AIR CONDITIONING -PACK FLOW CONTROL -FUNCTIONAL DESCRIPTION -ASCPC FLOW SCHEDULES
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777 AIRCRAFT MAINTENANCE MANUAL
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AIR CONDITIONING -PACK FLOW CONTROL -FUNCTIONAL DESCRIPTION -CTC FLOW CONTROL
Upper/Lower Valve Selection
If the airplane altitude is low, the cabin temperature controllers (CTCs) use the upper flow control and shutoff valves to control air flow into the packs. These are the altitudes:
*
Below 26,000 feet (7930 meters) on climb
*
Below 24,000 feet (7315 meters) on descent.
If the airplane altitude is high, the CTC use the lower flow control and shutoff valves to control air flow into the packs. These are the altitudes:
*
26,000 feet (7930 meters) or above on climb
*
24,000 feet (7315 meters) or above on descent.
If one of the valves fails, the CTC controls the other valve at all altitudes.
Flow Regulation
The CTC calculates four flow rates. The flow schedule from the ASCPC tells the CTC which flow rate to use. The usual air flow rate for the left and right packs is the same. The flow rate of one pack can be 10 percent more or 10 percent less than the other pack.
If the right CTC decreases its pack flow rate to keep pack operation in limits, the right CTC sends a signal to the left CTC. The left CTC adds this R CTC flow bias signal to the flow rate for the left pack. The total is the flow command. A single pack has a flow command limit of 81% of the total flow into the airplane. See the pack cooling and mix manifold temperature control section for more information about the functional description of the packs (SECTION 21-52).
The CTC compares the flow command with the measured pack flow. The difference makes a drive signal for the flow control and shutoff valve. The valve moves until the measured pack flow equals the flow command.
Flow Rate Adjustments
If any recirculation fans are inoperative when the CTC uses schedule 1, the CTC increases the flow rate. The total flow into the airplane increases 67 lbs/min for each fan that is off, 33.5 lbs/min per pack. This keeps the total airplane ventilation rate constant.
If the forward or aft cargo fire switch is armed, the CTC sets the schedule 2 flow rate (except when schedule 0 is active). The ASCPC does not send schedules 3 or 4 if a cargo fire switch is armed. Schedule 2 supplies sufficient fresh air for the airplane occupants and keeps smoke out of the passenger compartment and flight deck.
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AIR CONDITIONING -PACK FLOW CONTROL -FUNCTIONAL DESCRIPTION -CTC FLOW CONTROL
The CTC closes the flow control and shutoff valve for these conditions:
*
Flow schedule 0 from the ASCPC
*
Pack outlet overheat
*
Compressor discharge overheat
*
Pack failure.
Other Flow Control Functions
At times, the air flow into the pack is not enough even with a flow control and shutoff valve full open. For some low flow conditions, the pack operates in economy cooling mode to help increase the air flow. An ARINC 629 input signal to the ELMS puts the pack into the economy cooling mode. See the pack cooling and mix manifold temperature control section for more information about the economy cooling mode ( SECTION 21-52) .
The CTC can also tell the ASCPC to adjust the engine bleed air supply system to help increase pack air flow. ARINC 629 signals tell the ASCPC to reduce the engine bleed air temperature or increase the HPSOV set point. See the engine air supply section for more information about the engine bleed bias ( SECTION 21-52) .
APU Demand
When the APU supplies the air for pack operation, the CTCs do not use the flow schedules from the ASCPCs. The CTCs set the flow control valve to fully open. The CTCs send an APU DEMAND signal to the APU controller (APUC) to tell it the amount of air necessary for the ventilation and the heating or the cooling of the airplane. See the zone temperature control and indication section for more information on APU DEMAND (SECTION 21-61).
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AIR CONDITIONING -PACK FLOW CONTROL -FUNCTIONAL DESCRIPTION -CTC FLOW CONTROL
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AIR CONDITIONING -PACK FLOW CONTROL -FUNCTIONAL DESCRIPTION -START-UP AND SHUTDOWN
General
The flow control and shutoff valves open and close at specified rates. The rates are different for different conditions. When one valve starts to open, the OFF light in the pack switch goes out. When the two valves close, the OFF light comes on.
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