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This paper focuses on the details of a Cabin Heat eXchanger (CHX) Dryout in Columbus, the European Laboratory part of the International Space Station (ISS), and the unique and challenging conditions, which occur during a high beta angle phase. A particular case happening between DOY154 and DOY163 of the year 2013 will be analyzed. Furthermore the paper illustrates how these difficult conditions can be overcome by the Flight Control Team at the Columbus Control Centre (COL-CC). A CHX Dryout and some of the challenges, which have been encountered during this specific high beta angle phase, are described below. The CHX Dryout is a regular maintenance activity, which is performed every 6 weeks (+/1 week) to change from one CHX core to the other. This activity is done after an extended period of time of the active CHX Core being exposed to water, in order to prevent microbial and fungal growth on the CHX hydrophilic coating. Such fungus growth may cause hardware damage and possible crew health issues, therefore is particularly important that a Dryout is regularly scheduled. The activity is nominally preceded by a Wet cycle phase lasting 7 days, in which the set point of the CHX control law is decreases to induce more condensation in the active core and to flush the core prior to its deactivation. In the particular case described here, the wet cycle has been skipped due to some anomalies in the TCS subsystem, which resulted in the impossibility of changing the CHX inlet temperature set point. At initiation of the CHX Dryout, the alternate Columbus Water Separator Assembly (CWSA) is activated, and the active CHX core, through which water is flowing, is swapped from one to the other. During the Dryout, both CWSA1 & CWSA2 are left active for a minimum of 16 hours. Having both CWSA on for that long amount of time ensures collection and separation of the condensate from the CHX to be dried out. At the end of the Dryout, the CWSA corresponding to the dried core is deactivated. An increase of humidity in the cabin is observed in the first few hours of the Dryout, which results in a higher dew point in the cabin. During high beta angle period, the Heater Control Unit (HCU) temperatures in certain areas, which are constantly shaded, are lower than usual. To avoid condensation of the Columbus external shell, the difference between the dew point and the lowest temperature on any of the heater strings shall be maintained at least at 4.4◦C. While performing the Dryout during a high beta angle period, this requirement can be violated and corrective measures have to be taken to activate parts of the redundant HCU, supporting the active HCU in heating up the shell and avoid the risk of condensation. Another additional challenge in performing a Dryout during an high beta angle phase is the fact that, when the redundant HCU is turned on, all its 6 heater strings are enabled, so the initial power draw can be quite high and might exceed the power allocated to Columbus during such a critical power phase. Only certain strings need to be active, hence all should be disabled as soon as possible and, in case additional limitations are in place, different strings have to be activated in sequence to guarantee that the power limits are not violated and that there is no built up of condensation in Columbus. |
