Hollow composite fibers selectively permeable to water vapor

A hollow composite fiber which comprises:

• (a) an asymmetric, hollow, polymeric fiber comprising poly(vinylidene fluoride), sulfonated polysulfone, polysulfone, cellulose ester, or mixtures thereof having no selectivity of the water vapor in air or carbon dioxide, and having an inside diameter of from 130 to 900 microns and an outside diameter of from 450 to 1,500 microns, said asymmetric hollow fiber comprising (i) a porous layer including a plurality of intercommunicating cells, the largest dimensions of which are less than 125 microns, with at least 80% of said cells having dimensions ranging from 0.5 to 20 microns and the pore volume of said cells constituting from 30 to 70% of the total volume of said asymmetric hollow fiber and (ii) a dense layer defining either the external or internal wall member of the asymmetric hollow fibers having pores, the diameters of which are less than 0.5 micron and the thickness thereof ranging from 0.1 to 1 micron;
• (b) a polymer layer coated onto the dense layer (ii) of said asymmetric hollow fiber;
• and
• (c) said hollow composite layer having a permeability relative to water vapor greater than to air or carbon dioxide and, thus, essentially no selectivity for airborne oxygen or to nitrogen.