8.  Water Separator 350F Control System
A.  When cooling requirements are high, the temperature of the air as it leaves the air cycle machine may drop below the freezing point of water. The water separator 350F control system regulates air temperature into the separator to keep moisture from freezing on the water separator coalescer bag. (See figure 4.)
B.  Keeping water separator temperature above freezing is accomplished by taking hot air from upstream of the air cycle machine compressor and routing it back into the system at the muff at the air cycle machine turbine discharge. The water separator 350F control system regulates the quantity of air being bypassed.
C.  The water separator 350F control system consists of a water separator 350F sensor, control, and valve. The sensor is located on the right forward underside of the water separator, the control is mounted to the outboard side of the equipment bay near the forward end of the bay, and the valve is located forward of the air cycle machine in the duct taking off from the air cycle machine compressor inlet duct.
D.  The 350F sensor is one element of the 350F control. The control contains a bridge circuit, an amplifier, and a silicon-controlled rectifier actuator control. The sensor is utilized as a remotely located variable resistance for one leg of the bridge circuit. As temperature at the sensor changes, its resistance changes, unbalancing the bridge. The bridge circuit is connected to the amplifier. The amplifier interprets the signal received and then signals the actuator control to move the valve open or closed. When the bridge is balanced current is shut off to the valve and it maintains the position held. The bridge balances with temperature in the separator at approximately 350F.

Water Separator 350 F Control System Circuit  500 
21-51-0  Figure 4  Feb 15/69 
Page 8 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

E.  Some 35°F controls have built-in test circuits which provide a quick check of the 35° F control system. A test switch, two indicator lights and a test instruction placard have been added to these units. The test switch is a rotary type with five test positions and one flight position. When control system is not being tested, the switch must be returned to the flight position.
9.  Operation
A.  When air conditioning is turned on, the pack valves open and hot air from the pneumatic system enters the air conditioning system. Depending on cabin temperature requirements, the mix valves adjust to send the required portion of this air through the air cycle system. The remainder passes unchanged through a duct to the mixing chamber (Fig. 5).
B.  The air entering the system first passes through the primary heat exchanger where some heat is removed. This air, still at an elevated temperature, moves on to the air cycle machine where it passes through the compressor and the secondary heat exchanger before it reaches the turbine. The turbine then drives the compressor. As the air is compressed, its temperature and pressure rise. The secondary heat exchanger then lowers the temperature to approximately the value it was as it entered the compressor but at a higher pressure. The air expands as it drives the turbine and leaves the turbine at the coolest temperature of the cycle.
C.  Should the temperature of the air leaving the compressor reach 365 + 10°F (185 + 6°C), the compressor discharge overheat switch will close to energize the pack overheat relay. Energizing the relay completes circuits to close the pack valve, illuminate the PACK TRIP OFF and AIR COND master caution annunciators. Pushing the TRIP RESET switch will return the system to normal operation after the overheat condition has been corrected.
D.  Unless the secondary heat exchanger reduces the temperature of the air sufficiently, overheat of the turbine may occur. At a turbine inlet temperature of 210 + 10°F (93 to 104°C), the turbine inlet overheat switch will close. Closing of this switch will also energize the pack trip relay to close the pack valve, illuminate the PACK TRIP OFF light and AIR COND master caution annunciators. Pushing the TRIP RESET switch will return the system to normal operation after the overheat condition has been corrected.
E.  From the air cycle machine the air is ducted forward to the water separator. Excess moisture condenses from the air at the reduced temperature and is separated from the air and drained overboard. The water separator 35°F control system bypasses primary heat exchanger air around the air cycle machine, if needed, to prevent water freezing in the separator.
F.  Cold air leaving the water separator then travels to the mixing chamber.
G.  When the air conditioning switches are turned off the pack valve closes and the mix valve drives to full cold.
　
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