F89260 F89265
Bleed Air Precooler System Component Location  575 
36-12-0  Figure 1  Nov 15/78 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

500  Precooler Valve 
Feb 15/70  Figure 2 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  36-12-0 Page 3 

4.  Operation
A.  Temperature control of the bleed air is accomplished with an air-to-air heat exchanger, which uses engine fan bleed air as a cooling medium. A precooler valve limits the temperature of the air in the downstream ducts to approximately 4500F. A large part of the 8th-stage bleed air bypasses the heat exchanger in order to keep the bleed air pressure drop to a minimum. The 8th-stage air cannot completely bypass the heat exchanger because there may be times when 8th-stage bleed air temperature exceeds 4500F and can reach a maximum of 5200F (Fig. 3).
B.  Regulation of the cooling airflow through the precooler valve is provided by a self-contained thermostatic valve in the air discharge duct. This valve is actuated by a bimetallic strip which positions the moving element in response to the sensed temperature. The precooler valve is fully closed at a temperature of 3650F or less and fully open at a temperature of 4400F or more. Under some conditions of high airflow demand when both the air conditioning and wing TAI systems are using 13th-stage bleed air the capacity of the precooler valve is exceeded and duct air temperatures could reach as much as 5500F. The overheat trip system is set at 4900F. A 4500F limit control is provided to avoid tripouts. This thermostat is located in the bleed air duct downstream of the precooler valve. Whenever the temperature sensed by the thermostat exceeds 4500F it starts to bleed off the control pressure in the sense line to the 13th-stage modulating and shutoff valve and the 13th-stage pressure regulator. The reduction in control pressure causes the 13th-stage modulating and shutoff valve to modulate towards the closed position thereby reducing the total bleed airflow through the precooler. For detailed description and location of 4500F thermostat refer to Engine Bleed Air Compression Control System, 21-00-0.
C.  Duct overheat protection is provided by a thermal switch located downstream of the precooler. If the sensed temperature at the switch exceeds 490 +100F for more than 15 seconds the switch closes, causing the bleed valve to close and energizing the appropriate trip-off light on the flight crew's panel. The bleed air shutoff valve is also controllable from the flight deck and may be reset in flight after a trip-off, once the thermal switch has cooled and reopened.
500 
36-12-0 Page 4  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Jun 20/93 

505  Bleed Air Precooler System Schematic 
Aug 15/72  Figure 3  36-12-0 
Page 5 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

BLEED AIR PRECOOLER SYSTEM – TROUBLE SHOOTING
1. Bleed Air Precooler System Trouble Shooting Chart
TROUBLE  PROBABLE CAUSE  ISOLATION PROCEDURE  REMEDY 
Bleed air temperature above 4500F  Bleed air precooler valve failure  Bleed air heat exchanger airflow blocked  With engine idle and PACK switch OFF, visually check that precooler valve is closed. With engine speed increased to 1.5 EPR and PACK switch ON, visually check that precooler valve is full open. With precooler valve full open, check that bleed air temperature below 4500F.  Replace bleed air precooler valve (Ref 36-12-31)Replace bleed air heat exchanger (Ref 36-12-21) 

501 
Nov 15/78  36-12-0 
Page 101 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

BLEED AIR PRECOOLER SYSTEM - ADJUSTMENT/TEST
1.  Bleed Air Precooler System Test
A.  General
(1)  
The bleed air precooler system reduces temperature of high pressure bleed air supplied to the main pneumatic manifold. During ground operation, the bleed air precooler valve is not likely to open without operating the respective engine to near takeoff thrust. Since the precooler valve has proven to be extremely reliable in service and to minimize fuel consumption during testing it is recommended that valve be removed from airplane and be bench checked if a malfunction is suspected.

(2)  
A visual check that the valve does close while performing the Bleed Air Compression Control System - Adjustment/Test (21-11-0) should verify that precooler system is functioning normally.
　
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