504 Bleed Air System Schematic
May 15/80 Figure 2 49-52-02
Page 5
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
C. The three-way selector solenoid valve is located on lower right-hand side of the APU below the surge bleed valve. During start and acceleration mode, the single thermostat controls the fuel control limiter valve by bleeding off some of the compressor air through the normally open three-way selector solenoid valve. At 95%, the normally open three-way selector solenoid valve cuts off the compressed air from the fuel control limiter valve. The turbine discharge temperature is controlled by the APU control thermostat bleeding compressed air from the load control valve when the load control valve is open.
D. The overtemperature control switch in conjunction with the immersion thermocouple provides overtemperature protection for the APU in the load control mode. The thermocouple is mounted in the turbine exhaust port to sample stratified layers of the exhaust gas stream. It comprises four chromel-alumel thermocouples connected in parallel. The overtemperature control switch trip point is set at 704 to 718°C. In the event of an overtemperature (704 to 718°C), the sensing circuit in the overtemperature control switch will de-energize the thermostat selector solenoid and the bleed air valve solenoid. The bleed air valve closes, and the thermostat returns to the acceleration mode. Refer to 49-31-0 for additional information pertaining to thermostat operation in the acceleration mode. With the load removed, the overtemperature switch will reset itself when the EGT drops to a nominal 205°C below the switch trip point. This cycle will repeat until the bleed air switch is turned OFF.
4. Differential Air Pressure Regulator
A. The differential air pressure regulator provides relatively constant pressure control air to the bleed air valve, enabling the valve to operate independent from ambient conditions: the ambient pressure and temperature. The differential pressure regulator is mounted on the bleed air valve (Fig. 1). The regulator consists of a cover with an inlet port fitting and an air filter, a main housing containing a spring-loaded diaphragm assembly, metering valve, and adjustable relief valve. The regulating pressure of the valve is set to approximately 19 psig.
5. Surge Bleed Valve
A. The surge bleed valve is mounted on the turbine plenum directly below the bleed air duct attachment to the plenum (Fig. 1). It operates above 95% speed when the airplane is in flight and the APU bleed valve is closed. The valve prevents compressor surge at high altitudes.
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49-52-02 Page 6 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. May 15/80
6. Operation
A. When the APU is operating at or above 95% service speed, the bleed air circuit is armed. Positioning the bleed air switch, located on forward overhead panel, to ON will energize the bleed air valve solenoid and open the valve. For bleed air system component operation electrically, refer to 49-11-0. With the bleed air valve open, the APU bleed air is supplied to the airplane pneumatic system.
B. During extraction of APU bleed air, the APU control thermostat, operating in the load control mode, acts as an overload protection for the APU and does not function until an overload condition occurs. In the event of an overload, the EFT will rise. The thermostat will then function to modulate the bleed air valve closed to lower the EGT. When the EGT reaches the trip point of the overtemperature switch will de-energize the thermostat selector solenoid and the bleed air valve solenoid. This causes the bleed air valve to close (removes the load) and switches the thermostat to the acceleration mode. When the EGT drops a nominal 205°C below the switch trip point, the overtemperature switch will reset itself.
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