CHAPTER 11 SYSTEMS: THERMAL CONTROL—THE “CIRCULATORY SYSTEM” OF THE INTERNATIONAL SPACE STATION 206 called the Nitrogen Introduction Assembly contains two separate two-way valves used to control loop pressure at the pump. The first, called the Nitrogen Introduction Assembly Introduction Valve, provides access to the ISS nitrogen system to maintain or increase the loop pressure via an accumulator in the pump package of each loop. The other, called the Nitrogen Introduction Assembly Vent Valve, allows the nitrogen in each ITCS accumulator to be vented into the cabin to reduce loop pressure. M M Three Way Mixing Valve Low Temperature Loop Moderate Temperature Loop Pump Package Assembly Heat Loads Interface Heat Exchanger Three Way Mixing Valve Regenerative Three Way Mixing Valve Pump Package Assembly Heat Loads Interface Heat Exchanger Loop Crossover Assembly System Flow Control Assembly System Flow Control Assembly M Figure 10. Schematic of the LAB Internal Thermal Control System. In the LTL, cold water (dark blue) comes out of the IFHX and, if needed to adjust the temperature, mixes with warm water (purple) at the TWMV. The adjusted water flows to the heat loads. It returns through the System Flow Control Assembly, which regulates flow through the loads and the flow bypassing the loads to ensure steady flow rates throughout the system. After leaving the System Flow Control Assembly, flow proceeds via the Pump Package Assembly to the heat exchanger to reject the heat that the water has picked up. The MTL behaves the same way, cooling hardware that operates at higher temperatures. In contingency cases, the Loop Crossover Assembly can be opened between the loops to create one big loop using only one of the two Pump Package Assemblies. In the big loop configuration, the Regenerative TWMV allows warm water at the outlet of the MTL pump to warm the water flowing toward the MTL heat loads to ensure the MTL coolant is at the right temperature for the warmer loads. Not all modules in the US On-orbit Segment have crossover assemblies. The two ITCS loops in the LAB operate at different temperatures to support different cooling needs in various systems. The colder of the two, known as the Low Temperature Loop (LTL), operates at 9.4˚C (48.9˚F) whereas the Moderate Temperature Loop (MTL) operates at 17.2˚C (63.0˚F). These control temperatures are optimized for the cooling of different equipment in the LAB, but each TWMV is capable of controlling to a broad range of temperatures, limited by the capability of the heat exchanger that is providing the cooling and the heat provided by the equipment being cooled. The LTL services mostly the environmental and life support systems equipment and payload racks that require cooling to lower temperatures. The MTL services mostly avionics and electronics across a variety of systems. The dew point on the ISS is kept in the 8°C to 10°C (46°F to 50°F) range, meaning that water will condense on any surface colder than those temperatures. For that reason, the LTL lines are covered in thick insulation to prevent any moisture collection. Since the MTL operates at temperatures above the dew point, the MTL fluid lines are uninsulated. A pair of 8-port valves, together called the Loop Crossover Assembly, allow the two ITCS loops to be connected in series such that one pump can provide flow to a single, larger loop. This mode can be used if there is a problem with one of the pumps. In the single loop configuration, water flows through the LTL loads and then through the MTL loads. The water temperature at the outlet of the LTL loads is not usually warm enough to ensure that the fluid lines will remain above the ISS dew point therefore, the water must be warmed before flowing into uninsulated MTL fluid lines. A water-to-water heat exchanger, called the Regenerative Heat Exchanger, transfers heat from the warmest water in the system at the outlet of the MTL, to the cooler water at the outlet of the LTL to ensure water moving from the LTL lines to the uninsulated MTL lines is warmer than the ISS dew point. A three-way valve known as the Regenerative TWMV controls how much of the warmest MTL water passes through the Regenerative Heat Exchanger to control the resultant temperature of the water flowing out of the LTL and into the MTL. In the first years of LAB ITCS operation, the system was run as two separate loops, known as the dual-loop configuration. After an MTL pump failure in 2003, the ISS thermal community decided to preserve operational life on all remaining pumps by operating the ITCS in the