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757
MAINTENANCE MANUAL
The necessary air-cooling-pack-outlet temperature is sent as a signal to the automatic pack temperature controller by the zone temperature controller (Ref 21-61-00). This is the pack demand signal. The automatic-pack-temperature controller compares the pack demand signal to the pack-temperature-sensor signal. The difference between the signals causes an error. The error controls the signal that is sent to the ram air door actuators and the temperature control valve.The positions of the ram air doors and the temperature control valve are controlled by a program. From the 70 percent to 100 percent cooling demand, the ram air doors are fully open and the temperature control valve is fully closed. From 70 percent to 1 percent cooling demand, the ram air doors move to closed and the temperature control valve moves to open. At 0 percent cooling demand, the ram air doors are fully closed and the temperature control valve is fully open.When the airplane is on the ground, the ram air doors stay fully open. The temperature control valve position changes to control the air-cooling-pack-outlet temperature.The compressor outlet temperature sensor supplies input to the controller to prevent an overheat condition. The temperature sensor sends signals at temperatures of 400°F (204°C), 425°F (218°C), 450°F (232°C), and 490°F (254°C). a) At 400°F (204°C), the controller causes the ram air
doors to open.
b) At 425°F (218°C), the controller decreases the current to the torque motor of the flow-control-and-shutoff valve. This causes the valve to close, if the valve is in the high flow mode.
c) At 450°F (232°C), the controller increases current to electromagnetic control valve in the low limit valve. This opens the valve.
d) At 490°F (254°C), controller signals the flow control card to command the flow control and shutoff valve closed.
The pack temperature sensor supplies input to the controller to keep the water separator temperature between 145°F (63°C) and 35°F (1.7°C) (0°F [-18°C] min above 31,000 feet). The controller changes the positions of the ram air doors and the temperature control valve to stay between these limits.
NOTE: The PACK OUT temperature as shown on the EICAS
____ display can be lower than the water separator temperature.
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21-51-00
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757 MAINTENANCE MANUAL
7) The altitude switch supplies input to the controller to change the lower water separator temperature limit from 35°F (1.7°C) to 0°F (-18°C). The controller closes at 31,000 feet. This sends a signal to the controller to change the limit.
(c) The STBY-N mode 1) In the STBY-N mode, the backup-temperature-control card (Ref 21-53-00) and the standby-pack-temperature-controller controls the air cooling packs. When the L or R PACK selector is in STBY-N, the AUTO/MANUAL relays are not energized. This lets 115v ac power flow to the controller. It also supplies 28v dc to energize the low-limit-control-valve-control relay (ground is supplied in the backup-temperature-control card). The controller sends an open or a close signal through the control relay to the low-limit-control valve. The backup-temperature-control card causes the ram air doors to be fully open and the temperature control valve to be fully closed. Also, no current is supplied to the torque motor of the flow-control-and-shutoff valve.
2) The demand signals for the standby-pack-temperature controller are programmed to be 40°F (4.5°C) (for the pack temperature) and 450°F (232°C) (for the compressor outlet temperature). The controller compares the temperature sensor signals to the set demand signals. The difference of the signals causes two error signals. The greatest error signal controls the position of the low limit control valve.
(d) The STBY-W mode 1) In the STBY-W mode, the backup-temperature-control card (Ref 21-53-00) controls the air cooling packs. 28v dc is always supplied to the backup-temperature-control card. When the L or R PACK selector is in STBY-W, the AUTO/MANUAL relays are not energized. This lets the card send signals through the AUTO/MANUAL relays to the ram air doors and the temperature control valve.
2) In the STBY-W mode, the backup-temperature-control card causes the ram air doors and the temperature control valve to be fully open. The low-limit-control valve stays closed unless it is pneumatically opened.
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