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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  February 15, 2001

Air Systems -Bleed Air System Description
Bleed Air System Schematic
YA613 - YA687, YA801 - YA809

Copyright . The Boeing Company. See title page for details.
Air Systems -Bleed Air System Description 737 Flight Crew Operations Manual

YA721, YA722

Copyright . The Boeing Company. See title page for details.
2.20.4  July 28, 2000

Air Systems -Bleed Air System Description 737 Flight Crew Operations Manual
YC153 -YK104

Copyright . The Boeing Company. See title page for details.
September 26, 2003  2.20.5
Air Systems -Bleed Air System Description

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
YA613 - YA809

Copyright . The Boeing Company. See title page for details.
2.20.6  June 1, 2003

Air Systems -Bleed Air System Description 737 Flight Crew Operations Manual
YC153 -YK104

Copyright . The Boeing Company. See title page for details.
September 26, 2003  2.20.7
Air Systems -Bleed Air System Description 737 Flight Crew Operations Manual

Intentionally
Blank

Copyright . The Boeing Company. See title page for details.
2.20.8  June 1, 2003

737 Flight Crew Operations Manual

This Section Applies to YA613 - YA809
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.
Air Conditioning Pack
The flow of bleed air from the main bleed air duct through each air conditioning pack is controlled by the respective pack valve. Normally the left pack uses bleed air from engine No. 1 and the right pack uses bleed air from engine No. 2. A single pack is capable of maintaining pressurization and acceptable temperatures throughout the airplane up to the maximum certified ceiling.
The APU is capable of supplying bleed air for two packs on the ground, or one pack in flight. Most external air carts are capable of supplying adequate bleed air for two pack operation. Do not operate more than one pack from one engine at any time.
Airflow Control
With both air conditioning pack switches in AUTO and both packs operating, the packs provide “normal air flow”. However, with one pack not operating, the other pack automatically switches to “high air flow” in order to maintain the necessary ventilation rate. This automatic switching is inhibited when the airplane is on the ground, or inflight with the flaps extended, to insure adequate engine power for single engine operation. Automatic switching to “high air flow” occurs if both engine bleed air switches are OFF and the APU bleed air switch is ON, regardless of flap position, air/ground status or number of packs operating.
　
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