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8.  Fuel Drain
A.  The fuel drain permits the unburned fuel from the combustion chamber and turbine plenum to drain from the APU. The fuel drain has a drain union on the combustion chamber cap, a drain line, a plenum union, and a drain tee. For APUs with the fuel drain housing, the fuel drain has a fuel drain housing to increase the capacity of the fuel drain. The fuel drain housing has a drain seal, spring and screen. A warm airflow from the fuel drain shows that the fuel drain is not clogged.
9.  Fuel Heater
A.  The fuel heater is attached to the inside surface of the upper shroud, and is located between the fuel inlet and low pressure fuel filter (Fig. 2). It is used to prevent fuel ice from blocking the fuel filter by regulating the temperature of the fuel entering the APU. A check valve is installed in the fuel line upstream of the fuel heater to retain fuel in the APU fuel pump. This ensures a positive prime for starting.
B.  Fuel sensing flow control valve is part of the APU fuel heating system which operates automatically by action of the fuel temperature sensing air regulator valve. When the temperature of the fuel passing through this valve drops to a predetermined value, the valve will modulate open allowing hot air from the APU bleed port to circulate through the finned shroud around the fuel line passing through the fuel heater. A fuel sensing flow control check valve is installed in the fuel heating system to prevent fuel from entering the bleed air duct.
552 
49-31-0 Page 4  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Oct 20/93 

G00142
552  APU Fuel System Schematic 
Feb 15/80  Figure 2  49-31-0 
Page 5 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

10. Operation
A.  All components of the APU fuel system are controlled automatically. Placing the APU master switch to the START position opens the APU fuel valve to deliver fuel to the fuel control unit. For fuel system component operation electrically, refer to 49-11-0. The fuel control unit is full of fuel at all times since it is not drained at APU shutdown. The fuel pump delivers high pressure fuel to the fuel solenoid valve whenever the turbine is rotating. The valve solenoid is energized when the sequencing oil pressure switch actuates. This occurs when the engine oil pressure rises to approximately 4.5 psig. When the fuel solenoid valve is opened, high pressure fuel is delivered to the fuel atomizer and injected into the combustion chamber (Fig. 2). Engine speed and exhaust gas temperature are automatically controlled within established limits by the fuel control unit.
B.  FUEL CONTROL UNIT WITHOUT THE TIMED ACCELERATION CONTROL; The fuel control unit and the APU control thermostat function during the APU start cycle to increase the fuel pressure in proportion to an increase in compressor pressure. As the compressor pressure increases with the increase in compressor speed, control air will act on the diaphragm in the acceleration limiter valve to gradually close the fuel bypass valve. This will increase the fuel pressure to the fuel atomizer.
C.  FUEL CONTROL UNIT WITH THE TIMED ACCELERATION CONTROL; The fuel control unit and the APU control thermostat function during the APU start cycle to increase the fuel pressure in proportion to an increase in compressor pressure. When the APU speed is less than 20%, control air will act on the diaphragm in the acceleration limiter valve to gradually close the fuel bypass valve. This will increase the fuel pressure to the fuel atomizer. 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.
D.  Turbine exhaust gas temperature (EGT) is controlled by a single thermostat having two functions as follows:
(1)  
The thermostat functions as an acceleration control thermostat when the APU is accelerating to, or is operating below, 95% service speed. When turbine exhaust temperature exceeds 620 to 650°C (1150 to 1200°F) during this mode of operation, the thermostat opens and bleeds some of the control air from the acceleration limiter valve (Fig. 2). This allows the fuel control unit bypass valve to bypass fuel and thereby reduces the turbine EGT. When the turbine EGT is reduced below the limits noted above, the thermostat closes and stops bleeding control air from the acceleration limiter valve. This results in the closing of the fuel bypass valve.

(2)  
When the APU is operating at or above 95% service speed, the 95% switch actuates and the thermostat selector solenoid is energized closed, to transfer the thermostat to the load control mode and causes it to function as an overload protection for the APU. Refer to 49-52-01 for additional information pertaining to thermostat operation in the load control mode.
　
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