icing. (See figure 4-8.) Ice forming on the wire screen at the water separator discharge duct is detected by two pneumatic pickups located just before and after the water separator. These pickups sense a differential pressure across the water separator. If differential pressure is sensed across the water separator, the nonice and low- limit control valve will remain open until the temperature of the inlet air to the water separator is high enough to melt collected ice. When the ice is melted, the pressure differential returns to normal. In addition, the refrigeration pack low- limit control electrically signals the nonice and low-limit control valve when separator outflow drops to 0°F. REFRIGERATION PACK LOW-LIMIT CONTROL.— The refrigeration pack low-limit control (fig. 4-6) is located in the ECS compartment. It is mounted downstream from the water separator in a 6-inch duct of cooled discharged bleed air. The refrigeration pack low-limit control uses 28-volt dc power to energize its circuitry. A thermistor senses duct air temperature and compares it with an internally generated reference. The difference is amplified to modulate a torque motor in the nonice and low-limit control valve. (See figure 4-8.) The torque valve controls the regulated air supply (3) with a flapper valve (1), which controls the diaphragm pressure in a butterfly actuating linkage (12). The nonice and low-limit control valve can be returned to the differential pressure control mode by opening the cabin temperature high-limit thermostat (4). This causes the upper chamber (6) of the switcher valve (7) to be vented (17) and returned to its primary position. A check valve (5) is provided to prevent extraneous signals from affecting the nonice and low-limit control valve. REFRIGERATION UNIT CHECK VALVE.— The refrigeration unit check valve (fig. 4-7, detail A) is an insert-type check valve with a split flapper spring-loaded in the closed position. The valve, which is installed in the refrigeration unit to prevent hot bleed air from entering directly into the turbine, is located in a tee arrangement in the system just downstream of an orifice. Icing of the water separator will occur only at low altitudes where mass airflow and tempera- ture are relatively high. Only a small amount of high-temperature air is required through the orifice to melt such a deposit. However, at high altitude where the mass flow and bleed-air temperatures are low, the refrigeration pack low- limit control operates to open the nonice and low- limit control valve. When the nonice and low-limit control valve is open, high differential pressure across the bleed-air orifice permits the refrigera- tion unit check valve to open. This allows intermediate-temperature air to bypass the turbine, and thereby maintain water separator temperature above 0°F. (See figure 4-6.) CABIN AIR/WATER SEPARATOR, CO- ALESCER CONE, AND COALESCER BAG.— The water separator is a welded cylindrical aluminum container installed downstream from the turbine and fan assembly. Its purpose is to remove a portion of the moisture condensed during the air-expansion process within the expansion turbine. (See figure 4-6.) The water separator container holds a coalescer bag, which collects the finely dispersed fog-like moisture discharged from the turbine. The wet air flows through the coalescer cone and through louvered swirl vanes to cause the heavier water particles to be deposited by centrifugal force against the outer surface of the collector section. Accumulated water is drained through the sump in the bottom of the collector section. The partially dried air then leaves the water separator by way of the air outlet duct. The coalescer bag may be removed for cleaning through an access cover secured with a quick-disconnect band coupling to the water separator shell. WATER SEPARATOR ICE SCREEN.— An ice screen is located in the discharge end of the water separator to collect ice when moisturized airflow temperature is below the dew point temperature, or below 32°F. The condensed ice crystals gathered across the ice screen cause a pressure differential, which is sensed by the nonice and low-limit control valve. The nonice and low- limit control valve then increases the warm air supply through the mixing muff, the coalescer bag, and to the ice screen to melt collected ice. WATER SEPARATOR BYPASS VALVE.— The bypass valve is a spring-loaded valve mounted in the water separator container. A failure of the nonice and low-limit control valve could cause ice particles to build up in the water separator coalescer bag. This ice would block the cabin air system. To ensure that air is supplied to the cabin, the water separator bypass valve allows turbine air to bypass the coalescer bag. 4-12