.
Hydraulic reservoirs pressurization

.
Water tank pressurization

.
Aspirated TAT probe (as installed)

Switches on the air conditioning panel operate the APU and engine bleed air supply system.
Engine Bleed System Supply
Engine bleed air is obtained from the 5th and 9th stages of the compressor section. When 5th stage low pressure bleed air is insufficient for the bleed air system requirements, the high stage valve modulates open to maintain adequate bleed air pressure. During takeoff, climb, and most cruise conditions, low pressure bleed air from the 5th stage is adequate and the high stage valve remains closed.
Engine Bleed Air Valves
The engine bleed air valve acts as a pressure regulator and shutoff valve. With the engine bleed air switch ON, the valve is DC activated and pressure operated. The valve maintains proper system operating pressure and reduces bleed air outflow in response to high bleed air temperature.
Bleed Trip Sensors
Bleed trip sensors illuminate the respective BLEED TRIP OFF light when engine bleed air temperature or pressure exceeds a predetermined limit. The respective engine bleed air valve closes automatically.
Duct Pressure Transmitters
Duct pressure transmitters provide bleed air pressure indications to the respective (L and R) pointers on the bleed air duct pressure indicator. The indicator is AC operated.
Air Systems -Bleed Air System Description 737 Operations Manual

Isolation Valve
The isolation valve isolates the left and right sides of the bleed air duct during

normal operations. The isolation valve is AC operated.
With the isolation valve switch in AUTO, both engine bleed air switches ON, and
both air conditioning pack switches AUTO or HIGH, the isolation valve is closed.
The isolation valve opens if either engine bleed air switch or air conditioning pack
switch is positioned OFF. Isolation valve position is not affected by the APU bleed
air switch.

External Air Connection
An external air cart/source provides an alternate air source for engine start or air conditioning.
APU Bleed Air Valve
The APU bleed air valve permits APU bleed air to flow to the bleed air duct. The valve closes automatically when the APU is shut down. The APU bleed air valve is DC controlled and pressure operated.
With both the APU and engine bleed air valves open, and the engines operating at idle thrust, there is a possibility of APU bleed air backpressuring the 9th stage modulating and shutoff valve. This would cause the 9th stage valve to close.
DUAL BLEED Light
The DUAL BLEED light illuminates whenever the APU bleed air valve is open and the position of the engine bleed air switches and isolation valve would permit possible backpressure of the APU. Therefore, thrust must be limited to idle with the DUAL BLEED light illuminated.
Copyright . The Boeing Company. See title page for details.
2.20.2  December 1, 2000

Air Systems -Bleed Air System Description 737 Operations Manual
Bleed Air System Schematic

As installed
Copyright . The Boeing Company. See title page for details.
December 1, 2000  2.20.3
Air Systems -Bleed Air System Description
737 Operations Manual
Wing-Body Overheat
A wing-body overheat condition is caused by a bleed air duct leak. It is sensed by the overheat sensors located as shown.
Wing-Body Overheat Ducts and Lights
5
7 4
86 3 2
1
RECIRC FAN
OFF
AUTO 40 60 OVHT
80
PSI
0 100

TEST
L PACK ISOLATION R PACK
OFF VALVE OFF
AUTO CLOSE AUTO

HIGH
AUTO HIGH
WING
WING ANTI
OPEN
PACK PACK ANTITRIP OFF TRIP OFF ICE
ICE WING-BODY
WING-BODY WING-BODY
WING-BODY OVERHEAT
TRIP
OVERHEAT
BLEED
OVER HEAT OVER HEAT
BLEED
TRIP OFF RESET TRIP OFF
OFF OFF
ON ON 12
APU BLEED
LEFT LIGHT RIGHT LIGHT
FORWARD OVERHEAD PANEL
Sensors located: Sensors located:
Left engine strut.
6 Right engine strut.
1 Left inboard wing
7 Right inboard wing  leading edge.
2 leading edge.
Left-hand air
8 Right-hand air conditioning bay.
3 conditioning bay.
Keel beam.
4 Bleed duct from APU.
5
Copyright . The Boeing Company. See title page for details.
2.20.4  December 1, 2000

737 Operations Manual

Introduction
Conditioned air for the cabin comes from either the airplane air conditioning system or a preconditioned ground source. Air from the preconditioned ground source enters the air conditioning system through the mix manifold.
The air conditioning system provides temperature controlled air by processing bleed air from the engines, APU, or a ground air source in air conditioning packs. Conditioned air from the left pack, upstream of the mix manifold, flows directly to the flight deck. Excess air from the left pack, air from the right pack, and air from the recirculation system is combined in the mix manifold. The mixed air is then distributed through the left and right sidewall risers to the passenger cabin.
　
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本文鏈接地址：737-300機組操作手冊Flight Crew Operations Manual FCOM 2(11)