Performance of a Nafion Water Vapor Exchanger in an Amine Bed Test Loop

Thermal and environmental control systems for future exploration spacecraft must meet challenging requirements for efficient operation and conservation of resources. Maximizing the use of regenerative systems and conserving water are critical design considerations. This paper presents the results of testing a Nafion-based water vapor exchanger (WVX) in an amine bed test loop under conditions that simulate operation in a spacecraft life support system with a regenerative CO2 removal system. The WVX comprises an innovative assembly of Nafion tubes that achieves high water recovery (80-90%) with very low pressure losses (< 0.5 in. H2O). Nafion is an attractive material for this application because of its very high permeability for water vapor. However, Nafion is sensitive to poisoning by ammonia which is present in trace quantities in the outflow from amine-based CO2 removal beds. We measured the performance of a prototype WVX built by Creare in an amine-bed test loop at UTC Aerospace Systems. We found that water recovery efficiencies for short-duration tests were in the range 80-90%. These data come from tests run with flow rates and humidity levels that simulate the range of possible cabin conditions in future exploration spacecraft. These data are very consistent with the water recovery efficiencies measured in Creare’s laboratories without the amine beds. Pressure drop measurements by UTAS are also consistent with pressure drop data from Creare. These data are also consistent with separate effects tests in which we measured the water permeability of an unstructured Nafion tube bundle exposed to ammonia under accelerated test conditions. The accelerated tests can be used to extrapolate the lifetime of a Nafion WVX when used in conjunction with an amine bed.