If the temperatures are not up to minimum after 27 minutes, the GPC will issue an SM alert and display the data on the CRT. When the proper temperatures have been attained, the GPC will open for two minutes and then close the hydrogen and oxygen purge valves for fuel cells 1, 2 and 3 in that order. Thirty minutes after the fuel cell 3 purge valves have been closed (to ensure that the purge lines have been totally evacuated), the GPC will turn off the purge line heaters. This provides sufficient time and heat to bake out any remaining water vapor. If the heaters are turned off before 30 minutes have elapsed, water vapor left in the lines may freeze.
The manual fuel cell purge would be initiated by the flight crew using the switches on panel R12. In the manual mode, the three fuel cells must be purged separately. The fuel cell purge heater switch is positioned to on for the same purpose as in the automatic mode, and the flight crew verifies that the temperatures of the oxygen line and two hydrogen lines are at the same minimum temperatures as in the automatic mode before the purge sequence is initiated. The fuel cell purge valves 1 switch is positioned to open for two minutes, and the flight crew observes that the oxygen and hydrogen flow rates increase on the CRT. The fuel cell purge valves 1 switch is then positioned to close , and a decrease in the oxygen and hydrogen flow rates is observed on the CRT, indicating the purge valves are closed. Fuel cell 2 is purged in the same manner using the fuel cell purge valves 2 switch. Fuel cell 3 is then purged in the same manner using the fuel cell purge valves 3 switch. After the 30-minute line bakeout period, the fuel cell purge heater switch is positioned to off.
In order to cool the fuel cell stack during its operations, distribute heat during fuel cell starting, and warm the cryogenic reactants entering the stack, the fuel cell circulates a coolant-fluorinated hydrocarbon-throughout the fuel cell. The fuel cell coolant loop and its interface with the ECLSS Freon-21 coolant loops are identical in fuel cells 1, 2 and 3.
Where the coolant enters the fuel cell, the temperature of the F-40 coolant returning from the ECLSS Freon-21 coolant loops is sensed before it passes through a 75-micron filter. After the filter, two temperature-controlled mixing valves allow some of the hot coolant to mix with the cool returning coolant to prevent the condenser exit control valve from oscillating. The condenser exit control valve adjusts the flow of the coolant through the condenser to maintain the hydrogen-water vapor exiting the condenser at a temperature between 148 and 153 F.
The stack inlet control valve maintains the temperature of the coolant entering the stack between 177 and 187 F. The accumulator is the interface with the oxygen cryogenic reactant to maintain an equalized pressure between the oxygen and the coolant (the oxygen and hydrogen pressures are controlled at the dual gas regulator) to preclude a high-pressure differential in the stack. The pressure in the coolant loop is sensed before the coolant enters the stack.
The coolant is circulated through the fuel cell stack to absorb the waste heat from the hydrogen/oxygen reaction occurring in the individual cells. After the coolant leaves the stack, its temperature is sensed and the data transmitted to the GPC, to the fuel cell stack temp meter through the fuel cell 1, 2, 3 switch located below the meter on panel O2, and to the CRT display. The yellow fuel stack temp C/W and the backup C/W alarm lights on panel F7 and the SM alert light will be illuminated if fuel cell and stack temperatures exceed certain limits: below 172.5 F or above 243.7 F. The hot coolant from the stack flows through the oxygen and hydrogen preheaters, where it warms the cryogenic reactants before they enter the stack.
