Crew Compartment Cabin Pressurization

The cabin is pressurized to 14.7 psia, plus or minus 0.2 psia, and maintained at an average 80-percent nitrogen and 20-percent oxygen mixture by the air revitalization system. Oxygen partial pressure is maintained between 2.95 and 3.45 psi, with sufficient nitrogen pressure of 11.5 psia added to achieve the cabin total pressure of 14.7 psia, plus or minus 0.2 psia.

The pressurization system consists of two oxygen systems and two gaseous nitrogen systems. The two oxygen systems are supplied by the PRSD oxygen system, which is the same source that supplies oxygen to the orbiter fuel cell power plants. The PRSD cryogenic supercritical oxygen storage system is controlled by electrical heaters within the tanks and supplies the oxygen to the ECLSS pressurization control system at a pressure of 835 to 852 psia in a gaseous state. The gaseous nitrogen supply system consists of two systems with two gaseous nitrogen tanks for each system. The nitrogen storage tanks are serviced to a nominal pressure of 2,964 psia at 80º F. If the auxiliary gaseous oxygen supply tank is installed, it is serviced to 2,440 psia at 80º F and stores 67.6 pounds of gaseous oxygen to provide high flow along with gaseous nitrogen. It would maintain the crew cabin at 8 psi with oxygen partial pressure at 2 psia. For normal on-orbit operations one oxygen and nitrogen supply system is used. For launch and entry both oxygen and nitrogen supply systems are used in addition to repressurization of the airlock.

The heart of the cabin pressurization is the nitrogen/oxygen control and supply panels, the PPO 2 sensor, and crew cabin positive and negative pressure relief valves. The nitrogen/oxygen control panel selects and regulates primary (system 1) or secondary (system 2) oxygen and nitrogen. The primary and secondary nitrogen/oxygen supply panels are located in the lower forward portion of the midfuselage. The primary and secondary oxygen supply systems have a crossover capability, as do the primary and secondary nitrogen supply systems. If installed, the auxiliary oxygen supply system is also controlled by the supply panel.

The oxygen and nitrogen supply systems provide the makeup cabin oxygen gas consumed by the flight crew and nitrogen for pressurizing the potable and waste water tanks and repressurizing the airlock. An average of 1.76 pounds of oxygen is used per flight crew member per day. Up to 7.7 pounds of nitrogen and 9 pounds of oxygen are expected to be used per day for normal loss of crew cabin gas to space and metabolic usage. The potable and waste water tanks are pressurized to 17 psia.

Oxygen from the respective PRSD cryogenic oxygen supply system is routed to the atmosphere pressure control oxygen sys tem 1 and system 2 supply valves. The atmosphere pressure control oxygen system 1 and system 2 supply valves are controlled by a switch on panel L2. When the switch is positioned to open, the corresponding oxygen system valve opens to permit oxygen to flow through an oxygen restrictor at a maximum flow of 20 pounds per hour and to a heat exchanger in the Freon-21 coolant loop (oxygen system 1 through Freon coolant loop 1 and oxygen system 2 through Freon coolant loop 2), which warms the oxygen supply to the oxygen regulator of that system. A talkback indicator next to the switch indicates op when the valve is open. When the atm press control O 2 sys 1 or sys 2 switch is positioned to close, the valve is closed, isolating that oxygen supply system. The talkback indicator indicates cl . A check valve downstream of the heat exchanger prevents oxygen from flowing from one supply source to the other if the crossover valves are open. Downstream of the oxygen check valve is a manifold with an oxygen systems 1 and 2 crossover valve that would permit system 1 to system 2 or vice versa. The crossover valves are controlled by the atm press control O 2 xovr sys 1 and sys 2 switches on panel L2. When the respective switch is positioned to open, that oxygen supply system is directed to airlock supply oxygen 1 and 2 manual valves, airlock oxygen 1 and 2 extravehicular mobility unit, and eight face mask outlets. If both atmosphere pressure control oxygen pressure system 1 and system 2 crossover valves are opened, oxygen supply systems 1 and 2 are interconnected. When the respective atm press control O2 press sys 1 or sys 2 xover switch is positioned to close, that oxygen supply system is isolated from the crossover feature.

The oxygen supply systems are directed to their corresponding oxygen regulator inlet manual valve. When the valve is manually positioned to open on panel M010W, the oxygen supply system is directed to its oxygen regulator, which reduces that oxygen supply source pressure to 100 psi with a minimum flow rate capability of 75 pounds per hour. Each regulator is a two-stage regulator with the second stage functioning as a relief valve when the differential pressure across the second stage is 215 psi. The relief pressure is vented into the crew cabin. This regulated pressure passes through another check valve and is directed to its 14.7-psi cabin regulator inlet manual valve, 8-psi regulator and payload oxygen manual valve for Spacelab (if the Spacelab pressurized module is installed in the payload bay) on panel M010W. The check valve on each oxygen supply system between the oxygen regulator and 14.7-psia cabin regulator prevents the reverse flow of oxygen and nitrogen into the oxygen system since the 14.7-psi and 8-psi regulators in each system control the oxygen and nitrogen flow to the cabin as required to maintain the desired cabin pressure.

The two primary and two secondary gaseous nitrogen supply tanks are constructed of filament-wound Kevlar fiber with a titanium liner. Each nitrogen tank is serviced to a nominal pressure of 3,300 psia at 80º F with a volume of 8,181 cubic inches. The two nitrogen tanks in each system are manifolded together. The primary and secondary nitrogen supply systems are controlled by the atmosphere pressure control nitrogen supply valves in each system. Each valve is controlled by its corresponding atm press control N 2 sys 1 and 2 supply switch on panel L2. When a supply switch is positioned to open, that nitrogen supply system is directed to its corresponding atmosphere pressure control system regulator inlet valve. An indicator adjacent to the switch indicates barberpole when the motor-operated valve is in transit and op when the supply valve is open. When the supply switch is positioned to close, that nitrogen supply system is isolated from the nitrogen system regulator inlet valve, and the talkback indicator indicates cl.

The inlet valve in each nitrogen system is controlled by its respective atm press control N 2 sys 1 and 2 reg inlet switch on panel L2. When a reg inlet switch is positioned to open , that system's nitrogen source pressure is directed to the system's nitrogen regulator. A talkback indicator next to the reg inlet switch indicates barberpole when the motor-operated valve is in transit and op when the valve is open. When the reg inlet switch is positioned to close, the nitrogen supply pressure is isolated from the system's nitrogen regulator, and the talkback indicator indicates cl.

The nitrogen regulators in the primary and secondary supply system reduce the pressure to 200 psi. Each nitrogen regulator is a two-stage regulator with the second stage functioning as a relief valve. The second stage relieves pressure overboard at 245 psi.

The regulated pressure of each nitrogen system is directed to the nitrogen manual crossover valve, the water tank regulator inlet valve and the oxygen and nitrogen controller valve in each system.

The nitrogen crossover manual valve connects both regulated nitrogen systems when the valve is open and isolates the nitrogen supply systems from each other when closed. A check valve between the nitrogen regulator and nitrogen crossover valve in each nitrogen-regulated supply line prevents flow from one nitrogen source supply pressure to the other if the nitrogen crossover valve is open.

The partial pressure of oxygen in the flight crew cabin can be controlled automatically by one of two oxygen and nitrogen controllers. Two PPO2 sensors are located under the crew cabin flight deck mission support console. The PPO 2 A and B sensors provide inputs to the PPO2 control systems 1 and 2 controller and switches, respectively.

When a PPO2 contr switch is positioned to norm on panel M010W and the atm press control PPO 2 snsr/vlv switch on panel L2 is positioned to norm, electrical power is supplied to the corre sponding atm press control O 2 /N 2 cntlr vlv switches on panel L2 for system 1 or 2. When the atm press contlr vlv switch is positioned to auto, electrical power automatically energizes or de-energizes the corresponding nitrogen control valve and nitrogen-regulated supply. When the corresponding PPO 2 sensor determines that oxygen is required in the crew cabin to maintain the level at 3.5 psi, the nitrogen supply valve is automatically closed. When the 200-psi nitrogen supply in the manifold drops below 100 psi, the corresponding oxygen supply system flows through its check valve and 14.7-psi cabin regulator into the crew cabin. When the PPO2 sensor determines that the oxygen in the crew cabin is at 3.2 psi, the corresponding nitrogen supply system valve is automatically opened, the 200-psi nitrogen enters the oxygen and nitrogen manifold and closes the corresponding oxygen supply system check valve, and nitrogen flows through the 14.7-psi regulator into the crew cabin. The open and close positions of the O 2 /N 2 cntlr vlv sys 1 and 2 switch on panel L2 permit the flight crew to control the nitrogen valve in each system manually, and thus cabin pressure is controlled manually. The reverse position of the PPO 2 snsr/vlv switch on panel L2 allows controller B to system 1 and controller A to system 2.

If the 14.7-psi cabin regulator inlet manual valves of systems 1 and 2 are closed on panel M010W, the crew module cabin pressure will decrease to 8 psi. The PPO2 contr sys 1 and sys 2 switches on panel M010W are positioned to emer for the corresponding nitrogen system, which selects the 2.2-psi oxygen partial pressure. The corresponding PPO 2 sensor and controller, through the corresponding PPO 2 contr switch and the PPO2 snsr/vlv switch positioned to norm , provide electrical inputs to the corresponding O 2 /N 2 cntrl vlv switch. The electrical output from the applicable O 2 /N 2 cntrl vlv switch controls the nitrogen valve in that supply system in the same manner as in the 14.7-psi mode except that the crew module cabin oxygen partial pressure is maintained at 2.2 psi.

The oxygen systems 1 and 2 and nitrogen systems 1 and 2 flows are monitored and sent to the O 2 /N 2 flow rotary switch on panel O1. The rotary switch permits system 1 oxygen or nitrogen or system 2 oxygen or nitrogen flow to be monitored on the flow meter on panel O1 in pounds per hour.

PPO2 sensors A and B monitor the oxygen partial pressure and transmit the signal to the PPO 2 sensor select switch on panel O1. When the switch is positioned to sensor A, oxygen partial pressure from sensor A is monitored on the PPO 2 meter on panel O1 in psia. If the switch is set on sensor B, oxygen partial pressure from sensor B is monitored. The cabin pressure sensor transmits directly to the cabin press meter on panel O1 and is monitored in psia.

The red cabin atm caution and warning light on panel F7 is illuminated for any of the following monitored parameters:

A klaxon will sound in the crew cabin and the master alarm push button light indicators will be illuminated if the change in pressure versus change in time decreases at a rate of 0.05 psi per minute or greater. The normal cabin dP/dT is zero psi per minute, plus or minus 0.01 psi, for all normal operations.

The temperature and pressure of the primary and secondary nitrogen and emergency oxygen tanks are monitored and transmitted to the systems management computer. This information is used to compute oxygen and nitrogen quantities.

The two cabin relief valves are in parallel to provide overpres surization protection of the crew module cabin above 16 psid. Each cabin relief valve is controlled by its corresponding switch on panel L2. The cabin relief A switch controls cabin relief A, and the cabin relief B switch controls cabin relief B. When the switch is positioned to enable, the corresponding motor-operated valve allows the cabin pressure to a corresponding positive pressure relief valve that relieves at 16 psid and reseats at 15.5 psid. The relief valve maximum flow capability is 150 pounds per hour. A talkback indicator above the respective switch indicates barberpole when the motor-operated valve is in transit and op when the motor-operated valve is open. When the switch is positioned to close , the corresponding motor-operated valve isolates cabin pressure from the relief valve, and the talkback indicator indicates cl.

The crew module cabin vent isolation valve and cabin vent valve are in series to vent the crew cabin to ambient pressure. Approximately one hour and 30 minutes before lift-off, the crew module cabin is pressurized to approximately 16.7 psi for leak checks of the crew cabin. The cabin vent isolation valve is controlled by the cabin vent , vent isol switch on panel L2, and the cabin vent valve is controlled by the cabin vent, vent switch on panel L2. Each switch is positioned to open to control its respective motor-operated valve. When both valves are open, the cabin pressure is vented into the midfuselage. The maximum flow capability through the valves at 0.2 psid is 900 pounds per hour. A talkback indicator above each switch indicates the position of the respective valve-barberpole when the valve is in transit and op when it is open.

If the crew cabin pressure is lower than the pressure outside the crew cabin, two negative pressure relief valves in parallel will open at 0.2 to 0.7 psid, permitting flow of ambient pressure into the crew cabin. The maximum flow rate at 0.5 psid is zero to 654 pounds per hour.

The manual water tank nitrogen regulator inlet valve in each nitrogen-regulated supply system permits nitrogen to flow to its corresponding regulator and water tank nitrogen regulator isolation manual valve. The inlet and isolation manual valves are on panel M010W. The water regulator in each nitrogen system reduces the 200-psi supply pressure to 16 psi. Each regulator is a two-stage regulator with the second stage relieving pressure into the crew cabin at a differential pressure of 20 psi. The nitrogen pressurization system for the potable and waste water tanks is discussed later in this section.