21-51-00
 ALL  ú ú 01 Page 3 ú Oct 25/84
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 4.  Air Cycle Machine
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 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 (Fig. 1, detail A). 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.
 5.  Secondary Heat Exchanger
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 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, 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.
 6.  Thermal Sensing Units
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 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.
 B.  The turbine inlet overheat switch is located in the transition at the secondary heat exchanger and senses the temperature of air passing from the heat exchanger to the turbine of the air cycle machine.
 C.  The compressor discharge overheat switch senses temperature of air being discharged from the compressor to the secondary heat exchanger. The switch is installed on the compressor scroll (Fig. 1, detail B).
 7.  Check Valves
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 A.  Two check valves are provided for each air cycle system. One valve is in the cold air supply duct from the water separator to the mixing chamber and another is in the hot air supply duct to the mixing chamber. The two check valves prevent loss of air from the distribution system should a failure occur in the air cycle system area or the system is shut down in flight.
 8.  Water Separator
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 A.  Cold air leaving the air cycle machine passes through a muff to the water separator (Fig. 3). Moisture in the air at this reduced temperature begins to condense. The condensate is so finely atomized however, that it will follow along in the air stream unless a suitable method is used to collect it. The water separator is used to collect and remove the excess moisture from the air before it enters the distribution system.
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 21-51-00
 ALL  ú ú 01 Page 4 ú Mar 15/90
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 B.  The water separator is a cylindrical chamber consisting of an inlet and outlet shell assembly which houses a polyester coalescer (bag), a conical-shaped metal coalescer support, a bypass valve assembly, and a valve support guide. A coupling joins the inlet and outlet shell assemblies and secures the coalescer support. The outlet shell assembly contains a collection chamber, a baffle, a water spray extractor boss, and an overboard water drain. A boss is provided for the installation of the 35°F sensor. A bag condition indicator is also included which consists of a spring loaded piston and disk enclosed in a housing and a color coded cap.
 C.  The coalescer bag and its support are conically shaped with the small diameter at the upstream end. The support fits inside the bag and has louvers shaped to impart a whirling motion to air as it passes through. Air enters the separator around the outside of the bag, passes through the bag, then through the louvers. As the damp air passes through the bag, the bag is wetted and larger droplets of water are formed. These droplets along with the air are caused to whirl by the louvers of the support. As the air and moisture pass through the separator the centrifugal force keeps the heavier moisture close to the inside of the support until it reaches the collection chamber. A cylindrical baffle approximately the diameter of the outlet duct extends inside the separator at the downstream end. The water and air whirling in a greater diameter than the baffle find it necessary to make a double reverse turn in order to leave the separator. The turning does not appreciably affect airflow but the water being much heavier cannot make the turn and remains in the collection chamber. An overboard drain mates to an outlet in the equipment bay doors (Fig. 3).
　
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