500  Forward Outflow Valve Installation 
Feb 20/92  Figure 401 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  21-43-11 Page 401 

2.  Install Forward Outflow Valve
A.  Position valve in place and install clamps (Fig. 401).
NOTE: Ensure that arrow on valve body points forward.
B.  Make sure that bonding lug is free of dirt, oil, and finish, and install bonding jumper.
C.  Install electrical connector.
D.  Close FORWARD OUTFLOW VALVE circuit breaker on P6-4 circuit breaker panel.
E.  Test forward outflow valve. Refer to Pressurization Control System - Adjustment/Test, AC or DC Manual Mode Test.
F.  Install cargo compartment lining.
500 
21-43-11 Page 402  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Feb 01/74 

COOLING - DESCRIPTION AND OPERATION
1. General
A.  Cooling includes that portion of the air conditioning system which reduces the temperature of air received from the pneumatic system enough to meet any cooling requirement of the air conditioning system. The method of cooling this air consists of air-to-air type heat exchangers and an expansion turbine air cycle machine.
B.  Air from the pneumatic system is divided such that part of the air is passed through the cooling system with the remainder passing on to the mixing chamber. The cooled air is then mixed with the bypassed air to supply properly conditioned air to the distribution system. Two mix valves, one for each air cycle system, proportion the air.
C.  The cooling system is divided into two subsystems. Section 21-51-0, Air Cycle System, discusses the path of the air being cooled and the equipment used for cooling it. Section 21-52-0, Ram Air System, describes the system and its control which employs outside air as a cooling medium for the air cycle system. See figure 1 for location of cooling air and ram air equipment.
D.  The air cycle system supplies the cold air needed for mixture with the hot compressed engine bleed air to provide a selected temperature in the control and passenger cabins. Air from the Pneumatic System, Chapter 36, as regulated by the Engine Bleed Air Compression Control System, 21-11-0, is received into the air conditioning system. This air must be cooled a variable amount depending upon environmental conditions of the cabin. The temperature of air entering the cabins is regulated by the two mix valves. These valves send part of the air through the air cycle system to be cooled and bypass the remainder to be mixed with the cold air in the mixing chamber. See Temperature Control, 21-61-0.
E.  Since airflow through the air cycle system and hot air bypassing the system are supplied by the same duct, and airflow to the cabin is relatively constant, the mix valves not only regulate the quantity of cold air being supplied, but in regulating quantity, also influence temperature drop across the air cycle system. A water separator 35°F control system bypasses warm air around the turbine to mix with cold air from the turbine to prevent freezing in the water separator. As a result cold air at the water separator is never below freezing.
500 
Aug 15/67  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  21-50-0 Page 1 

Cooling Air and Ram Air Equipment Location  505 
21-50-0  Figure 1  May 01/74 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

F.  Another system which has an influence on the cooling ability of the air cycle system is the ram air system. Although this system is not actually a part of the air cycle system, it supplies a cooling medium for the heat exchangers and must necessarily be considered in a discussion of the air cycle system. The ram air system automatically reduces ram air flow when maximum cooling is not required.
G.  The mix valves may vary from full cold to hot depending on cabin requirements and the ram air system may be set at considerably less than maximum airflow. In describing how the air cycle system operates however, it is convenient to consider the mix valves in the full cold Position, hot valve closed-cold valve open, and the ram air doors full open.
H.  There are two air cycle systems on each airplane. The systems are virtually identical and are located on either side of the airplane centerline in a fairing beneath the center fuselage. The left system is considered to supply the control cabin and the right system the passenger cabin. The systems operate in parallel however and feed to a common manifold. During normal operation the control cabin utilizes only approximately twenty percent of the left system supply with the balance going to the passenger cabin. Either system operating alone is capable of maintaining acceptable temperatures when necessary.
　
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