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 7.  Supply and Exhaust Fan
______________________
 A.  The equipment cooling system utilizes two identical fans to induce airflow, a supply fan (left side just below the passenger cabin floor) and an exhaust fan (right side just below the passenger cabin floor). The fan motors are powered by 115 volt ac, 400 Hz, three phase power.
 B.  Each phase winding is protected by an overheat switch that will open the fan relay control circuit cutting off power to the fan should an overheat occur.
 8.  Fan Check Valve
_______________
 A.  A check valve is installed in parallel with each fan to provide a path for airflow when the fan is not operating. The valve consists of a housing and two semi-circular check valve flappers hinged about a pin.
 B.  Airflow in the direction of the arrow on the airflow indicator plate forces the flappers open. Reverse airflow forces the flappers to close and seat against the housing seating surface.
 9.  Fan Screen
__________
 A.  Wire mesh screens are installed in the ducts just upstream of the supply and exhaust fans. The screens prevent foreign material from damaging the fan impellers. Access panels are located in the ducts for access to the upstream side of the screens.
 10. Aft Equipment Cooling Fans
__________________________
 A.  There are two identical aft equipment cooling fans installed in the area aft of the bulk cargo compartment. The fans exhaust hot air from the aft equipment racks and dump the hot air overboard through the pressurization outflow valves.
 B.  The fans are powered from 115 VAC 3-Phase power.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400 MAINTENANCE MANUAL
 11. Air Cleaner___________
 A.  The air cleaner is a centrifugal swirl device, using inertia to separate particulate matter from the equipment cooling air. As the air flowing through the air cleaner is swirled, foreign material is thrown to the outside of the flow and purged.
 12. Low Flow Detectors__________________
 A.  Two low flow detectors, a forward rack detector (located in the input duct to the E1/E2 rack) and a flight deck detector (located in the flight deck supply duct) consist of a probe-like element with a self-heating thermistor. Airflow through the duct cools the heated detector probe. Low airflow in the duct cannot sufficiently cool the heated probe and indicates inadequate cooling.
 B.  When sufficient airflow for cooling does not exist the low flow detectors will generate the EICAS caution message EQUIP COOLING. In addition, when the air/ground relay system is in the ground mode, the ground crew warning horn will sound. The forward rack detector generates the EICAS status message EQUIP LOW FLOW and the flight deck detector generates the EICAS status message FLT DK LOW FLOW.
 13. Skin Temperature Switch_______________________
 A.  The skin temperature switch mounts on the airplane skin below the forward cargo compartment floor area. A set of contacts within the switch open and close in response to temperature increases or decreases. The switch will close when lower lobe skin temperature falls to 40°F (4°C) and open when skin temperature rises to 50°F (10°C).
 14. Differential Pressure Switch____________________________
 A.  A differential pressure switch is used to verify that there is cooling airflow in the override mode by measuring the pressure difference between the E1/E2 rack plenum and the equipment bay.
 B.  The differential pressure switch is located on the E1/E2 rack plenum. The pressure switch has two pressure sense tubes. One tube is open to ambient. The other tube is tied into the plenum. In the override mode with cooling airflow present, a differential pressure between the two sense tubes exists. When a pressure difference above a preset value is sensed, a pressure plate moves to actuate the electrical switch element. When the pressure difference decreases below a preset value, the pressure plate moves back to its original position and the electrical switch element relaxes.
 C.  If a differential pressure does not exist when the system is in override mode, the EICAS caution message EQUIP COOLING and the EICAS status message COOLING OVRD will be displayed. With the air/ground system in the ground mode the ground crew warning horn will sound.
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