(2)
Operation
(a)
The axial flow fan operates at a high shaft speed when supplied with aircraft electric power. The fan impeller drives air through the fan assembly. If the temperature of a winding reaches 400°F (204°C), the thermal switch on that winding de-energizes the fan power relay, removing power from the fan. If the cargo compartment ambient temperature is below 50°F (10°C), the EICAS status and maintenance message FWD CARGO FAN will be displayed. When the fan temperature decreases to 340°F (171°C), the thermal switch resets, allowing the fan to restart if the cargo compartment ambient temperature is below 50°F (10°C).
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A
757 MAINTENANCE MANUAL
SEE A
SEE B
HEAT FLOW
FAN
GRILL
(REF)
FORWARD CARGO
HEATING INTAKE
(REF) DUCT FWD INBD
FORWARD CARGO HEATING FLOW FAN
A
WIRE FILTER TEMPERATURE THERMAL SWITCH
INLET GRILL AND
FORWARD CARGO TEMPERATURE
THERMAL SWITCH
B
Forward Cargo Compartment Heating System
Figure 1
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A
757
MAINTENANCE MANUAL
B. Forward Cargo Temperature Thermal Switch
________________________________________
(1)
The Cargo Compartment Temperature Thermal switch provides temperature sensing and heating fan control. The switch closes whenever cargo compartment temperature falls below 50°F (10°C) providing power to the heating flow fan. The switch opens when cargo compartment temperature reaches 70°F (21°C).
(2)
The thermal switch is located in the mix bay compartment, mounted on the cargo compartment left side aft endliner panel.
C.
Forward Cargo Fan Current Sensor or Current Sense Relay
_______________________________________________________
(1)
AIRPLANES WITH THE FWD CARGO FAN CURRENT SENSOR; A fan current sensor is located on the left miscellaneous electrical panel, P34. The current sensor is part of the forward cargo heating indication system. The current sensor detects if the fan has electrical power. If the fan has a failure, the current sensor will provide a ground to the EICAS computers and the FWD CARGO FAN message will show.
(2)
AIRPLANES WITH THE FWD CARGO FAN SENSE RELAY; A current sense relay is located on the left miscellaneous electrical panel, P34. The relay is part of the forward cargo heating indication system. The relay detects if the fan has electrical power. If the fan has a failure, the relay will provide a ground to the EICAS computers and the FWD CARGO FAN message will show.
D.
Forward Cargo Heating Inlet Grill
_________________________________
(1) The grill in the aft of the forward cargo compartment covers the inlet to the forward cargo heating system. The grill includes a wire filter to keep dirt and debris out of the heat fan.
3. Operation (Fig. 2)
_________
A. Functional Description
______________________
(1)
The forward cargo compartment heating system draws air from the cargo compartment, warms the air with a heating flow fan then returns the warm air into the cargo compartment.
(2)
If the cargo compartment temperature falls below 50°F (10°C), the forward cargo compartment temperature thermal switch closes. This energizes the forward cargo fan relay providing power to the forward cargo heating flow fan. The fan draws cool air from the cargo compartment. This air then passes through the heating flow fan which warms the air and exhausts it back into the cargo compartment.
(3)
When the cargo compartment temperature reaches 70°F (21°C), the forward cargo compartment temperature thermal switch opens, interrupting power to the heating flow fan.
(4)
If the forward cargo compartment heating flow fan does not operate when the temperature thermal switch closes, the forward cargo fan current sensor (or current sense relay) provides a ground through the thermal temperature switch for EICAS. This displays the EICAS status and maintenance message FWD CARGO FAN.
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