Thermal Control Systems Overview
Anyone wishing to be a THOR, ASTRO, or PRO needs to be very familiar with the thermal
control systems onboard the ISS. The ISS has two Active Thermal Control Systems (ATCS),
which are the Internal Active Thermal Control System (IATCS) and the External Active Thermal
Control System (EATCS) and a Passive Thermal Control System (PTCS). The following
overview will detail the components of all three Thermal Control Systems (TCS).
Exhibit 1. ISS Thermal Control System Architecture
Thermal Control
System (TCS)
Active TCS
Internal ATCS
External ATCS
Passive TCS
Insulation
Coatings
Electric Heaters
However, before we start you should become familiar with the acronyms used for the
components of these systems, which are listed below:
Acronym
Long Form
TCS
Thermal Control System
IATCS
Internal Active Thermal Control System
EATCS
External Active Thermal Control System
LTL
Low Temperature Loop
MTL
Moderate Temperature Loop
CCAA
Common Cabin Air Assembly
CDRA
Carbon Dioxide removal Assembly
ExPRESS
Expedite the PRocessing of Experiments to Space Station
HX
Heat Exchanger
IFHX
Interface Heat Exchanger
MFCV
Manual Flow Control Valve
RFCA
Rack Flow Control Assembly
PPA
Pump Package Assembly
SFCA
System Flow Control Assembly
TWMV
Three Way Mixing Valve
LCA
Loop Crossover Assembly
RHX
Regenerative Heat Exchanger
MBSU
Main Bus Switching Units
DDCU
Direct Current- to Direct Current Converter Unit
PM
ATA
NTA
RBVA
TRRJ
ML
ORU
Pump Module
Ammonia Tank Assembly
Nitrogen Tank Assembly
Rotary Beam Valve Module
Thermal Radiator Rotary Joint
Multilayer Insulation
Orbital Replaceable Unit
What are the Three Major Functions of the Internal Active Thermal Control System?
The three major functions of the IATCS are heat collection, heat transportation, and heat
rejection. Each function employs various Orbital Replaceable Units (ORUs), which are
components that are able to be replaced if a malfunction occurs within the unit. The IATCS is
responsible for making sure that the electrical equipment and experiments onboard the ISS do
not overheat. This is accomplished by first collecting the waste heat generated by the equipment
and experiments and then transporting this waste heat to the External Active Thermal Control
System (EATCS), which then collects, transports, and dissipates this heat into space.
Exhibit 2. Overview of Internal Active Thermal Control System
Functions and its Orbital Replaceable Units
Heat Collection
Heat Transportation
Coldplates
SFCA
Heat
Exchangers
PPA
RFCA
NTA
MFCV
TWMV
Heat Rejection
IFHX
How is Heat Collection Accomplished on the IATCS?
The IATCS has two separate loops of piping with water flowing through them that are used to
collect waste heat. One loop is called the Low Temperature Loop (LTL) and the other is the
Moderate Temperature Loop (MTL). Each loop has different water temperatures to service
different equipment. The LTL was designed to carry water at 4.4 degrees Celsius, but it is
presently running at 10.5 degrees Celsius. The MTL was designed to carry water at 17.2 degrees
Celsius, which is also the present temperature of the water. All the temperatures on the IATCS
will be recorded in Celsius. However, it is easy to convert Celsius to Fahrenheit by the formula
F = 1.8 times the temperature in Celsius plus 32. For example, 10.5 degrees Celsius is 1.8 times
10.5 plus 32, which equals 50.9 degrees Fahrenheit. Not only is the water temperature
maintained in the loops, but also the pH of the water. The pH is running at 8.4, which is slightly
basic.
The Low Temperature Loop (LTL) nominally services two pieces of equipment. They are the
Common Cabin Air Assembly (CCAA), which is similar to an air conditioner, and a Carbon
Dioxide Removal Assembly (CDRA), which is used as a carbon dioxide scrubber on the ISS.
Waste heat from the CCAA and the CDRA is collected by Coldplates and heat exchangers.
ORUs are bolted down to Coldplates that collect the waste heat from the ORU. The Coldplate
has channels with water flowing through them. The cool water from the loop enters the
Coldplate through a gamah fitting on one end of the Coldplate. As the water flows through
channels in the Coldplate, it becomes warmer due to conduction and forced convection of the
heat moves the waste heat in the channels. This process allows the ORU or equipment to become
cooler and the Coldplate’s water to become warmer. The warmer water exits the Coldplate
through a gamah fitting that is an inlet to one of the loops on the IATCS.
Exhibit 3. Coldplate
Heat exchangers also collect heat from ORUs or experiments by removing the heat in the air
through conduction and then transporting the heat away through the MTL or LTL by convection.
Exhibit 4. Heat Exchanger on the Common Cabin Air Assembly
Heat Exchanger
In the Billings Gazette on July 2, 2010 an article titled Logistical issues delay end of shuttle
program states, “ NASA announced today that the final shuttle mission will be rescheduled for
February 2011. The delay was in part due to having to get a heat exchanger ready for launch.”
The heat exchanger is an Orbital Replaceable Unit (ORU).
The Moderate Temperature Loop (MTL) collects waste heat from all the rest of the
equipment and experiments on the ISS that is not serviced by the Russian Space Agency. The
equipment and experiments are located in the three research modules: Destiny, JEM, and
Columbus, along with airlocks and nodes.
Besides the temperature differences, and the different equipment that the two loops service, the
appearances of the loops are different. The LTL pipes have insulation wrapped around it, while
the MTL pipes do not have insulation. The reason for the insulation on the LTL is that if the
water inside the loops is at 4.4 degrees Celsius this is below the dew point temperature, in which
water cannot hold all its water vapor so that water condenses out onto the pipes. The dew point is
not a problem for the MTL because its temperature of 17.2 degrees Celsius, which is above the
dew point.
Exhibit 5. Top Piping is LTL with Insulation, Bottom Piping is MTL
LTL
MTL
In the research modules, experiments are placed in ExPRESS racks, which stand for Expedite the
PRocessing of Experiments to Space Station. Each ExPRESS rack has a Rack Flow Control
Assembly (RFCA) that regulates the flow of water through that particular rack. If experiments
are generating a lot of waste heat, then the flow of water is increased. This variation of flow of
water is controlled by computer software, but it has a manual override in case electrical power or
connectivity to the computer is lost. Each ExPRESS rack also has a Z panel in the front of it.
The Z panel allows for easy interface with the ISS systems available to the payloads or
experiments. Z panels have a connection for the experiments to both the MTL and the LTL,
along with electrical power, vacuum, and gaseous nitrogen.
Exhibit 6. ExPRESS Rack
Z Panel
Exhibit 7. Z panel in ExPRESS RACK
Just like the RFCA has a manual valve to control the flow of water to the ExPRESS racks so
does the LTL and MTL have Manual Flow Control Valves (MFCVs). This is because the
ORUs on a loop generate a known quantity of heat so that the MFCV can be set on Earth and not
have to be adjusted. A standalone temperature sensor is used to check that the correct
temperature is being maintained for the water in the loop.
Exhibit 8. Manual Flow Control Valve (MFCV)
In summary, heat collection is accomplished by water acquiring waste heat from equipment and
experiments by conduction and convection. The waste heat is collected in Coldplates and Heat
Exchangers aligned with the LTL and the MTL. The two loops service different equipment and
experiments that produce waste heat. The experiments are located on ExPRESS racks, in which
each have a RFCA to control the flow of water to the rack. The greater the flow of water to the
ExPRESS racks the more waste heat can be collected and carried away.
How is Heat Transportation Accomplished on the IATCS?
The IATCS transports heat through water in the MTL and LTL loops. The flow of water in the
loops is maintained and controlled by three devices, which are the System Flow Control
Assembly (SFCA), the Pump Package Assembly (PPA), and the Three Way Mixing Valve
(TWMV).
The System Flow Control Assembly (SFCA) controls the water flow or pressure for the entire
LTL or MTL system. There is one SFCA for the LTL and another for the MTL. The SFCA is
commanded by computer software. However, there is a manual control valve called Header
Pressure Control Valve and a Pump Inlet Shutoff Valve. The Header Pressure Control Valve has
variable speeds, but the Pump Inlet Shutoff Valve has either a close or open position.
The Pump Package Assembly (PPA) maintains the water pressure for each loop. There is a
separate PPA for the LTL and another for the MTL. The PPA works by passing the incoming
water from a loop through a fine filter that can filter out particles as small as 2 microns. Then
the water passes through a gas filter that filters out any gases dissolved in the water. Both filters
have a sensing device to tell you if they need to be changed. In the PPA, there is an Accumulator
that helps keep the water at a steady pressure. After the water flows through the Accumulator, it
flows to the inlet of the pump impeller (a small metal disc the size of a quarter that rotates
extremely fast to propel the water through the pump assembly). The water after leaving the
pump impeller passes through a course filter to catch any debris that might have occurred if the
pump itself looses a part.
Exhibit 9. Pump Impeller
TD_520_014
Figure 2-17. Impeller scale schematic
The Accumulator in the PPA has three important functions. It insures a positive pressure to the
pump impeller, by the use of gaseous nitrogen in a bellow system, in which gaseous nitrogen is
used to apply or release pressure on the water passing through the bellows of the Accumulator.
The Accumulator can add more water to the loop if there is a loss of water, and it accommodates
for the expansion or contraction of water in the loop due to changes in temperatures. Gaseous
nitrogen is obtained for the Accumulator from the Nitrogen Interface Assembly (NIA). When
the NIA adds nitrogen to the Accumulator this increases the pressure on the water side of the
Accumulator, which in turn increases the water pressure in the loop. The NIA is computer
software controlled, however it does have two manual open or close override values. By
manually opening up the Vent Valve you can relieve pressure in the loop by venting out the
nitrogen in the Accumulator directly to the cabin of the ISS. The Isolation Valve on the NIA
puts pressure on the bellows of the Accumulator by adding nitrogen if the switch is open. A
caution warning above these switches warns of an asphyxiation hazard. This danger occurs if
you have the Vent Valve open (in which you are allowing nitrogen to enter the cabin) and the
Isolation Valve open at the same time. Because that means you are putting nitrogen into the
Accumulator at the same time you are allowing the nitrogen in the Accumulator to enter the
cabin. The danger is of allowing too much nitrogen into the cabin that can cause asphyxiation.
Exhibit 10. The Vent Valve on the Left and the Isolation Valve on
the Right and for the Nitrogen Interface Assembly
Exhibit 11. Top of Rack is the Pump Package Assembly
Bottom is the System Flow Control Assembly
Gas Filter
Fine Filter
Accumulator
Pump Inlet Shutoff
Header Pressure
Control Valve
The Three Way Mixing Valve (TWMV) mixes the heated water from the equipment or
experiments with cooler water in the loop. The TWMV is like a facet that mixes cold and hot
water to give you the temperature of the water that you desire. After water leaves the TWMV, it
is at the set temperature for the LTL or MTL, whichever it is located within.
Exhibit 12. Three Way Mixing Valve
Redundancy of the IATCS is achieved through an amazing attribute of the IATCS, which is
that the two loops can functioning both separately or together as one loop and still maintain the
set temperatures for both loops. The combining of the two loops into one is accomplished by a
single valve in the correct position on the Loop Crossover Assembly (LCA). This allows the
two loops to operate as one and therefore, they would require only one PPA or SFCA to be in
working order. The LCA therefore provides a means of redundancy for the IATCS system. If
there is a malfunction in one of the loops, then you are able to bypass the malfunctioning
equipment and substitute it with the same equipment in the other loop. The use of a
Regenerative Heat Exchanger (RHX) and a Regenerative Three Way Mixing Valve allows
the temperatures of both loops when combined to function at their set temperature points.
Exhibit 13. The Loop Crossover Assembly
In Summary, heat transportation is facilitated in each loop by its System Flow Control Assembly
(SFCA), the Pump Package Assembly (PPA), and the Three Way Mixing Value (TWMV).
When necessary the two loops can become one, which provides an important means of
redundancy for the IATCS. This redundancy is accomplished by the Loop Crossover Assembly
(LCA), the Regenerative Heat Exchanger (RHX), and the Regenerative Three Way Mixing
Valve.
How is Heat Rejection Accomplished on the IATCS?
Heat rejection means taking the heat from the water of the Internal Active Thermal Control
System (IATCS) and transferring it to the External Active Thermal Control System (EATCS).
This process of heat rejection occurs on the truss of the ISS in the Interface Heat Exchangers
(IFHXs). Each IATCS loop has its own IFHXs. Therefore, you can consider an IFHX
belonging to both the IATCS and the EATCS. The IFHX is made up of 45 layers. Twenty-three
of these layers have the heated water from the LTL or MTL and twenty-two of these layers have
anhydrous ammonia that is contained in the loops of the EATCS. The layers of water and
ammonia alternate in the IFHX so that the heated water transfers its heat to the cooler ammonia.
Since there is anhydrous ammonia in the IFHX, it needs to be located on the outside of the ISS,
because anhydrous ammonia is toxic.
In summary, the IATCS collects, transports, and rejects waste heat from the equipment and
experiments onboard the ISS. The heat is collected by water travelling in two loops that carry
water at different set temperatures. The loops can function separately or be combined by the use
of a Loop Crossover Assembly (LCA). This LCA provides an important factor of redundancy to
the IATCS in case of a malfunction in one of the loops. The warmed water travels to Interface
Heat Exchangers (IFHXs), where it transfers heat to anhydrous ammonia in the External Active
Thermal Control System (EATCS).
What are the Three Major Functions of the External Active Thermal Control System?
The three major functions of the External Active Thermal Control System (EATCS) are heat
collection, heat transportation, and heat rejection. Each function employs various Orbital
Replaceable Units (ORUs), which are able to be replaced if a malfunction occurs within the unit.
The EATCS is responsible for making sure that the waste heat from the IATCS is rejected into
space and that any electrical equipment on the outside of the ISS does not overheat. The EATCS
collects the heat from the Interface Heat Exchanger (IFHX) and Coldplates of equipment and
transfers it to the radiators on the truss.
Exhibit 14. Overview of External Active Thermal Control System
Functions and its Orbital Replaceable Units
Heat Collection
Heat Transportation
IFHX
PM
Coldplates
Valves
Heat Rejection
Radiators
TRRJ
ATA
NTA
RBVA
How is Heat Collection Accomplished on the EATCS?
The EATCS has two separate loops of piping with a hundred percent of liquid anhydrous
ammonia (ammonia without any water in it) flowing through them that are used to collect waste
heat. One loop is called Loop A and the other is Loop B. Each loop is responsible for collecting
the waste heat from one of the IATCS loops. Loop A is responsible for collecting the waste heat
from the LTL Interface Heat Exchanger, and Loop B is responsible for collecting the waste heat
from the MTL Interface Heat Exchanger. Loop A and B also collect waste heat from externally
mounted Coldplates. These Coldplates collect heat from the Main Bus Switching Units (MBSU)
and the Direct Current-to Direct Current Converter Units (DDCU). The MBSU distributes the
electricity generated by the solar panels to the interior of the ISS. The DDCU converts the power
from the solar panels into the electricity that is distributed by the MBSU.
Exhibit 15. Direct Current-to Direct Current Converter Unit
Sitting on top of a Coldplate that is attached to Frame of Truss
DDCU
Coldplate
Frame to Truss
Anhydrous ammonia is used instead of water in the EATCS loops, because on the outside of the
ISS temperatures get extremely cold. Water freezes at zero degrees Celsius, but anhydrous
ammonia does not freeze until – 107.9 degrees Celsius. However, even ammonia with this low
freezing point is in danger of freezing on the outside of the ISS where temperatures could drop to
-157 degrees Celsius.
The Interface Heat Exchangers (IFHX) and the Coldplates located on the outside of the ISS
are the ORUs that collect heat in the EATCS. The IFHX collects the heat from the LTL and
MTL of the IATCS. The Coldplates collect waste heat from the MBSU and DDCUs that are
located on the truss system of the ISS. The Coldplates of the EATCS are slightly different than
the Coldplates of the IATCS. The EATCS’s Coldplates have anhydrous ammonia running
through its channels instead of water found in the IATCS’s Coldplates. The EATCS’s
Coldplates also have fins that increase their surface area because their channels transfer waste
heat through radiation between the DDCU or MBSU and their Coldplates. In the ammonia lines
themselves the heat is transferred by conduction.
How is Heat Transportation Accomplished on the EATCS?
Heat transportation is accomplished on the EATCS by the use of a Pump Module, Control
Valves, Ammonia Tank Assembly (ATA), Nitrogen Tank Assembly (NTA), and Rotary Beam
Valve Module (RBVA). These components work together to allow the liquid anhydrous
ammonia in Loop A and B to flow through the system of pipes on the outside of the ISS.
The Pump Module (PM) on the EATCS is similar to the Pump Package Assembly of the
IATCS. Both systems have Control Valves and an Accumulator that functions similar to the
Accumulator on the IATCS. The EATCS’s Accumulator performs the same three functions as
the IATCS’s Accumulator, which are to keep the fluid pressure steady, to replace any fluids that
are lost (water on the IATCS and anhydrous ammonia on the EATCS), and to compensate for
any expansion or contraction of the fluids that are being transported in the loops. The EATCS’s
Accumulator has a fourth function, which is to keep the ammonia in the liquid state. Also the
EATCS’s Accumulator has ammonia and nitrogen on each side of its bellows, while the PPA
Accumulator has water and nitrogen. The PM and the PPA have similar pump impellers that
keep the fluids moving throughout its loops.
Exhibit 16. Pump Module for EATCS
Accumulator
Control Valve
Electric Heater Wrap
The Ammonia Tank Assembly (ATA) is part of the EATCS. Its purpose is to supply Loop A
and Loop B with 100% anhydrous ammonia at the correct pressure so that it flows as a liquid
within the loops. The regulation of pressure of the ammonia entering the loops is accomplished
by the use of gaseous nitrogen from a Nitrogen Tank Assembly (NTA). The NTA consists of
nitrogen under high pressure so that it can efficiently control the pressure of the ammonia in the
ATA, which it is connected to. Therefore, it is the nitrogen in the NTA that applies the pressure
to the ammonia to keep it circulating throughout the loops.
Exhibit 17. Ammonia Tank Assembly for EATCS
Ammonia Tank
Isolation Vent
Vent Valve
Valve where Ammonia Exits
the Pump Module
Exhibit 18. Nitrogen Tank Assembly for EATCS
The Rotary Beam Valve Module (RBVM) controls the flow of ammonia in the loops to the
radiators. This valve can shut down the ammonia flowing into the radiators. The radiators are
the final destination of Loop A and B. It is in the deployed radiators that the waste heat, which is
collected in both the IATCS and the EATCS is radiated out to space. Each radiator has two
RBVMs.
How is Heat Rejection Accomplished on the EATCS?
Radiators are used to reject the heat from the EATCS into space. The radiators are fully
deployable and retractable along with the ability to swivel so that they can maximize heat
rejection into space. They are made of honeycomb aluminum panels. These panels measure six
feet by ten feet and carry a total of 1680 square feet of piping in Loop A and B.
Exhibit 19. Radiator During Deployment Testing
Form website: http://www.boeing.com/defensespace/space/spacestation/systems/docs/ISS%20Active%20Thermal%20Control%20System.pdf
The Thermal Radiator Rotary Joint (TRRJ) allows the radiator beam to rotate. When the
radiators are in the sunshine they are turned with their edges toward the sun to maximize heat
radiation away from the radiators and into space. When the Earth eclipses the sun from the ISS
then the radiators are turned to face the Earth, so that the radiators can receive some of the heat
radiated from Earth. The direction and deployment of the radiators is controlled by the Thermal
Control System Software, which is an intricate computer program that monitors temperatures
within the radiators.
Exhibit 20. Thermal Radiator Rotary Joint
Thermal Radiator
Rotary Joint
In summary, heat is collected in Loop A and Loop B by liquid anhydrous ammonia travelling to
Interface Heat Exchangers and Coldplates to collect waste heat. The heat is transported through
Loop A and B by the use of Control Valves, Pump Modules, Ammonia Tank Assembly,
Nitrogen Tank Assembly, and Rotary Beam Valve Modules all of which are used to maintain the
temperature and pressure of the ammonia flowing in the loops. The final outcome of the EATCS
is to radiate the accumulated heat in Loop A and B into space through deployable radiators that
can swivel on a Thermal Radiator Rotary Joint.
What is the Passive Thermal Control System?
The Passive Thermal Control System on the ISS has no fluids running through it, which is in
contrast to the Active Thermal Control System. The Passive Thermal Control System has three
components that are commonly found in heating and air-conditioning systems on Earth. The
three components are Multi-Layer Insulation (MLI), surface coatings, and electrical heaters.
These components are employed both inside and outside the ISS to insure that appropriate
temperatures are maintained. The following website gives a good description of how the Passive
Thermal Control System on the ISS works.
http://science.nasa.gov/science-news/science-at-nasa/2001/ast21mar_1/
The Multi-Layer Insulation (MLI) is specially designed for the ISS. It can keep heat in for
warmth and keep heat out for cooling. It is made of highly reflective aluminized Mylar, Kapton,
and Dacron. The reflectivity of the insulation reflects the sun’s rays off the ISS so that the ISS
does not heat up excessively, even when temperatures climb to 121 degrees Celsius. When the
ISS is surrounded by freezing temperatures, even as low as -157 degrees Celsius, the ISS is able
to stay warmer by the MLI’s ability to keep the cold temperatures from penetrating the ISS. The
MLI not only keeps the heat out and warmth in, but it also helps to keep harmful radiation waves
out along with protecting the ISS from space debris. Most of the outside of the ISS is covered
with MLI. However, MLI is also used inside the ISS as a protection for crew from contact with
hot equipment.
Surface Coatings and Paints vary throughout the ISS inside and out to provide for the different
requirements of reflectivity, absorptivity, and emissivity. Each of these three concepts is
important in understanding the contributions of coatings and paints to thermal control.
Reflectivity is the ability of a material to reflect or not absorb the radiant energy falling upon it.
A material high in reflectivity is aluminum, because it reflects the radiant energy coming from
the sun. The Multi-Layer Insulation is aluminized to reflect the suns radiant energy, so that the
ISS does not become too hot. Absorptivity is the opposite of reflectivity. A material has good
absorptivity if it has the ability to absorb radiant energy falling upon it. Good absorptivity
occurs when you paint an object black. Solar panels on the ISS are a dark color so that they can
absorb the radiant energy from the sun. Emissivity varies from absorptivity because emissivity
is the ability of an object to emit radiant energy or to radiate. Light colors are good emitters of
radiation. For example, the radiators on the ISS are white, which helps them to emit their radiant
energy.
Electric Heaters just like those found on Earth are used to maintain proper temperatures inside
and outside the ISS. There are over 300 electrical heaters used on the ISS. An example where a
heater is needed is on the piping of the loops in the EATCS. Anhydrous Ammonia has a low
freezing point, but sometimes temperatures do go below its freezing point. Therefore, electrical
heaters are wrapped around the loops to keep them from freezing when temperatures drop (see
Exhibit 15 Pump Module).
There are three types of heaters used on the ISS. They are operational heaters, survival heaters,
and shell heaters. The type of heater employed depends on what the heater is needed to
accomplish. If a component might need heat while it is operating, such as the pipes carrying the
ammonia in the EATCS then the heater type is operational. If the heat is needed when the
component is turned off, as a Coldplate might need if its ORU is not operational, then the heater
type is survival. Finally, if a heater is needed to prevent condensation from forming around
equipment or experiments in modules, then the heater type is shell. Shell heaters could be used to
prevent condensation on the Low Thermal Loop of the IATCS when insulation alone is not able
to prevent condensation.
In summary, the Passive Thermal Control System is similar to the insulation, surface coatings,
paints, and electrical heaters used on Earth. Along with the IATCS and the EATCS the PTCS
makes sure that crew, equipment, and experiments on the ISS are maintained at optimal
temperatures.