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
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
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.