Shuttle Reference Manual Space Shuttle Orbiter Systems Water Spray Boilers

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The water spray boiler system consists of three identical independent water spray boilers, one for a corresponding auxiliary power unit and hydraulic system. The boilers are located in the aft fuselage of the orbiter. Each water spray boiler cools the corresponding power unit lube oil system and hydraulic system by spraying water onto their lines; the water then boils off, cooling the lube oil and hydraulic fluid. The water (steam) that boils off in each water spray boiler exits its own steam duct, located to the right of the vertical stabilizer.

Each water spray boiler stores water in a bellows-type storage tank pressurized by gaseous nitrogen to provide positive water expulsion that feeds the boiler. The hydraulic fluid passes through the boiler three times, and the lube oil of the auxiliary power unit passes through the boiler twice in a set of tubes. The hydraulic fluid tubes are sprayed with water from three water spray bars, and two water spray bars spray the power unit lube oil. Separate water feed valves allow independent control of the hydraulic fluid spray bars and power unit lube oil spray bars. Redundant electrical controllers provide completely automatic operation.

The boiler system maintains auxiliary power unit lube oil temperature at approximately 250º F and the hydraulic fluid in the range of 210º to 220º F.

The three auxiliary power units and hydraulic pumps and water spray boilers are in operation five minutes before lift-off, operate throughout the launch phase and are shut down after the first orbital maneuvering system thrust period. One power unit/hydraulic system and corresponding water spray boiler are operated briefly one day before deorbit during a checkout of the orbiter flight control system, which includes the orbiter aerosurfaces.

One auxiliary power unit is restarted before the deorbit thrusting maneuver. The two remaining units are started after the deorbit thrusting maneuver and operate continuously through entry, landing and landing rollout to provide hydraulic pressure for positioning the orbiter aerosurfaces during the atmospheric flight portion of entry; deploying the nose and main landing gear, main landing gear braking and nose wheel steering; and positioning the three space shuttle main engines after landing rollout. The corresponding water spray boilers are also in operation during this period.

Each water spray boiler is 45 by 31 inches long by 19 inches wide. Each boiler, including controller and vent nozzle, weighs 181 pounds. They are mounted in the orbiter aft fuselage between Xo 1340 and 1400, at Zo 488 minus 15, and at Yo plus 15. Insulation blankets cover each boiler. The boiler's water capacity is 142 pounds.

The gaseous nitrogen pressure for each water spray boiler is contained in a corresponding 6-inch spherical pressure vessel. The pressure vessel contains 0.77 pound of nitrogen at a nominal pressure of 2,400 psi at 70º F. The gaseous nitrogen storage system of each water spray boiler is directed to its corresponding water storage tank. Each storage vessel contains sufficient nitrogen gas to expel all the water from the tank and allow for relief valve venting during ascent.

The nitrogen shutoff valve between the pressure valve and water storage tank of each boiler permits the pressure to reach the nitrogen regulator and water tank or isolates the nitrogen supply from the water tank. Each nitrogen valve is controlled by its respective boiler N2 supply 1,2 or 3 switch on panel R2. The nitrogen shutoff valve, which is latched open or closed consists of two independent solenoid coils that permit valve control from either the primary or secondary controller.

A single-stage regulator is installed between the nitrogen pressure shutoff valve and the water storage tank. The gaseous nitrogen regulator for each water spray boiler regulates the high-pressure nitrogen between 24.5 and 26 psig as it flows to the water storage tank.

A relief valve is incorporated inside each nitrogen regulator to prevent the water storage tank pressure from exceeding 33.5 psig from heat soakback during operations or in the event of a failed-open nitrogen regulator. The gaseous nitrogen relief valve opens between 30 to 33.5 psig.

The water supply for each boiler is stored in a positive-displacement aluminum tank containing a welded metal bellows separating the stored water inside the bellows from the nitrogen expulsion gas.

Non-redundant pressure and temperature sensors located downstream from the gaseous nitrogen pressure vessel and on the water tank for each boiler transmit the pressures and temperatures through the A controller to the systems management general-purpose computer. The computer computes the pressure, volume and temperature and transmits the water tank quantity to panel F8 for each boiler. The APU fuel/H20 qty switch on panel F8 is positioned to allow the water quantity of each boiler to be displayed on the APU fuel/H20 qty 1,2 or 3 meter. Thus, water quantity is available only when the A controller is powered.

Downstream of the water storage tank, the feedwater lines to each water boiler split into two parallel lines: one line goes to the hydraulic fluid flow section and one to the lube oil section of the auxiliary power unit. A hydraulic-fluid water feed valve is installed in the water line to the hydraulic fluid section, and a power unit lube oil water feed valve is installed in the water line to the lube oil section of the power unit. Each valve is controlled independently by the boiler controller.

The two boiler controllers are operated by the respective boiler cntlr pwr/htr 1,2 and 3 switches on panel R2. When the applicable switch is positioned to A, the A controller for that boiler is powered; if it is positioned to B, the B controller is powered. The off position of the applicable switch removes electrical power from both controllers.

The boiler contlr 1,2 and 3 switches on panel R2 enable (provide the automatic control functions) the specific controller A or B selected for that boiler by the boiler contlr pwr/htr 1,2 and 3 switches on panel R2. When the applicable controller A or B is enabled for that boiler, a ready signal is transmitted to the corresponding APU/hyd ready to start talkback indicator (along with other prerequisites from the auxiliary power unit and hydraulic system) on panel R2 if the following additional conditions are met: gaseous nitrogen shutoff valve is open, steam vent nozzle temperature is greater than 130º F, and hydraulic fluid bypass valve is in the correct position with regard to the hydraulic fluid temperature.

The core of each water spray boiler is a stainless steel crimped-tube bundle. The hydraulic fluid section is divided into three 17-inch-long passes of smooth tubes (first pass—234 tubes, second pass—224 tubes and third pass—214 tubes). The lube oil section of the auxiliary power unit comprises two passes with 103 crimped tubes in its first pass and 81 smooth tubes in the second pass. The tubes are 0.0125 of an inch in diameter with a wall thickness of 0.010 of an inch. Crimps located every 0.24 of an inch break up the internal boundary layer and promote enhanced turbulent heat transfer. Although the second pass is primarily a low-pressure drop return section, approximately 15 percent of the unit's lube oil heat transfer occurs there.

Three connected spray bars feed the hydraulic fluid section, while two spray bars feed the power unit's lube oil section in each boiler.

When the orbiter is in the vertical position on the launch pad, each boiler is loaded with up to 3.5 pounds of water, which is referred to as pool mode operation. When each auxiliary power unit/hydraulic system and water spray boiler is in operation five minutes before lift-off, the power unit tube bundle and hydraulic tube bundle are immersed in the boiler water precharge pool mode operation. Liquid level sensors in each water boiler prevent the water feed valves from pulsing to avoid water spillage or loss. As the vehicle ascends during launch, the lube oil system of the auxiliary power unit heats up, eventually the boiler water precharge boils off, and the boiler goes into the spray mode. The hydraulic fluid usually does not heat up enough during ascent to require spray cooling.

The enabled controller of the operating water spray boiler monitors the hydraulic fluid and lube oil outlet temperature from the auxiliary power unit. The hydraulic fluid outlet temperature controls the hydraulic-fluid water feed valve, and the power unit's lube oil outlet temperature controls the lube oil water feed valve. Signals are based on a comparison of the hydraulic system fluid temperature to its 208º F set point and of the lube oil of the power unit to its 250º F set point. When the respective water feed valve opens, instantaneous flows of 10 pounds per minute maximum through the hydraulic section and 5 pounds per minute maximum through the lube oil section of the power unit enter the water boiler through the corresponding spray bars to begin evaporative cooling of the hydraulic fluid and auxiliary power unit lube oil. The steam is vented out through the overboard steam vent.

The separate water feed valves modulate the water flow to each section of the tube bundle core in each water spray boiler independently in 200-millisecond pulses that vary from one pulse every 10 seconds to one pulse every 0.25 of a second.

Because of the unique hydraulic system fluid flows, control valves are located in the hydraulic system fluid line section of each water spray boiler. Normally, hydraulic system fluid flows at up to 21 gallons per minute; however, the hydraulic system experiences one- to two-second flow spikes at up to 63 gallons per minute. If these spikes were to pass through the boiler, pressure drop would increase ninefold and the boiler would limit the flow of the hydraulic system. To prevent this, a relief function is provided by a spring-loaded poppet valve that opens when the hydraulic fluid pressure drop exceeds 48 psi and is capable of flowing 43 gallons per minute at a differential pressure of 50 psi across the boiler. A temperature controller diverter valve allows the hydraulic fluid to bypass the boiler when the fluid temperature decreases to 190º F. At 210º F, the controller commands the diverter valve to direct the fluid through the boiler. When the hydraulic fluid cools to 190º F, the controller again commands the diverter valve to route the fluid around the boiler.

Each water boiler, water tank and steam vent is equipped with electrical heaters to prevent freeze-up in orbit. The water tank and boiler electrical heaters are activated by the corresponding boiler cntlr pwr/htr 1, 2 and 3 switches on panel R2. The A or B position of each switch selects the A or B heater system and is automatically controlled by the corresponding A or B controller. The steam vent heaters are also activated by the boiler cntlr pwr/htr 1, 2 and 3 switches but only if the boiler cntlr 1, 2 or 3 switch on panel R2 is on. The water tank and boiler heaters are cycled on a 50º F and off at 55º F. The steam vent heaters are not operated continuously in orbit; they are activated approximately two hours before auxiliary power unit startup. The steam vent heaters are cycled on at 150º F and cycled off at 175º F.

When the auxiliary power unit/hydraulic combination is started for atmospheric entry and the hydraulic fluid and power unit lube oil flow commences and fluid temperatures rise, spraying is initiated as required. During the lower part of entry, when the boiler temperature reaches 188º F, the water spray boiler returns to the pool mode. The spray bars begin discharging excess water to fill the boiler. When the water reaches the liquid level sensors, the spray is turned off so that the boiler is not overfilled. During entry, because the orbiter's orientation is different from that of launch, the boiler can hold up to 14 pounds of water.

Each water spray boiler transmits data to the systems management summary CRT for display. Data displayed for each boiler consist of water quantity, gaseous nitrogen pressure, gaseous nitrogen regulated pressure, bypass valve status, gaseous nitrogen and water tank temperature, and boiler temperature.

The boiler system controllers are powered up at launch minus 4 hours. The boiler water tanks are pressured at T minus one hour and 10 minutes in preparation for auxiliary power unit activation. The boiler system controllers activate heaters on the water tank, boiler and steam vent to ensure that the water spray boiler is ready to operate for launch.

Auxiliary power unit start is delayed as long as possible to save fuel. At T minus six minutes, the pilot begins the power unit prestart sequence—confirming that the water spray boiler is activated—activates the auxiliary power unit controllers and depressurizes the main hydraulic pump. Depressurizing the main pump reduces the starting torque on the auxiliary power unit. The pilot then opens the auxiliary power unit fuel tank valves and looks for three APU ready to start gray talkbacks. At T minus five minutes, the pilot starts the three power units by setting the APU cntl switches to start/run and checks the hydraulic power gauges for an indication of approximately 600 to 1,000 psi. Then the pilot pressurizes the main pump and looks for approximately 3,000 psi on the gauges. All three hydraulic main pump pressures must be greater than 2,800 psi by T minus four minutes, or the automatic launch sequencer will abort the launch.

The auxiliary power units operate during the ascent phase and through the first orbital maneuvering system thrusting period. At the conclusion of the main engine purge, dump and stow sequence, the auxiliary power units and water spray boilers are shut down. The same sequence applies for a delayed OMS-1 thrusting period. If an abort once around is declared, the auxiliary power units are left running but the hydraulic pumps are depressurized to reduce power unit fuel consumption. The auxiliary power units are left running to avoid restarting hot power units for deorbit and re-entry.

Six hours after lift-off, the heater gas generator/fuel pump heaters of the auxiliary power units are activated to operate for the remainder of the orbital mission. The fuel and water line heaters of the power units are also activated to prevent the lines from freezing as the units cool down.

A few hours after lift-off, the landing gear isolation valves on hydraulic systems 2 and 3 are opened so that the pumps can circulate hydraulic fluid through the lines. Because these valves will not open or close unless the pressure in the line is at least 100 psi, the main hydraulic pump or hydraulic circulation pump must be active. The hydraulic system 1 landing gear isolation valve is left closed because of the danger of inadvertently lowering the landing gear while the vehicle is in orbit.

Two hours after lift-off, the steam vent heaters of the water spray boilers are turned on and left on for about 1.5 hours to eliminate all ice from the boiler steam vents.

While the vehicle is in orbit, the hydraulic circulation pumps are in the GPC mode (controlled by the general-purpose computers): automatically activated when hydraulic line temperatures become too low and automatically deactivated when the lines warm up sufficiently.

On the day before deorbit, one auxiliary power unit is started to supply hydraulic pressure for checkout of the flight control system. (Hydraulic pressure is needed to move the orbiter aerosurfaces during checkout.) The associated water spray boiler controller is activated, and landing gear isolation valves 2 and 3 are closed. Then one auxiliary power unit (selected by the Mission Control Center) is started. The hydraulic main pump is set to normal pressure (approximately 3,000 psi), and aerosurface drive is checked. After about five minutes, the checks are complete and the power unit is shut down. Normally, the auxiliary power unit does not run long enough to require water spray boiler operation. The landing gear isolation valves on hydraulic systems 2 and 3 are reopened after the auxiliary power unit is shut down.

At 2.5 hours before the deorbit thrusting period, the steam vent heaters of the water spray boiler are activated to prepare the boiler systems for operation during entry. At about the same time, the landing gear isolation valves on hydraulic systems 2 and 3 are closed, and the circulation pumps are turned off.

At 45 minutes before deorbit, the water tanks of the boiler systems are pressurized, the APU controllers are activated, and the main hydraulic pumps are set at low pressure. The pilot opens the fuel tank valves of the auxiliary power units and looks for three gray APU/hyd rdy talkbacks. The pilot then closes the fuel tank valves. This procedure takes place while the pilot is in contact with the ground so that flight controllers can observe the status of the auxiliary power units. Five minutes before the deorbit thrusting period, one power unit (selected by Mission Control) is started to ensure that at least one unit will be operating for entry.

The hydraulic pump is left in low. This auxiliary power unit operates through the deorbit burn. At 13 minutes before entry interface (400,000-foot altitude), while the orbiter is still in free fall, the other two auxiliary power units are started and all three hydraulic pumps are pressurized (norm). Two space shuttle main engine hydraulic isolation valves are cycled open, then closed, to ensure that the engines are stowed for entry. Two minutes later, if required, the aerosurfaces are put through an automatic cycle sequence to make sure warm hydraulic fluid is available in the aerosurface drive units.

After touch down, a hydraulic load test may be conducted to test the response of the auxiliary power units and hydraulic pumps under high load (i.e., high flow demand) conditions. This test cycles the orbiter aerosurfaces with one hydraulic system at a time in depressed mode (the remaining two power units and hydraulic pumps have to drive all the aerosurfaces). This is typically done on the first flight of a new vehicle. The main engine hydraulic isolation valves are opened again, and the engines are moved to the transport position. At this point, the hydraulic systems are no longer needed, and the auxiliary power units and water spray boilers are shut down.

The contractor for the water spray boilers is Hamilton Standard Division, United Technologies Corp., of Windsor Locks, Conn.