The vapor membrane dehumidification device and method for air cycle environment control systems uses a semipermeable vapor membrane for water vapor, air separation. The vapor membrane has a high water to air permselectivity. When air containing water vapor is passed over or through the semipermeable fibers of the membrane countercurrently or crosscurrently to a purge air stream the water vapor may permeate through the fibers. Using a purge air stream of lower water vapor partial pressure relative to the feed moist air stream, the water vapor is moved to the purge air stream due to the partial pressure differential of water vapor and then expelled overboard with the purge air stream. The purge air stream can be replaced with any source of air internal or external to the aircraft such as air from the aircraft cabin.
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