H18532 H18518
 Pack Valve Schematic  506 
21-11-0Page 4   Figure 2 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Nov 15/71 

501  Pack Valve Control Circuit 
Aug 01/74  Figure 3 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  21-11-0 Page 5 

3.  Modulating and Shutoff Valve
A.  The modulating and shutoff valve adds 13th stage bleed air when 8th stage bleed is inadequate. The modulating and shutoff valve regulates the flow of 13th stage bleed air into the system and is controlled by a pneumatic signal received from the pressure regulator. The unit consists of a pneumatic actuator and butterfly valve. (See figure 4.) The actuator is divided into two pressure chambers, an opening chamber and a closing chamber, and by a diaphragm assembly. The opening chamber is connected by tubing to the pressure regulator, and the closing chamber is connected by tubing to the pneumatic manifold downstream of the heat exchanger. The diaphragm assembly operates the butterfly through a connecting rod and linkage. The butterfly valve consists of a butterfly disk located within the airflow duct of the valve body. A port on the valve body upstream of the butterfly supplies unregulated air to the pressure regulator.
B.  During operation, pressure from the pressure regulator is supplied into the opening chamber, where the pressure on the diaphragm overcomes the spring and moves the butterfly toward the open position. Simultaneously the downstream pressure sensed in the closing chamber will supplement the closing spring and tend to drive the valve toward the close position. As the bleed air manifold pressure approaches the differential between the control pressure and closing spring pressure the valve modulates.

H28430
Modulating and Shutoff Valve Schematic  500 
21-11-0  Figure 4  Nov 15/66 
Page 6 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

4.  Pressure Regulator
A.  This unit is a variable differential pressure regulator. It is spring-loaded in the open position and regulates outlet pressure to a constant differential above ambient when duct and control pressures are low. At this time the calibration diaphragm assembly rests against the stop in the actuator cover. When duct or control pressure is higher, the calibration diaphragm assembly moves the hydraulic damping bellows assembly and recalibrates the outlet pressure by compressing the regulation spring according to the levels of sensed duct and control pressures. Stroke of the calibration diaphragm assembly is limited by a stop in the actuator body to limit the level to which outlet pressure can be increased. (See figure 5.)
B.  A bleed orifice vents outlet air to ambient to provide a bleed flow for the control orifice. A test port is provided in the inlet port.

H28434
500  Pressure Regulator Schematic 
May 01/99  Figure 5  21-11-0 
Page 7 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

5.  450°F Thermostat
A.  A 450° thermostat is provided in the engine bleed air compression system. This thermostat is located in the bleed air duct downstream of the precooler. The thermostat is a direct acting bleedoff, bimetallic sensing probe.
B.  Under most operating conditions the precooler system keeps the air temperature below 450°F but when there is a high airflow demand from the thermal anti-ice and air conditioning systems, the precooler system limits may be exceeded and the air temperature may continue to rise when all systems are functioning properly. The 450°F thermostat operates between 450 and 490°F to prevent trips from the bleed overheat switch during these conditions.
C.  A tube from the thermostat connects to the control line for the modulating and shutoff valve (13th stage). At 450°F an ambient port in the thermostat begins to open reducing control pressure to the valve which causes it to move towards closed. This reduction in pressure causes the modulating and shutoff valve to modulate towards the closed position thereby reducing the total airflow through the precooler. When the thermostat is operating the vent opening to ambient may vary anywhere between full closed and full open depending on temperature of the air.
6.  Operation
A.  With the exception of the solenoid valves, the flow control and bleed stage switchover functions are entirely pneumatic. (See figure 6.) When the system is operating on 8th-stage bleed, the actuator regulates the airflow in response to control signals generated by the flow-sensing venturi control servo. This valve controls the pressure in the head of the actuator and responds primarily to the pressure difference sensed by the venturi. If the regulated airflow tends to increase, the venturi airflow will also increase causing the control pressure to decrease and move the actuator toward the closed position.
　
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