(h)  
The manual mode

 1)  If an electrical failure occurs, the valve can be manually operated. When the manual control rod in solenoid B is manually pushed in, solenoid B is open. The valve will operate in the normal flow mode. Solenoid B will stay open until the manual control rod is electrically or manually moved to the closed postion.
 2)  If a pneumatic failure occurs, the valve may be manually closed. When the cam in the mechanical manual override valve is moved to the closed position, it moves the actuator rod. The actuator rod moves the ball in the ball valve assembly. When the ball is moved, it closes the bleed port to solenoid B and opens the bleed port to ambient. Thus, the air in the pneumatic actuator bleeds to ambient and the butterfly plate closes.
 3)  The cam must be manually moved to the open position before the valve can be operated again. The position of the cam is shown by the visual position indicator.
 B.  Primary Heat Exchanger (Fig. 5)______________________
 (1)  
Description

 (a)  
The primary heat exchanger is a plate-fin, single-pass, cross-flow, air-to-air heat exchanger. The core, which is a plate-fin heat-transfer matrix, is brazed together as a unit. The housing assembly is welded to the core. The housing assembly has bolt holes to connect the primary heat exchanger to the secondary heat exchanger and the plenum/ diffuser assembly. All the parts of the heat exchanger are made of aluminum alloys.

 (2)  
Function

 (a)  
Hot air from the flow-control-and-shutoff valve enters the top of the primary heat exchanger. It flows first to the aft end of the heat exchanger. Then it flows forward, through some of S-turns, to the heat exchanger outlet. Cold air flows at a right angle to the hot air. The air temperatures change as the heat transmits through the thin plate fins.

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757
MAINTENANCE MANUAL

SEE C
FWD
SEE B
SEE A
PLENUM
ACCESS DOOR DIFFUSER
A
MATE WITH RAM AIR PLENUM
MATES WITH PRIMARY HEAT EXCHANGER
AIR FROM
COLD AIR FLOW CONTROL
INLET VALVE
MATES WITH SECONDARY HEAT EXCHANGER (ACCESS DOOR)
MOUNT 
AIR TO COMPRESSOR  AIR FROM COMPRESSOR  COLD AIR 
MATES WITH AIRCRAFT 
MOUNT  RAM INLET DUCT 
PRIMARY HEAT EXCHANGER 
TO AIR 
B  REHEATER 

SECONDARY HEAT EXCHANGER
C

Heat Exchangers and Plenum/Diffuser Assembly
Figure 5

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757 MAINTENANCE MANUAL
 (b)  An access door on each side of the heat exchanger supplies an access to clean it.
 C.  Secondary Heat Exchanger (Fig. 5)________________________
 (1)  The secondary heat exchanger is almost the same as the primary heat exchanger. One difference is that the secondary heat exchanger is wider. Also, it has a heavy fin on the cold air inlet side, to prevent damage from hail. Hot air for the secondary heat exchanger comes from the compressor outlet on the air cycle machine.
 D.  Plenum/Diffuser Assembly (Fig. 5)________________________
 (1)  
Description

 (a)  
The plenum/diffuser assembly has two main parts; the plenum and the diffuser. The diffuser, which is the inner part, is made of aluminum alloy and some insulation. The fan in the air cycle machine and a space around the fan are in the diffuser. The plenum, which is the outer part, is made of fiberglass. An access door in the plenum supplies access to clean the primary heat exchanger.

 (2)  
Function

 (a)  
The plenum/diffuser assembly connects the air cycle machine, the primary heat exchanger, and the ram air outlet duct. It lets air flow from the primary heat exchanger outlet to the ram air outlet door. The plenum makes the air from the primary-heat-exchanger outlet flow to the diffuser inlet. The air-cycle-machine fan blows air through the diffuser to the ram air exhaust duct. The space around the fan, when necessary, lets more air flow through the diffuser. The plenum and the space are made so that air cannot recirculate back into the plenum.
　
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