APU Operation
The APU starts and operates up to the airplane maximum certified altitude.
The APU supplies bleed air for both air conditioning packs on the ground or one pack in flight. Both transfer busses can be powered on the ground or in flight.
APU Fuel Supply
YA613 - YA722, YC153 - YK104 Fuel to start and operate the APU comes from the left side of the fuel manifold when the AC fuel pumps are operating. If the AC fuel pumps are not operating, fuel is suction fed from the No. 1 tank. During APU operation, fuel is automatically heated to prevent icing.
Copyright . The Boeing Company. See title page for details.
Engines, APU -APU System Description 737 Flight Crew Operations Manual

YA801 - YA809 Fuel to start and operate the APU comes from the left side of the fuel manifold when the AC fuel pumps are operating. A DC operated APU fuel boost pump is installed to ensure positive fuel pressure to the APU fuel control unit. During APU start and operation, the pump operates automatically when the APU fuel control unit senses low fuel pressure. The pump shuts off automatically when an AC fuel pump pressurizes the fuel manifold. If the AC and DC fuel pumps are not operating, fuel is suction fed from the No. 1 tank. During APU operation, fuel is automatically heated to prevent icing.
APU Engine and Cooling Air
APU engine air routes to the APU through an automatically operated air inlet door located on the right side of the fuselage. APU exhaust gases discharge overboard through an exhaust muffler.
Air for APU cooling enters through a cooling air inlet above the APU exhaust outlet. This air circulates through the APU compartment, passes through the oil cooler and vents through the exhaust outlet.
Electrical Requirements for APU Operation
APU operation requires the following:
.
APU fire switch on the overheat/fire panel must be IN

.
APU fire control handle on the APU ground control panel must be IN

.
battery switch must be ON.

YA801 - YA803 Electrical power to start the APU comes from No. 1 transfer bus or the airplane battery. With AC power available, the starter generator uses AC power to start the APU. With no AC power, the starter generator uses battery power to start the APU.
YA613 - YA722, YA809 - YK104 Electrical power to start the APU comes from No. 1 transfer bus or the airplane main battery. With AC power available, the starter generator uses AC power to start the APU. With no AC power, the starter generator uses battery power to start the APU.
Moving the battery switch to OFF on the ground or in the air automatically shuts down the APU because of power loss to the electronic control unit.
Copyright . The Boeing Company. See title page for details.
7.30.2  September 26, 2003

Engines, APU -APU System Description 737 Flight Crew Operations Manual
APU Start
The automatic start sequence begins by moving the APU switch momentarily to START. This initiates opening of the air inlet door. When the APU inlet door reaches the full open position the start sequence begins. After the APU reaches the proper speed, ignition and fuel are provided. When the APU is ready to accept a bleed air or electrical load the APU GEN OFF BUS light illuminates.
Note: When the APU is started using battery power only, there is no indication
on the electrical metering panel that the APU generator has come on line
and is ready to be selected. Both the frequency and voltage readings are
zero until the APU generator is placed on line.
Note: During the APU start cycle, the APU EGT indication may fluctuate from
0o to 1100o C prior to normal EGT rise and the LOW OIL PRESSURE light
may cycle on and off several times. These indications have no adverse
effect on starting the APU. It is not necessary to monitor EGT during start.
If the APU does not reach the proper speed with the proper acceleration rate within the time limit of the starter, the start cycle automatically terminates. The start cycle may take as long as 120 seconds. Automatic shutdown occurs in the event of EGT exceedance.
If the start fails or the APU GEN OFF BUS light fails to illuminate by the end of the start cycle, a system failure has occurred and the FAULT light illuminates.
Operate the APU for one full minute before using it as a bleed air source. This one minute stabilization is recommended to extend the service life of the APU.
APU Shutdown
Operate the APU for one full minute with no bleed air load prior to shutdown. This cooling period is recommended to extend the service life of the APU. When the APU switch is moved to OFF, this time delay is met automatically.
Moving the APU switch to OFF trips the APU generator, closes the APU bleed air valve and extinguishes the APU GEN OFF BUS light. Shutdown occurs automatically after 60 seconds. When the APU speed decreases sufficiently during shutdown, the fuel valve and inlet door close. If the fuel valve does not close, the FAULT light will illuminate after approximately 30 seconds. An immediate shutdown can be accomplished by pulling the APU fire switch.
　
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