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757 MAINTENANCE MANUAL
 AFT CARGO COMPARTMENT HEATING SYSTEM - DESCRIPTION AND OPERATION
________________________________________________________________
 1.  General (Fig. 1)_______
 A.  Aft cargo heating is provided by recirculating the aft cargo compartment air by a fan and passing the air through an electrical heater before returning to the aft cargo distribution system. Operation of the fan and heater is automatic with both selected ON/OFF in response to a signal from a remotely located temperature switch. Heater element and duct overheat protection are provided and are integral with the heater.
 2.  Component Details_________________
 A.  Aft Cargo Heater
________________
 (1)  
The heater is an electrical resistance type, which heats air forced past heat exchange surfaces. The heater operates on 200 Vac nominal supply voltage.

 (2)  
The heater is installed in line with a 6 inch O/D duct in a pressurized area. The unit is capable of heating aft cargo compartment air under any ground or flight condition. The outside surface of the heater will not exceed 185°F at normal operating conditions.

 (3)  
An overtemperature switch senses the temperature of the heat exchanger surface. The switch opens at a temperature that limits the heat exchanger surface temperature to 400°F maximum steady state and 450°F transient overshoots. The switch will shut off the fan when these temperatures are reached.

 (4)  
An overtemperature switch, downstream of the heater, senses duct air temperature. The switch opens when the air temperature is equal to or in excess of 185°F.

 B.  
Aft Cargo Heating Flow Fan

__________________________
 (1)  
Physical Description

 (a)  
The axial flow fan is a single stage vaneaxial fan that integrates the motor into the fan. The fan's simple design provides ease of construction and optimizes unit life and reliability.

 (b)  
The axial flow fan consists of a single cast aluminum alloy housing containing an impeller and an end bell (both of aluminum alloy, two sealed ball bearings, and the stator assembly and rotor.

 (c)  
The fan housing contains an inner housing which encases the stator assembly and rotor. Vanes are integrally cast between the outer and inner housings. The vanes optimize low noise levels, provide structural support, and transfer heat from the motor to the airflow and outer housing for motor cooling.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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757
MAINTENANCE MANUAL

SEE A
SEE B
SEE C
HEATER
FLOW FAN
AFT CARGO HEATER
A
WIRE FILTER
FWD
AFT CARGO FLOW FAN
TEMPERATURE CONTROL
B
SWITCH INLET GRILL AND AFT CARGO TEMPERATURE THERMAL SWITCH
C
Aft Cargo Heating Components
Figure 1

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757 MAINTENANCE MANUAL
 (d)  
The drive motor is a three-phase, squirrel cage type induction machine. Electromagnetic circuitry is used to reduce noise generated by electrical field saturation. The stator assembly consists of epoxy and mechanically bonded silicon steel laminations on which a three-phase, four-pole winding is distributed into 36 slots. The special bonding of the laminations prevents stator teeth vibration at line frequency or any harmonic. All 12 coils in each phase are continuously wound with no intercoil connections. Magnet wires terminate in silver-brazed connections to the leadwires.

 (e)  
Three miniature indirect-acting thermostats, which serve as thermal protective devices, are connected in series, one located over a coil in each phase.

 (f)  
The rotor consists of a silicon steel lamination core with copper rotor bars and end rings cast in place.
　
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