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increased airflow requirements. This mode has priority over all
other modes.
ACS Mode
In each ACS mode, the ECU opens the IGVs as necessary to
supply air to the air conditioning system. These are the four
ACS modes:
* One pack inflight
* One pack ground
* Two pack, normal
* Two pack, high.
The IGV positions in the ACS modes are different for each
airplane model. The ECU finds the airplane model from two
signature pins on the airplane ECU connector.
It is recommended that you operate the APU with the two air
conditioning packs when maximum cabin cooling is necessary.
Use the two air conditioning packs as an alternative to the one
air conditioning pack operation. The results of the operation of
two air conditioning packs will be cooler cabin temperatures,
lower APU fuel burn, better APU hot section life and lower APU
noise. Refer to Service Letter 737-SL-49-060 for more data on
these results.
APU BLEED AIR SYSTEM - INLET GUIDE VANES - FUNCTIONAL DESCRIPTION
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Control
After the mode selection, the ECU uses inlet temperature, inlet
pressure, and airplane specific data (flight or ground
conditions) to find the commanded IGV angle.
The ECU sends control signals to the servo valve on the IGV
actuator to open or close the guide vanes. The LVDT in the IGV
actuator supplies vane position feedback to the ECU.
APU BLEED AIR SYSTEM - INLET GUIDE VANES - FUNCTIONAL DESCRIPTION
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APU BLEED AIR SYSTEM - INLET GUIDE VANES - FUNCTIONAL DESCRIPTION
APU BLEED AIR SYSTEM - INLET GUIDE VANES - FUNCTIONAL DESCRIPTION
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Purpose
The three pressure sensors measure load compressor
pressures and convert these pressures into a electric signals.
The electric signals are sent to the ECU.
These are the three pressure sensors:
* Inlet pressure (P2) sensor
* Total pressure (PT) sensor
* Differential pressure (DP) sensor.
Location
The P2 sensor is attached to the APU inlet. The PT and DP
sensors are above the surge control valve (SCV) on the APU
inlet.
Functional Description
The P2 sensor measures the APU inlet pressure of the load
compressor. The ECU uses this data to control APU functions.
The PT sensor measures load compressor total discharge
pressure.
A DP sensor measures the difference between the total
discharge pressure and the static pressure in the diffuser.
Differential pressure equals total pressure minus static
pressure.
The ECU uses PT and DP to calculate load compressor airflow.
The ECU uses load compressor airflow, inlet temperature, and
IGV position to operate the surge control valve.
APU BLEED AIR SYSTEM - P2, PT, AND DP PRESSURE SENSORS
EFFECTIVITY
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APU BLEED AIR SYSTEM - P2, PT, AND DP PRESSURE SENSORS
APU BLEED AIR SYSTEM - P2, PT, AND DP PRESSURE SENSORS
EFFECTIVITY
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Purpose
The surge control valve (SCV) releases air from the load
compressor. The SCV makes sure there is a minimum flow of
air through the load compressor. This prevents a surge. If a
surge does occur, the SCV opens to help the load compressor
recover.
Physical Description
The SCV is a butterfly type valve. The surge control valve
actuator is on the top of the valve. A two-stage servo valve
controls the actuator. A visual indicator on the valve gives the
position of the valve.
Location
The valve is in the surge bleed duct on the right side of the APU.
Functional Description
The ECU controls a torque motor on the servo valve. This motor
sends high pressure fuel from the APU fuel system to open or
　
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