A device based on the transport of water through a membrane to a vacuum has been developed for dehumidifying a stream of air in the life-support system of a spacecraft or space suit. The device could also be adapted to terrestrial use in dehumidification of air in an air-conditioning stream or drying of air or another gas in a chemical processing stream. The design of this device is an advance in that it decreases (relative to prior designs) the weight, power consumption, and volume of the dehumidifier in the life-support system or other gas-circulation system in which the device is used. In the case of a spacecraft or space suit, the design thereby also increases safety and health margins. Although the membranes in the device must be replaced periodically and a vacuum source is essential for its operation, no other dehumidifier works as well in a spacecraft or space suit.

Water Vapor Diffuses from flowing vent gas through water permeable membranes into a vacuum.

Unless controlled, the concentrations of CO2 and H2O in respirable air in a spacecraft or space suit quickly reach unacceptably high levels. Heretofore, life-support systems in spacecraft and space suits have included solid metal hydroxides for depleting exhaled CO2, and condensers integrated with cooling sources for removing excess moisture. To enable the condensers to function, such systems must also include microgravitational phase separators and coolant/heat sink subsystems, all of which contribute to weight, volume, and consumption of power.

The present device operates without need for either a coolant/heat sink subsystem or a microgravitational phase separator. Moreover, in the original outer space application, the vacuum needed for operation is available naturally, so that it is not necessary to incur the cost, weight, and power penalty of a vacuum source. Hence, in comparison with breathing air systems of prior design, it is possible to simplify breathing loop interfaces, reduce weight and volume, and decrease the amount of power expended, thereby also saving on the cost of fuel.

The device contains one or more modules in which spaces in a vent-layer assembly alternate with spaces in a vacuum-layer assembly and the vent and vacuum spaces are separated by flat sheets of a polyelectric membrane material that is permeable by water (see figure). The vent-layer assembly includes a vent frame, a metal foam insert, and screens. The frame is rectangular and is sized to optimize the pressure drop characteristics of the vent-gas and water-vapor flows. The metal foam insert is bonded to the top and bottom of the frame and sandwiched between a pair of screens that provide additional support so that the membrane is not torn against the surface of the metal foam. Flow holes connect the central section of the frame to header slots and allow humid gas to enter the frame without leaking into the vacuum space. Dovetailed O rings grooved to fit the perimeter of the frame retain a gasket during assembly of the module. The thickness of the vent layer is chosen to be the minimum thickness needed to maintain top and bottom gasket seals.

The vacuum-layer assembly is similar to the vent-layer assembly except that its flow holes are on the short ends and there is no header for a vacuum-layer, eliminating the need for another set of gaskets on each layer. Inasmuch as the flow of water vapor is a fraction (typically 4 percent by volume) of the flow of the gas to be dehumidified (hereafter called vent gas for short), the vacuum frame can be made thinner than the vent frame to minimize overall volume. This frame is so thin that a single gasket, held in place by a retaining ring, seals against layers above and below the vent layers. The number and size of vacuum holes satisfy two constraints: (1) the minimum equivalent area limits the back pressure of vapor; and (2) the maximum equivalent area restricts the vent flow if one or more membranes rupture. Top and bottom end plates, sized to provide sufficient stiffness, apply an even pressure to the gaskets to prevent leakage. These plates also include ports through which the vent gas flows into and out of the module, plus ports for testing the vent gas.

Modules like the one described above are stacked in alternating layers, beginning and ending with a vacuum frame. The modular nature of the device provides flexibility for changing the membrane area to satisfying requirements for the conditions of a particular application.

This work was done by Karen Murdoch and C. H. Miller of United Technologies for Johnson Space Center. MSC-22878


NASA Tech Briefs Magazine

This article first appeared in the April, 2002 issue of NASA Tech Briefs Magazine.

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