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What is the Internal Active Thermal Control System (IATCS)?
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- Exhibit 6. Interface Heat Exchanger (IFHX)
- The Interface Heat Exchanger
- Exhibit 7. Coldplate (Black Cushion) on Main Bus Switching Unit
What is the Internal Active Thermal Control System (IATCS)? There are two systems an internal and external system that makes up the ATCS. The internal loop is made up of systems of pipes in which water is circulated. Water is a good conductor of heat. As the water circulates throughout the interior of the ISS, it accumulates waste heat from the equipment. Water is used because it is not toxic or flammable. There are two internal loops, one for low temperatures (4.40 degrees Celsius) and one for moderate temperatures (17.2 degrees Celsius). These two loops can act independently or as a single loop. The system is set up so that when it is acting as a single loop it still maintains the difference in temperatures for the low temperature loop and the moderate temperature loop. Visually you can tell the two loops apart because the Low Temperature Loop (LTL) has insulation wrapped around it to prevent condensation of water, since the temperature of the loop is below the dew point. This insulation prevents condensation. The water transports its waste heat through the loops to the Interface Heat Exchangers (IFHX) located on the exterior of the station. The Interface Heat Exchangers transfers the heat from the internal water loops to the External Active Thermal Control System. Exhibit 6. Interface Heat Exchanger (IFHX) 
The Interface Heat Exchanger takes heated water and cool ammonia in and outputs cool water and heated ammonia. What is the External Active Thermal Control System (EATCS)? There are two independent loops on the External Active Thermal Control System (EATCS). Loop A is at the starboard side (right) of the ISS and its Interface Heat Exchanger connects to the Low Temperature Loop. Loop B is at the port side (left) of the ISS and its Interface Heat Exchanger connects to the Moderate Temperature Loop. Anhydrous ammonia is used in these external loops. Ammonia is used because of its extremely low freezing point of -77.2 degrees Celsius, at standard atmospheric pressure. This is important since when the ISS is not experiencing daylight, temperatures get extremely cold in space. Both loop A and B collect waste heat from five Interface Heat Exchangers (IFHX) and four coldplates that are externally mounted on the ISS. The coldplates are part of the externally mounted Main Bus Switching Units (MBSU) and the Direct Current-to Direct Current Converter Units (DDCU). The coldplates collect the waste heat generated from the MBSU and the DDCU so that they do not overheat and stop functioning. There are 2 Main Bus Switching Units (MBSU) and 2 Direct Current-to Direct Current Converter Units on loop A and B. The DDCU converts power and the MBSU distributes electricity to the interior and the exterior of the ISS. If a malfunction occurs within the DDCU or MBSU then electricity to the ISS is diminished. Electricity is needed to run heaters and Pump Modules (PM), that are components of the EATCS. Electrical heaters are wrapped around the external loops to maintain temperatures when the ISS is eclipsed by earth from the sun. Without these heaters the ammonia in the loops could freeze because temperatures outside the ISS go below the -77.2 degrees Celsius, the freezing point of ammonia. The Pump Modules control the flow of ammonia, which helps to regulate the temperature and pressure of ammonia inside the loops. Normal operating temperature in the loops is 2.7 degrees Celsius and normal operating pressure is 300 psia. A Pump & Control Valve Package (PCVP) contains the necessary components to regulate and monitor the temperature and pressure of the ammonia within the loops. Exhibit 7. Coldplate (Black Cushion) on Main Bus Switching Unit 
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