Challenges to Cabin Humidity Removal Presented by Intermittent Condensing Conditions

On-orbit temperature and humidity control (THC) is more easily accomplished when the THC hardware is either consistently dry (i.e., no humidity control is occurring), or consistently wet. The system is especially challenged when intermittent wet/dry conditions occur. The first six years of on-orbit ISS operations have revealed specific concerns within the THC system, specifically in the condensing heat exchanger and the downstream air/water separator. Failed or degraded hardware has been returned to ground and investigated. This paper presents the investigation findings, and the recommended hardware and procedural revisions to prevent and recover from the effects of intermittent condensing conditions. INTRODUCTION The control of temperature and humidity in an enclosed environment presents particular challenges when applied under micro-gravity conditions. The removal of humidity is especially accommodated by gravity in earth-bound applications, but must be enabled using additional machinery when gravity is absent. In the case of the U.S. modules of the International Space Station (ISS), which utilize a Common Cabin Air Assembly (CCAA) to accomplish temperature and humidity control (THC), ground testing and analysis were particularly geared toward addressing conditions of maximum humidity production. This approach confirmed that the temperature and humidity could be controlled within required limits for reasonable maximum heat and humidity burdens within the respective ISS cabins. However, after six years of on-orbit operating experience, the greatest challenges experienced to date arise from low or intermittent heat and humidity loads. The following report summarizes the issues surrounding intermittent condensing conditions within the air side of the ISS CCAA. Figure 1 shows a schematic cutaway of the CCAA, with the process air entering on the left and exiting on the right. The air is drawn into the THC Return Ducting by the Inlet ORU (i.e., the fan), and then travels through either the Heat Exchanger (HX) or a bypass manifold (as diverted by the Temperature Control and Check Valve, or TCCV) as necessary to maintain the desired cabin temperature. The process air then passes across the temperature sensors and enters the downstream distribution ducting. The condensate from the HX is drawn into the Water Separator (WS), which separates the water from entrained air, and delivers it into a water bus (not shown). The entrained air is returned at the end of the CCAA just upstream from the distribution ducting. [insert Figure 1] Normally over the course of long-term operations, the objective is to operate a CCAA in condensing mode for about one month, and then to allow a dryout period for the sake of microbial control on the HX fins. For the central U.S. Laboratory Module (USL), where continuous THC operations are required, two CCAA’s (“Port”, and “Starboard”) alternate their duty cycles to accommodate this long-term cycle. In the ISS Airlock (A/L), the need for THC is not continuous, which intrinsically accommodates the periodic dryout for microbial control. Thus, there is a certain, limited, quantity of wet/dry cycling which is intentional Intermittent condensing conditions in the CCAA HX could also occur, for the most part unintentionally, if the humidity-control function is being accomplished elsewhere onboard ISS. This is often the case, as the preference to-date has been to control ISS humidity from the Russian Segment (RS). Under this operating regime, condensation might only occur sporadically in the USL or A/L CCAA’s, particularly during periods of high latent loads. Intermittent condensation within the CCAA HX also manifests itself as a periodic dryout in the CCAA Water Separator (WS). The concerns associated with periodic dryout (also called “wet/dry cycles”) of the CCAA HX and