the plate-fin single pass crossflow type. The heat collected from the hot air ducted from the engine, is absorbed by the cool air ducted from the fan. The cooling air is then discharged overboard after passing through the precooler valve.
3.  Precooler Valve
A.  The precooler valve is attached to the discharge port of the heat exchanger. (See figure 2.) The purpose of the valve is to control the flow of cooling air through the heat exchanger to regulate the temperature of the bleed air. The unit modulates the area as a function of temperature. Temperature modulation is obtained through a fixed plate containing several apertures, and a moving plate of similar construction. The moving plate is positioned by a bi-metalic element so that the openings in the modulating plate coincide with those in the fixed plate at high temperature and the solid portions cover the holes at low temperature. The area changes linearly between open and closed as the temperature decreases.
　
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