Purpose
Three pressure transducers measure load compressor pressure conditions.
Location
The pressure transducers are on the forward lower face of the pneumatic cluster.
Functional Description
The inlet pressure sensor (P2) measures ambient pressure upstream of the load compressor. The APUC uses this data to control some APU functions.
The total pressure sensor (PT) measures load compressor discharge pressure. A differential pressure sensor (DP) measures the difference between the total pressure and the static pressure in the diffuser (differential pressure equals total pressure minus static pressure).
The APUC uses PT and DP to calculate load compressor airflow to operate the surge control valve.
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APU AIR -APU SURGE BLEED SYSTEM -PRESSURE SENSORS

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APU AIR -APU SURGE BLEED SYSTEM -FUNCTIONAL DESCRIPTION

General
The APU surge bleed system releases the air from the load compressor that the airplane pneumatic system does not use. This keeps air moving through the load compressor and prevents compressor surge.
Corrected Air Flow and Surge Margin Set Point
The APUC calculates a corrected airflow and a surge margin set point.
The corrected airflow is the amount of air that flows through the load compressor. The APUC uses total pressure (PT) and differential pressure DP) to calculate the corrected airflow.
The surge margin set point is the minimum amount of air that should flow from the load compressor for the APU operating conditions.
The APUC uses these inputs to calculate the surge margin set point:
*
Total pressure (PT)

*
Inlet pressure (P2)

*
Inlet temperature (T2)

*
IGV position

*
Pneumatic mode

*
APU shutoff valve position

*
ADP condition.

The corrected airflow must be more than the surge margin set point to prevent a compressor surge. If the corrected airflow is less than the surge margin set point, the APUC opens the surge control valve. This increases the airflow from the load compressor to the minimum level.
For the ECS pneumatic mode, the usual position for the surge control valve is fully closed. For the duct pressurization, main engine start, ADP, and ADP/ECS pneumatic modes, the usual position for the surge control valve is between fully closed and fully open. For these pneumatic modes, the surge control valve opens more if the corrected airflow goes below the surge margin set point.
For the idle pneumatic mode, the usual position for the surge control valve is fully open.
Surge Control Valve Actuator Control
The APUC sends control signals to the servo valve on the SCV actuator to open or close the valve. The LVDT in the surge control valve supplies valve position feedback to the APUC.
APU Start and Shutdown
During APU start, the surge control valve is in the fully open position. The valve closes when APU speed is 95 percent RPM.
During APU shutdown, the surge control valve fully opens when you move the APU selector to the OFF position.
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BOEING PROPRIETARY -Copyright # -Unpublished Work -See title page for details.

APU CONTROL SYSTEM -INTRODUCTION

Purpose
The APU control system uses an APU controller to control these APU functions:
*
Starting and ignition

*
Fuel metering

*
Surge control

*
Inlet guide vane (IGV) control

*
Data Storage

*
Protective shutdown

*
BITE/fault reporting

*
APU indication.

System Components
The other control system components are:
*
The speed sensor (2)

*
The inlet temperature sensor.

These sensors supply data to the APUC for control functions.
Location
The APUC is on the E7 rack above the aft galley. The two APU speed sensors and the inlet temperature sensor are on the sides of the APU.
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APU CONTROL SYSTEM -INTRODUCTION

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