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UPPER SHROUD
EXHAUST PORT
APU CONTROL
THERMOSTAT

CONTROL LINE FOR THE BLEED AIR
BLEED AIR LINE TO THE APU CONTROL THERMOSTAT
PRESSURE LINE FOR THE COMPRESSOR DISCHARGE
3-WAY
SOLENOID
DRAIN VALVE
3
LINE PLENUM UNION
DRAIN TEEDRAINUPPER UNIONSHROUD DRAIN
B 1 UNION
EXHAUST
PORT

4
DRAIN REDUCER
DRAIN LINE FUEL DRAIN HOUSING
PLENUM UNION
3
AND DRAIN TEE
DRAIN
UNION

DRAIN
LINE B2
1
AIRPLANES WITHOUT THE ETC  3 APUs WITHOUT THE FUEL DRAIN HOUSING
2 AIRPLANES WITH THE ETC 4 APUs WITH THE FUEL DRAIN HOUSING
APU Fuel System Component Location
Figure 1 (Sheet 2)

A42861
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////////////////////////A / GTCP85-129 SERIES /737-300/400/500/ ENGINES/MAINTENANCE MANUAL ////////////////////////
C.  AIRPLANES WITHOUT THE TAFCU; The acceleration limiter valve controls the quantity of fuel for acceleration during the APU start. The acceleration limiter valve has a diaphragm, piston and guide assembly, bypass valve, spring, adjustment screw, nut, and bypass filter. Air supplied by the compressor controls the acceleration limiter valve. The air is supplied from an orificed tee. The orificed tee is found forward of the combustor unit on the lower right side of the diffuser housing.
 D.  AIRPLANES WITH THE TAFCU; The acceleration limiter valve controls the quantity of fuel for acceleration when the APU speed is less than 20%. The acceleration limiter valve has a diaphragm, piston and guide assembly, bypass valve, spring, adjustment screw, nut, and bypass filter. Air supplied by the compressor controls the acceleration limiter valve. The air is supplied from an orificed tee. The orificed tee is found forward of the combustor unit on the lower right side of the diffuser housing. When the APU speed is more than 20%, the timed-acceleration head on the fuel control unit supplies a specified quantity of fuel. The timed-acceleration head is made to supply a specified quantity of fuel for a specified APU speed during the acceleration.
 E.  The governor controls the fuel flow to the fuel atomizer during the APU usual operation. The governor supplies the fuel to keep a constant APU speed during different load conditions. The governor has a shaft and sleeve assembly, governor cage, flyweights, spring, adjustment screw, and nut. High-pressure fuel is on all sides of the flyweights. The flyweights move the sleeve to cover the shaft ports for control of the bypass of the fuel. This will give the correct quantity of fuel to the fuel atomizer.
 5.  APU Control Thermostat (Airplanes without the ETC)
______________________
 A.  The APU control thermostat is part of a single thermostat control system. The APU control thermostat is an acceleration control thermostat and a load control thermostat. The single thermostat control system has a thermostat, 3-way solenoid valve, thermocouple, and overtemperature control switch. The thermostat is installed in the turbine exhaust port. The thermostat has a housing, spring-loaded ball valve, thermostatic core, and pneumatic line connection. The thermostat is connected through the 3-way solenoid valve to the acceleration limiter valve. The 3-way solenoid valve is usually open. During the acceleration of the APU engine to 95% speed, the thermostat is an acceleration control thermostat. The thermostat discards a specified quantity of control air to prevent a turbine overspeed and a high EGT. When the APU speed is more than 95%, the 3-way solenoid valve closes. This changes the thermostat to the load control thermostat. When the EGT is 650°C (1200°F), the APU control thermostat opens to bleed air from the bleed air valve. This will permit the bleed air valve to move to the closed position and lower the EGT.
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