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ENGINE BLEED AIR COMPRESSION CONTROL SYSTEM - DESCRIPTION AND OPERATION
1.  General
A.  The air source for the air condition system is the pneumatic manifold covered in Chapter 36, Pneumatic. The air compression control system meters air from the manifold to meet air conditioning airflow requirements. Whenever the output of eight stage pressure is insufficient to meet the air conditioning needs, the control system permits bleed air from the 13th stage to enter the manifold and supplement the airflow output of the 8th stage bleed.
B.  There are two identical and parallel engine bleed air compression control systems. Each bleed control system consists of a pack valve, a pressure regulator, and a modulating and shutoff valve, all interconnected with pressure sensing lines (Fig. 1). The modulating and shutoff valve is located on the 13th stage engine bleed duct, upstream of the precooler heat exchanger. The pack valves are located in the air conditioning pack bay. The engine bleed air compression control system is pneumatic in operation as shown by figure 2.
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G42314F46496
Engine Bleed Air Compression Control System Component Location  500 
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2.  Pack Valves
A.  The pack valves have the purpose of metering the air into the air condition system. The pack valves are pneumatically driven and electrically controlled, with a solenoid override for APU operation (Fig. 2). The pack valves are located in the air condition pack bay and the control switches are located on the forward overhead panel and marked L PACK and R PACK. The units are normally closed flow control and shutoff valves.
B.  Butterfly shutoff operation is accomplished by energizing solenoid A to seat the solenoid ball on the inlet pressure port, thus closing off the actuator air supply and venting the actuator. The actuator spring will then move the butterfly to the closed position.
C.  When solenoid A is energized to seat the ball on the vent port and open the actuator supply pressure port, the control orifice and actuator will be supplied with air pressure for airflow control operation (Fig. 3). The pressure is supplied from an inlet upstream pressure tap and is regulated to a desired value by the pilot regulator. Air then passes through the control orifice and builds up pressure in the actuator chamber, moving the butterfly toward the open position. When the airflow increases to the scheduled value, the venturi control servo bleeds off air through its ball valve at the correct rate to maintain the actuator pressure at the proper level for the desired flow. If the cabin pressure decreases, the bellows will expand and change the venturi control servo spring balance which produces the desired lower airflow rate.
D.  For APU operation, solenoid B is energized to allow inlet pressure to act on the servo reset piston. As the piston moves the venturi control balance is reset to control to a high airflow rate as required for APU operation.
E.  On Passenger/Cargo Convertible Airplanes the left pack valve has two modes of operation (normal and low flow). Electrical power to operate the pack valve comes from the air conditioning switch (L PACK) and goes to the SMOKE CLEARANCE switch which is a multiple contact switch. With the air conditioning switch ON and SMOKE CLEARANCE switch in NORMAL the pack valve drives open. (Refer to Passenger Cabin Conditioned Air Distribution System, 21-23-0, Fig. 4). When the SMOKE CLEARANCE switch is placed in the CARGO-P/C UNPRESS position, power is moved from the valve open contact to the valve closed contact and at the same time a circuit is set up to solenoid C (low flow mode). This causes pressure to be removed from the actuator chamber and pressurizes chamber A. As a result instead of the pack valve going full closed it remains slightly open (Fig. 2).
F.  A pack valve closed limit switch provides a ground for the pack valve closed relay to drive mix valve full cold after the pack valve closes. The switch is actuated by the valve butterfly bell crank. The latch type solenoid A is provided with a manual control, such that the solenoid valve and latch can be operated manually.
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