10–7. SYSTEM DESCRIPTION (cont)

TM 1-1520-238-T-7 10–46 10–7. SYSTEM DESCRIPTION (cont) 10–7 (b) The NIU (fig. 10–22) receives pressurized air from the PAS manifold. The pressure switch senses the pressure and closes, sending a signal to the NIU monitor which activates a timing circuit. The timing circuit provides a 10 second delay to allow the NIU to come on line before energizing the NITROGEN INERT MONITOR indicator. (c) Air flows through the heat exchanger where it is cooled to approximately 25° F (-3.9° C) of the ambient air temperature. The air exiting the heat exchanger passes over a thermistor. If the temperature is below 40° F (4.4° C), the thermistor turns the fan off. If the temperature is above 140° F (60° C), the thermistor turns the fan on. Before entering the pressure regulator, the air enters the filter/water separator and condensation is drained overboard. The pressure regulator regulates the air pressure to 25 ±3 psi. (d) Regulated air enters the air separation module and is directed to one of two sieve cannisters. The selected sieve canister traps approximately 71% of the oxygen molecules and outputs the nitrogen-rich air through a check valve to the pressure transducer. A portion of the nitrogen-rich air is fed back through a metering orifice of the other cannister to flush the trapped oxygen molecules out the overboard exhaust. The pressure transducer monitors the pressurized output. If the pressure is insufficient, the pressure transducer fails the NITROGEN INERT MONITOR indicator. The fuel cells receive an output air mixture of 93.5% to 94.0% nitrogen and 6.0% to 6.5% oxygen. TO FUEL CELL 6.0-6.5% OXYGEN 93.5-94% NITROGEN ROTARY VALVE PAS MANIFOLD (21% OXYGEN) EXHAUST OVERBOARD VENT 115 VAC M SIEVE CANISTER 1 AIR SEPARATION MODULE 90º ROTATION PRESSURE REGULATOR FILTER/WATER SEPARATOR THERMISTOR PRESS TO TEST FAN 115 VAC PRESSURE SWITCH HEAT EXCHANGER PRESSURE TRANSDUCER NITROGEN INERT MONITOR INDICATOR (AFT AVIONICS BAY) SIEVE CANISTER 2 M70-213 Figure 10–22. NIU Functional Block Diagram f. Pressure Refueling/Defueling System. (1) Purpose. The purpose of the pressure refueling/defueling system is to provide a means pressure refueling or suction defueling of the forward and aft fuel cells. (2) System Operation. (a) The refueling panel provides a means of controlling refuel operations and monitoring fuel quantity during refueling/defueling operations.