AIR CYCLE SYSTEM - DESCRIPTION AND OPERATION
1. General
A.  Air conditioning cooling is provided by two air cycle systems. The systems also remove excess moisture from the air.
B.  Air cycle system equipment is contained in equipment bays located on either side of the airplane centerline at the underside of the center fuselage. (Fig. 1)
C.  The cooling devices used in the air cycle system consist of a primary heat exchanger, a secondary heat exchanger, and an air cycle machine. The heat exchangers are of the air-to-air type with heat being transferred from the air going through the air cycle system to air going through the ram air system. The air cycle machine consists of a turbine and a compressor. Air passing through the secondary heat exchanger and expanding through the turbine drops in temperature as the energy is extracted. The expanding air releases energy to drive the compressor. The compressed air increases the temperature to the secondary heat exchanger, thus improving the heat transfer efficiency of the secondary heat exchanger.
D.  Protection of the air cycle machine is provided by two thermal switches. One thermal switch senses compressor discharge temperature. The other thermal switch senses turbine inlet temperature. Actuation of either thermal switch will cause the pack value to close.
E.  As the air cools its moisture content condenses. The moisture is atomized so finely however, that it will stay in suspension without a moisture removing device. The water separator collects this atomized moisture and removes it from the air cycle system air after it has left the air cycle machine.
F.  Moisture entering the water separator is kept from freezing by a 35°F control system. A 35°F sensor in the water separator and a 35°F control regulate a control valve in a duct between the primary heat exchanger exit and the water separator inlet. The valve opens to add warm air if the turbine discharge temperature approaches the freezing temperature of water.
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2.  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 unit is rectangular in shape and is located between two sections of the ram air duct. The heat exchangers are of the true 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 as it passes between the sandwiched plates and fins to the other plenum chamber, 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.
3.  Air Cycle Machine
A.  The air cycle machine is a cooling unit consisting of an expansion turbine on a common shaft with a compressor. The shaft is bearing mounted in a housing to support the rotating turbine and compressor. A wick extends from the shaft to the bottom of the oil sump formed by the housing for lubrication of the moving parts. A filler plug and sight gage are provided on each side of the housing, with a magnetic oil drain plug on the bottom.
B.  The air cycle machine is located in the equipment bay between the duct leading from the primary heat exchanger and the duct to the water separator. (See figure 1.) A duct from the compressor and another to the turbine connect to the secondary heat exchanger. The turbine mounts are connected to airplane structure through serrated plates and washers to provide location adjustment. There is one air cycle machine for each air cycle system.
4.  Secondary Heat Exchanger
A.  The secondary heat exchanger is identical to the primary heat exchanger. It is located in the equipment bay forward of the primary heat exchanger. Air from the air cycle machine compressor outlet enters the forward inboard connection of the secondary heat exchanger, passes between the sandwiched plates and fins, then returns to the air cycle machine turbine inlet from the forward outboard exchanger connection. There is one secondary heat exchanger for each air cycle system.
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501  Air Cycle System Component Locaton 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

5.  Thermal Sensing Units
A.  Two thermal switches are in the cooling system, and directly affect the air cycle system operation. One thermal switch senses compressor discharge temperature. The other thermal switch senses turbine inlet temperature. When an overheat condition exists in either position the affected thermal switch will cause the pack valve to close. Other thermal sensing switches located in the air cycle system are covered under ram air system and temperature control system.
　
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