Revised Cabin Loop Concept for COLUMBUS APM 941306
The redesign of the international Space Station Freedom (SSF) and funding constraints in the ESA member states caused a redirection of the development effort for the Attached Pressurised Module (APM). For the ECLSS the most important changes are the reduction in length of the module in order to make it compatible with the ARIANE V capabilities and the more severe cost constraints. As a result new concepts for the cabin loop were investigated leading to a decrease in cabin loop power consumption, mass and volume and a reduced development effort due to a lower number of items.
In the previous concept a module internal loop with a flow rate of 864m3/hr and an Intermodule Ventilation (IMV) flow rate for air revitalisation to the station with 240m3/hr were installed. The revised boundary conditions with a reduced overall massflow rate of 540m3/hr allows the combination of the cabin loop and the IMV with limited impact on the total power consumption. The pressure rise needed to drive the cabin loop air flow can also be used to drive the IMV airflow. Different options for such a cabin loop concept were established and analysed with respect to configuration, power consumption and mass. Another intention was to reduce the number of hardware items needed or at least to use the same hardware in several places.
Several different options that allow to eliminate the IMV fans were identified. However, at least two identical cabin fans are required. The location of the filters proved to be a design driver. Prevention of microbial contamination of the Condensing Heat Exchanger (CHX) and the cabin loop components located downstream the CHX requires an air filtration upstream the CHX also for the IMV supplied air. This can be achieved either by a dedicated filter for the IMV supplied air plus distributed filters in the air return grids or by a centralised filter. In the latter case the fire detection for the cabin can be performed by sensors located in the air return ducts,whereas in the first case these sensors have to be located in the cabin itself. The control of the air flows is achieved with variable speed fans and either modulating valves or a 3-way valve. For modified IMV boundary conditions it is also possible to operate without feedback control.
All options revealed significant savings in power, mass, envelope and development costs. The selection of the baseline option also has to consider additional system aspects as e.g. the detailed configuration of the endcone located components, the potential accommodation of smoke sensors on the cabin walls and additional ducts from rack to the endcone. Another aspect to be considered is the assignment of hardware responsibilities to subcontractors as e.g. the filter housing as integral part of the CHX.