(3)
The Pack Flow Control
(a)
The amount of air that flows through the air cooling pack is controlled by the flow control cards. Each flow control card controls one flow-control-and-shutoff valve. The flow-control-and-shutoff valves permit a controlled amount of air into each air cooling pack.
(b)
The flow-control-and-shutoff valves open when the L or R PACK selector, on the pilot's overhead panel P5, is not in the OFF position. The selector sends a signal to the flow control card to open the flow-control-and-shutoff valve. The flow control card causes the flow-control-and-shutoff valves to go into the normal flow mode or the high flow mode.
(c)
When the L or R PACK control selector is not in the OFF position, the flow control card supplies a momentary current of 28v dc to solenoid B of the flow-control-and-shutoff valve. This current opens solenoid B so that the valve will open pneumatically. Thus, the valve operates in the normal flow mode. When the PACK control selector is put to OFF, the flow control card supplies a momentary current of 28v dc to the close side of solenoid B. This closes solenoid B, which causes the valve to close.
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(d)
(e)
(f)
(g)
(h)
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MAINTENANCE MANUAL
When the flow control card sends 28V DC power to the high flow
solenoid (solenoid A), the solenoid is energized. If solenoid
A and solenoid B are energized at the same time, then the valve
will move to the high flow mode position.
Do the following steps to enable high flow with either the
engines as the pneumatic supply or with the APU or a ground
source as the pneumatic supply:
With the engines providing the pneumatic supply, the flow
control card automatically sends the power to enable high flow
when all of these conditions occur:
1) The PACK control selector is in AUTO or STBY.
2) The opposite side engine is on.
3) An overheat condition does not occur.
4) The high flow inhibit circuit has not failed.
5) One of these conditions occurs:
a) The recirculation fan on the same side as the flow-control-and-shutoff valve is off. b) The flow-control-and-shutoff valve for the other pack
is closed.With the APU or a ground air source providing the pneumatic supply, the flow control card automatically sends the power to enable high flow when all of these conditions occur: 1) The PACK control selector is in AUTO or STBY. 2) An overheat condition does not occur. 3) The high flow inhibit circuit has not failed. 4) This condition does not occur:
a) If the airplane is on the ground, the left and right engine pressure regulating and shutoff valves (PRSOV) are closed.
The high-flow-inhibit circuit fails when the engine speed card sends the "engine-on" signal and the "engine-off" signal, at the same time. To reset the flow control card after a high-flow-inhibit-circuit failure, turn the PACK control selector to OFF and then back to AUTO or STBY.A compressor overheat condition occurs when the compressor discharge temperature is more than 490°F (254°C). At 490°F (254°C), the compressor outlet overheat switch closes and sends an overheat signal to the flow control card. Also at 490°F(254°C), the compressor outlet sensor sends an overheat signal through the pack temperature controller to the flow control card. The flow control card then latches in the overheat position and causes the following conditions to occur: 1) The flow-control-and-shutoff valve closes. 2) The PACK OFF and INOP lights, on the P5 panel, come on. 3) The advisory messages L (R) PACK OFF and L (R) PACK TEMP
show on the EICAS display.
4) The AUTO/MANUAL relays are not energized (this prevents control of the air cooling pack by the automatic-pack-temperature controller).
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A
757
MAINTENANCE MANUAL
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