The purpose of the bleed air precooler system is to control the temperature of the high pressure bleed air which is supplied to the manifold system to approximately 415°F. Two precooler systems are provided, one for engine No. 1 and one for engine No. 2. Each precooler system consists of an air-to-air heat exchanger, four-inch-diameter precooler control valve, a precooler control valve sensor and a solenoid valve installed in the sense line between the valve and sensor.
The precooler is an air-to-air heat exchanger located at top right center of each engine and is of the plate-fin single pass crossflow type. The heat collected from the hot air ducted from the engine is absorbed by the cool air ducted from the engine fan after passing through the precooler control valve. The cooling air is then discharged overboard.
The precooler valve is attached to the discharge port of the engine fan. The purpose of the valve is to control the flow of cooling air through the heat exchanger to regulate the temperature of the bleed air. This is a pneumatically actuated, temperature controlled, spring loaded open butterfly modulating valve. A remote pneumatic temperature sensor is used to produce a valve position that is proportional to the sensed temperature. The valve is closed at low sensed temperature and open at high sensed temperatures. Supply pressure from the pneumatic manifold is applied to the supply port on the valve. This pressure is ported to the base of two reference pressure regulators. Supply air flows through the reference pressure regulator metering valve and through an orifice to Chamber "A". When the sensed pressure reaches the desired level, the spring force is overcome and the metering valve moves toward the seat to maintain the outlet pressure at a constant level. The regulated outlet pressure from the actuator reference pressure regulator is routed through Orifice "A" to a control nozzle and butterfly actuator. The regulated outlet pressure from the servo reference pressure regulator is routed through a valve driven feedback control orifice to the remote temperature sensor and to the servo. The remote temperature sensor functions as a variable orifice producing a control pressure in servo Chamber "B" that is inversely proportional to sensed temperature thereby driving Chamber "B" pressure up with decreasing temperature and down with increasing temperature. When the remote temperature sensor is closed, Chamber "B" pressure is at its maximum, the control nozzle is closed by the pressure force on Diaphrgm "B" and the actuator pressure is at its maximum value which is sufficient to overcome the valve opening spring and drive the butterfly fully closed. A decrease in servo Chamber "B" pressure permits the load of the servo spring to pull the lever away from the control nozzle which produces a decrease in the actuator chamber pressure. As the actuator pressure decreases, the actuator opening spring force strokes the butterfly in the opening direction until a force balance exists between actuator and spring force. As the butterfly strokes toward the opening direction, the feedback spring produces an increasing tension load on the lever.
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.
Precooler Control Valve Sensor
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.
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.
C.Duct overheat protection is provided by an overheat (thermal) switch located downstream of the precooler but upstream of the precooler control valve sensor. If the sensed temperature at the switch exceeds 490 (± 10)°F the switch closes, causing the pressure regulator and shutoff valve to close and energizing the appropriate trip-off light on the flight crew's panel. The pressure regulator and shutoff valve is also controllable from the flight deck and may be reset in flight after a trip-off, once the overheat switch has cooled and reopened.
D.Ground wing thermal anti-ice (WTAI) solenoid valve operation is initiated by actuating the WTAI control switch in cockpit to ON when the bleed air system is on and the airplane is on the ground. The solenoid valve, which is installed in the sense line from the precooler control valve to the precooler valve sensor, is energized to the open position and will vent the control fuctions of the precooler control valve thus causing the valve to drive full open. The solenoid valve remains open as long as the WTAI switch is ON and the airplane is on the ground. The valve will close if the WTAI switch is positioned OFF or the airplane leaves the ground. The valve will remain off during flight.