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737-300/400/500MAINTENANCE MANUAL
 B.  The pack valve consists of a 3-1/2 inch diameter butterfly valve, an upstream venturi, a pneumatic actuator, and pneumatic control elements. The assembly also includes solenoids that permit manual closing of the valve and selection of flow modes by activating an air conditioning pack switch on the overhead (P5-10) panel (Fig. 1).
 C.  When Solenoid C is electrically energized to the open position (or manually operated by pulling out on the manual control rod), the ball valve actuating rod is retracted and latched. Regulated air pressure forces the ball to unseat and air flows to the actuator and the high flow servo valve. As air pressure increases and overcomes the actuator spring force, the valve opens. The valve opening allows flow to the venturi sense port and downstream sense port. The resulting differential pressure transmitted to the servo valve opens or closes the servo valve to maintain the scheduled bleed airflow. If cabin pressure decreases, the servo valve bellows expands, changes the servo spring balance and restores the desired airflow rate.
 D.  The high flow schedule from the APU is possible when the airplane is on the ground or in flight. When the PACK switch is at HIGH and the APU BLEED switch is at the ON position on the P5-10 panel, the APU/high flow solenoid A is energized. When the solenoid A opens, pressure is applied to the APU/high flow servo piston which tends to close the servo and increase the pressure to the valve actuator. This opens the valve and increases the airflow.
 NOTE: The high flow schedule will occur in flight when a PACK TRIP OFF
____
 light comes on or a pack is manually turned off.

 E.  If the PACK switch is positioned to HIGH but the APU BLEED switch is positioned to OFF, solenoid A will not be energized and the high flow servo will be controlled by solenoid C. The airflow maintained will be lower than that with APU operation.
 F.  Normal flow operation results from energizing solenoid B (PACK AUTO switch). Energizing solenoid B inactivates the APU/high flow servo and activates the auto flow servo. Airflow operation is controlled in the same manner for both servos.
 G.  Placing the PACK switch in the OFF position energizes the closing coil of solenoid C, which cuts off air pressure to the actuator and servo valve.
 H.  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 bellcrank.
 3.  Primary Heat Exchanger
______________________
 A.  The primary heat exchanger is the first unit of the air cycle system through which engine bleed air passes to be cooled. The heat exchanger is a counterflow plate-fin type.
 B.  Hot air enters one plenum chamber from the pneumatic duct at the aft inboard side of the exchanger. It is cooled, then leaves by way of the air cycle machine duct connected to the aft outboard side of the heat exchanger. There is one primary heat exchanger for each air cycle system.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
AIR CYCLE MACHINE
MUFF SEE B
35`F CONTROL
SEE A TURBINE INLET
OVERHEAT SWITCH
WATER
SEPARATOR SECONDARY HEAT EXCHANGER
35`F TEMPERATURE
SENSOR

PRIMARY HEAT EXCHANGER
TURBOFAN VALVE
35`F CONTROL
VALVE

TURBOFAN
WATER SPRAY VALVE
INJECTOR AIR CYCLE MACHINEFWD MIX VALVE
AIR CYCLE MACHINE COMPRESSOR PACK VALVE TURBINE
A DUAL RAM DOOR
RAM DOOR BLEED FULL OPEN FULL OPEN
RECIRC FAN OFF
AUTO
40 60 OVHT 80
PSI 0 100 TEST
L PACK R PACK
ISOLATION
OFF VALVE OFF
CLOSE

AUTO AUTO HIGH HIGH
AUTO
WING WING
COMPRESSOR
PACKPACK OPENANTI ANTITRIP OFFTRIP OFFICE
ICE WING-BODY
DISCHARGE
WING-BODY
TRIP
OVERHEATOVERHEAT
OVERHEAT
BLEED BLEED
SWITCH
TRIP OFF TRIP OFF
COMPRESSOR
RESET
SIGHT GAGE  OFF OFF
ON ON
DRAIN PLUG
1 APU 2
TURBINE  BLEED
AUTOOFF SCHED STANDBY MANUAL
FWD
FAIL DESCENT
P5-10 AIR CONDITIONING MODULE
B
Air Cycle System Component Location
Figure 1

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