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 C.  A temperature switch and temperature sensor are installed together in a box on the compartment LH sidewall. The temperature switch controls operation of the electric heaters. The forward cargo temperature sensor provides for compartment temperature indication on EICAS. The temperature sensor is treated as an additional zone by the zone temperature controller for BIT and indication (Ref 21-61-00).
Heaters
_______
 A.  There is an air heater installed in each of the two branch ducts which split from the equipment cooling exhaust duct. Thermal switches within each heater will open the heater control relay should an overheat condition occur.
 B.  Line-to-line power (200-volts) is used to power the air heaters. The left heater is powered from the 115 vac bus 1. The right heater is powered from the 115 vac bus 3. The control circuit is powered by 28 volt dc from the 28 vdc bus 2.
Temperature Control Switch
__________________________
 A.  The temperature switch is installed next to the forward cargo temperature sensor on the compartment LH sidewall. The temperature sensor blower draws compartment air across the temperature switch. A set of contacts within the switch open and close in response to temperature increases or decreases. The switch closes when temperature falls below 40°F (4°C). The switch reopens when temperature increases to 50°F (10°C).
Operation
_________
 A.  Functional Description (Fig. 2)
 (1)  On the ground, forward cargo compartment heating is provided only by the equipment cooling system (Ref 21-58-00). During warm day ground operations equipment cooling exhaust exits through the ground exhaust valve. When outside ambient temperature drops to 40°F (4°C) the equipment cooling skin temperature switch closes sending power to close the ground exhaust valve and open the inboard exhaust valve. With the inboard exhaust valve open air warmed by electrical equpment will flow to the heat ducts and be dispersed to the compartment.
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A
747-400
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BSEE HEATERS COMPARTMENT FORWARD CARGO  HEAT COMPARTMENT FORWARD CARGO 
SEE  A  SEE  C  COOLING EXHAUST EQUIPMENT  BYPASS DUCT COOLING EQUIPMENT EXHAUST GROUND COOLING EQUIPMENT  A VALVE (REF) EXHAUST INBOARD 
HEATER 
BOX SENSOR 
SENSOR TEMPERATURE 
B 

COVERPLATE TEMPERATURE
SWITCH
TEMPERATURE SENSOR ASSEMBLY
C
Forward Cargo Compartment Heating System Equipment LocationFigure 1
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28V DC BUS 2

 

 

HEATER CURRENT SENSOR (P414)
Forward Cargo Compartment Heating System Schematic Figure 2
C76633
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 (2)  
For the heaters to operate the forward cargo air conditioning system must be off , the equipment cooling inboard exhaust valve must be open, and at least two engines must be at greater than 50% power. When compartment temperature falls to 40°F (4°C) the temperature switch closes and the heaters energize. The system supplies heat until compartment temperature increases to 50°F (10°C). At this time, the temperature switch opens, removing power to the heaters.

 (3)  
If the heaters are on when the equipment cooling inboard exhaust valve closes, the inboard exhaust valve control relay will de-energize and cause the heater control relay to de-energize. This removes power to the heaters. The inboard exhaust valve will close in the equipment cooling automatic bypass (fan failure) mode or the override mode (Ref 21-58-00).
　
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