Cabin Air Revitalization

There are five independent air loops in the cabin: the cabin itself, three avionics bays and inertial measurement units. The cabin pressure atmosphere is circulated by the air revitalization system. The air circulated through the flight crew cabin picks up heat, moisture, odor, carbon dioxide and debris with additional heat from electronic units in the crew cabin. The cabin air is drawn through the cabin loop and through a 300-micron filter by one of two cabin fans located downstream of the filter.

Each cabin fan is controlled by its respective cabin fan A and B switch on panel L1. Normally, only one fan is used at a time. The cabin fans are located under the middeck floor.

The cabin air from the cabin fan is ducted to the two lithium hydroxide canisters, where carbon dioxide is removed and activated charcoal removes odors and trace contaminants. An orifice in the duct directs a specific amount of cabin air through each lithium hydroxide canister. The canisters are also located under the middeck floor. They are changed alternately every 12 hours through an access door in the floor. For a flight crew of seven, the lithium hydroxide canisters are changed alternately every 11 hours. Replacement canisters are stored under the middeck floor between the cabin heat exchanger and water tanks.

Cabin air is then directed to the crew cabin heat exchanger located under the middeck floor and cooled by the water coolant loops. Humidity condensation that forms in the slurper of the cabin heat exchanger is removed by a fan separator that draws air and water from the cabin heat exchanger. The moist air is drawn from the slurper into the humidity separator fan, where centrifugal force separates the water from the air. The fan separator removes up to approximately 4 pounds of water per hour. The water is routed to the waste water tank, and the air is ducted through the exhaust for return to the cabin. There are two fan separators controlled individually by humidity sep A and B switches on panel L1. The humidity sep A switch controls humidity separator fan A, and the B switch controls humidity separator fan B. Normally, only one fan separator is used at a time. The relative humidity in the crew cabin is maintained between 30 and 65 percent in this manner. A small portion of the revitalized and conditioned air from the cabin heat exchanger is ducted to the carbon monoxide removal unit, which converts carbon monoxide to carbon dioxide.

Based on the crew cabin volume of 2,300 cubic feet and 330 cubic feet of air per minute, one volume crew cabin air change occurs in approximately seven minutes, and approximately 8.5 air changes occur in one hour.

A bypass duct carries cabin air around the cabin heat exchanger and mixes it with the revitalized and conditioned air to control the crew cabin air temperature in a range between 65 and 80 F. When the cabin temp cntlr switch on panel L1 is positioned to 1 , it enables controller 1. The rotary cabin temp cool/warm switch on panel L1 selects and automatically controls the bypass valve by diverting zero to 70 percent of the air flow around the cabin heat exchanger depending on the position of the cool/warm rotary switch. The controllers are attached to a single bypass valve by an actuator arm. If controller 1 malfunctions, the actuator arm linkage must be removed from controller 1 by the flight crew at panel MD44F and connected manually to controller 2 before the cabin temp cntlr switch on panel L1 is positioned to 2. This enables controller 2 and permits the rotary cool/warm switch to control controller 2 and the single bypass control valve. The cabin temp cntlr switch's off position removes electrical power from both controllers, the cabin temp cool/warm switch and automatic control of the single bypass valve.

The cabin heat exchanger outlet temperature is transmitted to the cabin hx out av bay rotary switch on panel O1. When the switch is positioned to cabin hx out , the temperature can be monitored on the panel O1 air temp meter. The cabin heat exchanger outlet temperature provides an input to the yellow av bay/cabin air caution and warning light on panel F7. The C/W light is illuminated if the cabin heat exchanger outlet temperature is above 65 F or if the cabin fan delta pressure is 2.8 inches of water or above 7.1 inches of water.

The air from the cabin heat exchanger and the bypassed air come together in the supply duct and are exhausted into the crew cabin through consoles and middeck and various station duct outlets into the crew cabin.

If cabin temperature controllers 1 and 2 or the cabin temp cool/warm rotary switch is unable to control the single bypass valve, the flight crew can position the single bypass valve actuator drive arm to the desired position and pin the bypass valve in place at panel MD44F. The full cool position at panel MD44F establishes the maximum cabin air flow rate to the cabin heat exchanger, the 2/3 cool position establishes a flow rate that provides approximately two-thirds of the maximum cooling capability, the 1/3 cool position establishes a flow rate that provides approximately one-third of the maximum cooling capability, and the full heat position establishes the minimum cabin air flow rate to the cabin heat exchanger.

The cabin air is also used to cool the three avionics equipment bays and some of the electronic avionics units in the avionics bays in addition to the three IMUs. Each of the three avionics equipment bays in the middeck has a closeout cover to minimize air interchange and thermal gradients between the avionics bay and crew cabin; however, the closeout cover for each avionics equipment bay is not airtight. The electronic avionics units in each avionics bay meet outgassing and flammability requirements to minimize toxicity levels. Each of the three avionics equipment bays has identical air-cooled systems. Two fans per avionics equipment bay are controlled by individual avionics bay fan A and B switches on panel L1. Normally, only one fan is used at a time. When the A or B switch for an avionics bay is positioned to on, the fan draws cabin air from the floor of the avionics bay, through the applicable air-cooled avionics units, through connectors at the back of the applicable air-cooled units, to the cabin fan inlet, through a 300-micron filter and to the cabin fan. The cabin fan outlet directs the air through that avionics bay heat exchanger. The water coolant loops flow through the heat exchanger to cool the fan outlet air, and the cooled air is returned to the avionics bay. A check valve in the outlet of the fan that is not operating prevents a reverse flow through that fan. The air outlet from the fan in each avionics bay is monitored and transmitted to the cabin hx out av bay 1,2,3 rotary switch on panel O1. When the rotary switch is positioned to av bay 1, 2 or 3 , that avionics bay's fan outlet temperature can be displayed on the air temp meter on panel O1. The air outlet temperature of each avionics bay also provides an input to the yellow av bay/cabin air C/W light on panel F7. This light is illuminated if any of the avionics bay outlet temperatures are above 135 F. The off position of the A or B switch removes power from that avionics bay fan.

The three IMUs are cooled by one of three fans drawing cabin air through a 300-micron filter and across the three IMUs. The fan outlet air flows through the IMU heat exchanger and is cooled by the water coolant loops before returning to the crew cabin. Each IMU fan is controlled by the IMU fan A, B, C switches on panel L1. The on position turns the corresponding fan on and the off position turns it off. Normally, one fan is sufficient because one fan cools all three IMUs. A check valve is installed on the outlet of each fan to prevent a reverse air flow through the fans not operating.

If the payload bay contains the Spacelab pressurized module, a kit is installed to provide ducting for the flow of cabin air from the middeck through the airlock and tunnel to the module. The humidity separators, cabin fans, cabin heat exchanger, avionics bay heat exchangers, IMU heat exchanger, waste water tank, lithium hydroxide filters, carbon monoxide unit, and waste and potable water tanks are located beneath the middeck crew compartment floor.