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 4.  Pneumatic Actuator (Fig. 1)
__________________
 A.  The pneumatic actuator controls the shutoff valve. The pneumatic actuator has a cylinder, piston and rod assembly, spring, and cap. The pneumatic actuator is installed on the flange of the cooling air duct. The pneumatic actuator rod extends when the pneumatic pressure from the bleed air duct is more than the spring force. A filter in the pneumatic keeps all unwanted materials from the pneumatic actuator. The filter can be replaced or used again.
 5.  Cooling Fan (Fig. 1)
___________
 A.  The cooling fan supplies a positive, cool airflow through the oil cooler and the APU electrical generator. The cooling fan is an axial-flow fan that is installed in a T-shaped housing. The accessory gear train turns the cooling fan. One arm of the housing gives the support for the cooling fan bearings and the mounting flange for the gearbox. The other arm of the housing is part of the cooling air duct. The fan inlet screen is attached to the top of the housing for the cooling fan. The leg of the T-shaped housing is the cooling fan outlet. The leg supplies the mounting pad for the crossover duct.
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////////////////////////A / GTCP85-129 SERIES /737-300/400/500/ ENGINES /MAINTENANCE MANUAL ////////////////////////
SEE  C 
BLEED AIR DUCT 
SEE  B  BLEED AIR VALVE  SEE  A 
A 
SHUTOFF VALVE PNEUMATIC ACTUATOR  CAP  COOLING AIR INLET DUCT FOR THE  HORIZONTAL WEB UPPER SHROUD 
CROSSOVER DUCT FOR THE COOLING AIR

COOLING AIR FAN
FWD
OIL COOLER
B

APU Cooling Air System Component Location
Figure 1 (Sheet 1)

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
UPPER SHROUD
FWD 
C 
PNEUMATIC LINE TO THE 
PNEUMATIC ACTUATOR 
BLEED AIR 
DUCT 

PNEUMATIC LINE TO THE PNEUMATIC ACTUATOR
BLEED AIR
DUCT

REDUCER GASKET FILTER
GASKET
PNEUMATIC LINE FILTER
SEE D
CHECK VALVE (TO FUEL HEATER) (REF)
D
APU Cooling Air System Component Location
Figure 1 (Sheet 2)

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6.  Cooling Air Crossover Duct (Fig. 1)
__________________________
 A.  The airflow from the cooling fan moves through the crossover duct to the oil cooler and the APU electrical generator.
 7.  Operation
_________
 A.  When the APU operates, the cooling fan supplies the cooling air. The accessory gear train in the gearbox turns the cooling fan. The cooling fan supplies stable air while the airplane is on the ground. When the airplane is in flight, the cooling fan supplies ram air. The pneumatic actuator gets the pneumatic pressure from the bleed air duct to hold the shutoff valve open. The cooling air from the cooling fan is supplied to the APU electrical generator and the oil cooler. To get to the APU shroud, the cooling air moves through the APU electrical generator and oil cooler. In the APU shroud, the cooling air keeps the outer temperature of the APU low. From the APU shroud, the cooling air goes out through the APU exhaust system. When the APU stops, the pneumatic actuator closes the shutoff valve. When the shutoff valve closes, it will keep all flames from an APU fire in the APU shroud.
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