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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
SEE A
GROUND WTAI LOW FLOW SOLENOID
FAN AIR
PRECOOLER CONTROL VALVE
PRECOOLER CONTROL 
VALVE SENSOR 
PRSOV 
DOWNSTREAM 
SENSE LINE
PRECOOLER 
FAN AIR EXHAUST 
PRECOOLER  FWD 
CONTROL VALVE 
SUPPLY LINE 

Engine Bleed Air Precooler System Component Location
Figure 1

450`F/232`C THERMOSTAT
E02928
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL

Engine Bleed Air Precooler System Control Schematic
Figure 2

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E03025
A 737-300/400/500MAINTENANCE MANUAL
 4.  Solenoid Valve (Fig. 2)
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 A.  A solenoid valve, nonlatching type, is installed in the sense line from the precooler control valve to the precooler control valve sensor. It is a simple open-closed ball type valve which is actuated to the open position when the wing TAI system switch is positioned ON during airplane ground operation. The valve vents the sense line and allows the precooler control valve to open fully.
 5.  Precooler Control Valve Sensor (Fig. 2)
______________________________
 A.  The sensor is located downstream of the precooler . The purpose of the sensor is to control the precooler control valve butterfly position. The sensor is a bleed-off pneumatic thermostat which provides a pneumatic output signal that varies inversely with the sensed temperature. As sensed temperature increases, the hermetically sealed oil within the sensing tube expands and acts upon the bellows assembly. The resulting force against the bellows causes the output rod to move the metering valve off the seat and thereby produce flow through the control orifice and out the metering valve to ambient. The airflow from the constant supply pressure source develops a pressure drop across the control orifice. As the temperature increases, the metering valve opens up further, allowing more airflow through the control orifice with an associated larger pressure drop. The result is a decrease in output signal in response to an increase in sensed temperature. Similarly, when sensed temperature decreases, fluid contraction causes the output rod and metering valve to move in the closing direction. This reduces the pressure drop across the control orifice as sensed temperature decreases. Therefore, the metering valve is modulated to provide a discrete output signal pressure for each sensed temperature which in turn modulates the precooler valve open and closed.
 6.  Operation
_________
 A.  Temperature control of the bleed air is accomplished with an air-to-air heat exchanger, which uses engine fan bleed air as a cooling medium. A precooler control valve limits the temperature of the air in the downstream ducts to approximately 415°F.
 B.  Regulation of the cooling airflow through the precooler control valve is provided by the precooler control valve sensor in the bleed air discharge duct. The precooler valve is fully closed at a temperature of 390°F or less and fully open at a temperature of 440°F or more. When the bleed air temperature starts to exceed 390°F the precooler control valve sensor will start to modulate its metering valve open which in turn reduces the controlling pressure in the precooler control valve and will cause the valve to start modulating toward open. As the temperature increases the valve will continue to open until a stable temperature below 440°F is obtained. Conversely, if the bleed air temperature becomes lower the valve will modulate toward closed.
　
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