Condensing heat exchangers (CHX) are used in many applications, including space life support systems, to control temperature and humidity. Temperature control is achieved by transfer of the heat load to a circulating coolant. Simultaneously, humidity control is provided by cooling the air below its dew point, and separating the condensed water from the gas flow. In space, the condensate does not drain from the heat exchanger because of the absence of gravity. To overcome this problem, slurping condensing heat exchangers have been developed that combine a hydrophilic coating on the air flow passages and an additional slurping section added to the air outlet of the heat exchanger to achieve efficient air-water separation.
For short missions such as those typical for shuttle flights, microbial proliferation in the coatings has not been a major issue, despite the fact that the coatings are continuously moist and an ideal breeding ground for microbial species. For longer missions, such as those proposed for the International Space Station, hydrophilic coatings containing biocides are required to prevent microbial growth and proliferation.
A hydrophilic, antimicrobial coating has been developed for the condensing heat exchanger and filter assembly (CHFXA) for the Columbus Orbital Facility of the International Space Station. The coating is a single-phase, alumina-based, ceramic coating containing silver oxide as a biocide. It can be applied to complex shaped components using conventional coating processes. A significant amount of biocide can be included in such single-phase coatings without apparent detriment to physical properties. The development and characterization of this coating are described here.