A.The engine bleed air distribution system is a manifold of ducts and valves which carries hot compressed air from the engine compressor to the air conditioning packs and to the air-dependent units and subsystems in other systems. Air source, pressure, and temperature controlling components are part of the system. Ducts from the engines pass through control components and join the respective left or right wing manifolds. A crossover duct, forward and below the center wing section front spar joins the left and right wing manifold sections to form the complete cross-ship pneumatic manifold. The air control components connecting the pneumatic manifold to related systems are covered as part of the systems they serve.
B.The primary supply of pneumatic air is from the mid (5th) compressor stage of each engine, through a check valve. When 5th-stage bleed air pressure is not high enough, high (9th) stage bleed air is used. Switching from mid to high stage bleed on each engine is controlled automatically by the high stage valve and regulator. The pressure regulating and shutoff valve (PRSOV) controls the pressure of air supplied to the precooler within the range of 34 and 50 psig. The high stage air is controlled to within the range between 25 and 33 psig (25 and 35 psig on airplanes with improved regulators) by the high stage regulator. Air from either stage is routed through a precooler on each engine before entering the pneumatic manifold. The precooler control valve, controlled by a downstream temperature sensor, regulates manifold air temperature by controlling the amount of fan air flow through the precooler.
C.The pneumatic manifold is separated into left and right sections by an isolation valve. A center section contains the APU duct. Flow of APU air into the center section is controlled by the APU bleed air valve. A ground service pneumatic connector is in the right section. Ground pneumatic sources bypass the pressure regulating valve and must have pressure regulation equipment.
D.Switches on the pilot's forward overhead panel control solenoids in the pneumatically operated bleed air regulators and APU bleed air valve. The engine fire switches, when actuated, cause the regulators and valve to close.
E.Normally the APU check valve prevents engine bleed pressure from backing into the APU bleed system. Should the check valve fail open, with the APU bleed valve open, engine bleed pressure could be detrimental to the APU. The DUAL BLEED light on the overhead panel illuminates when the APU bleed valve is open and either Engine No. 1 PRSOV or Engine No. 2 PRSOV and isolation valve are open. The light would be on during an APU engine start. After engine start and the APU bleed valve switch is placed to OFF, the light goes out indicating that the APU bleed valve has closed.
F.The pneumatic system is used to start the engines on the airplane. Bleed air from a pneumatic source flows in the reverse direction through the pneumatic system to the engine start system.
G.The bleed air is diverted to the engine start system by the pressure regulating and shutoff valve (PRSOV). Each engine uses a bleed air regulator to pneumatically control the PRSOV. The bleed air regulator eliminates the control pressure to the PRSOV actuator which ensures the PRSOV will remain closed during starting.
A.The pneumatic distribution system connects air supply sources from the APU, engines, and ground air source to user systems through the pneumatic manifold system and their appropriate control valves. The pneumatic manifold system extends from the engine at one wing to the crossover duct in the air conditioning bay below the center wing section to the other engine at the opposite wing. An electrically actuated isolation valve in the crossover duct separates the left and right side system. The APU bleed air duct is connected to the crossover duct on the left side of the isolation valve and the pneumatic ground service connection is connected to the crossover duct on the right side of the isolation valve.
B.From the engine, bleed air source is ducted primarily from the 5th-stage port where it passes through the 5th-stage check valve before it is ducted together with the 9th-stage (high pressure) duct. Air for nose cowl thermal anti-ice (TAI) is tapped off downstream of the 5th-stage check valve. Bleed air from the 9th-stage ports are ducted through the 9th-stage manifold and high stage valve before it is joined together with the 5th-stage duct. The air is then ducted through the PRSOV to regulate the pressure and the precooler heat exchanger to regulate the temperature before it is discharged into the strut duct. The strut duct is connected to the engine starter duct and to the wing leading edge duct. Air for wing thermal anti-ice (TAI) is tapped off from the wing leading edge duct through the wing thermal anti-ice valve. The left and right wing leading edge ducts are routed inboard inside the wing leading edge fairings where the ducts are joined to the crossover duct in the air conditioning bay.
C.Bleed air source can also be supplied with use of the APU. The APU bleed air duct runs under the passenger floor from the APU along the left side of the aft cargo compartment, then inside the keel beam through the wheel well to the air conditioning bay where it joins the crossover duct on left side of the isolation valve. The APU bleed air (shutoff) valve, a pneumatically actuated solenoid controlled valve installed in the duct downstream of the APU, controls the airflow from the APU. A reverse flow APU check valve installed in the duct section downstream of the APU bleed air (shutoff) valve prevents damage to the APU compressor from bleed air backflows from the engine(s) or ground air source. A pressure relief valve is installed in the duct section between the APU bleed air valve and APU check valve to relief excess pressure in the pneumatic manifold.
D.A three-inch diameter tap with an integral check valve for pneumatic ground service connection is installed on the right side of the crossover duct to allow system pressurization from an external ground air source.
E.There are two duct pressure transmitters installed in the crossover duct to monitor duct pressure on either side of the isolation valve.
F.The pnematic manifold serves as the central reservoir for the supply of pressurized air for air conditioning, hydraulic system pressurization, water tank pressurization and turbofans through their applicable control valves from pressure taps in the pneumatic duct.
