Air Conditioning Block Diagram  558 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

B.  The engines furnish 8th- or 13th-stage air, depending on engine power setting and air conditioning demand, for air conditioning during normal operation. Bleed air from the engines enters the pneumatic system, where the temperature is normally regulated to approximately 370°F and limited to 450°F. A thermoswitch in the pneumatic system will prevent excessively hot air from entering the air conditioning system by closing the bleed air shutoff valve. Airflow to each air conditioning system is initiated by a pack valve which includes features for regulating airflow. The compression subsystem explains how airflow is controlled.
C.  An air cleaner system is provided to purge the engine bleed air of impurities. The system consists of two similar subsystems which function automatically whenever engine bleed air is used for air conditioning, and the airplane is on the ground or the flaps are extended. Each subsystem consists of an inertial type air cleaner and purge line, an electrically controlled pneumatically operated purge valve and an electrical control system. The impurities collected by the cleaner are vented overboard through the ram air exhaust louvers. For a more detailed description of the air cleaner system, refer to Air Cleaner System - Description and Operation.
3.  Distribution
A.  Two separate and independent systems distribute air to the passenger and control cabins. The individual (gasper) air distribution system routes only the cold air from the air cycle system while the conditioned air distribution system routes a mixture of the cold and hot air to the cabins.
B.  The gasper system provides each crewmember and passenger a method for cooling his local area to a value different from that provided by normal air conditioning automatic control. Air is received from the air cycle system and ducted to each individual station. An adjustable nozzle at each station allows the individual a choice anywhere between no supplementary cold air and full system capacity cold air. With all gasper outlets open approximately 20% (maximum) of the total cabin inflow air passes through the gasper system.
C.  The conditioned air distribution system routes temperature controlled air to the passenger and control cabins. One duct system supplies the control cabin and another entirely separate duct system supplies the passenger cabin. Both systems originate at the main distribution manifold with ducts extending forward. Riser ducts near the forward end of the passenger cabin connect to an overhead duct extending fore and aft the full length of the passenger cabin. A nozzle at the bottom of the duct releases a balanced flow of air directly into the cabin.
4.  Pressurization Control System
A.  The pressurization control system includes pressurization control, pressurization relief valves, and pressurization indicating and warning.
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B.  Cabin pressure is controlled by positioning a cabin pressurization outflow valve to meter cabin air exhaust. The valve operates electrically from any of three different control systems. Two of these control systems, AUTO and STANDBY, are electronic and provide automatic pressure control, while the third, MANUAL, requires manual switch operation for adjustment. Normal control is from the AUTO system with the STANDBY system acting as a backup. The MANUAL system functions as an override over both automatic systems.
C.  Pressurization relief valves include two safety relief valves, which prevent overpressurizing the airplane, and a vacuum relief valve which prevents pressure inside the airplane becoming appreciably less than ambient. In addition to the relief valves, pressure equalization valves are installed in both cargo compartments to provide a quick method of allowing cargo compartment pressure to vary, within limits, with cabin pressure. Blowout panels are used in both cargo compartments to protect against a sudden differential in pressure between the cargo compartment interior and exterior.
D.  Indicators are provided to allow monitoring cabin altitude, differential pressure, and rate of pressure change. A cabin altitude warning system sounds a horn if cabin altitude exceeds approximately 10,000 feet.
5.  Cooling
A.  All cold air required for air conditioning is provided by an air cycle system. Passing bleed air through a primary heat exchanger, an air cycle machine, and a secondary heat exchanger cools the air sufficiently to handle any cooling situation required. A ram air system provides coolant air for the heat exchangers. A ram air modulation control system automatically regulates the supply of coolant air during flight to obtain required cooling with minimum aerodynamic drag from the system. A turbofan draws air through the system for ground operation. A water separator removes excess moisture from the cooled air. Various thermal switches, thermostats, sensors, and valves are included with the system to provide automatic protection and warning of an air cycle system malfunction.
　
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