NOVELTY - Fuel cell system (700) includes an oxidant delivery assembly coupled to the electrochemical conversion assembly. The assembly manipulates at least one property of oxidant delivered from an oxidant source and prior to use of the oxidant in an electrochemical conversion assembly. The oxidant delivery assembly has a compressor (722), a heat exchanger (730), a water vapor transfer unit (732), and a housing (702) defining a chamber (705, 706). A controller monitors and varies the extent to which the oxidant delivery assembly modifies the oxidant.    USE - For mobile (e.g. vehicular) applications.    ADVANTAGE - The fuel cell system facilitates manipulation of one or more airflow properties without having to resort to approaches that require significant increases in system weight, redundancy, volume or complexity. The system has minimum presence of liquid water in the air exiting the air delivery system, since the vaporization process effected by the combined operation of the water vapor transfer unit and the heat exchanger promotes the formation of vapor. The system is of reduced complexity, eliminates extra pressure-reduction device that promotes the higher reactant pressures sometimes needed in fuel cell applications, and accommodates different types of compressors with minimum adjustments.    DETAILED DESCRIPTION - Fuel cell system comprises an electrochemical conversion assembly to combine an oxidant and a reductant. An oxidant delivery assembly is coupled to the electrochemical conversion assembly. It manipulates at least one property of oxidant delivered from an oxidant source and prior to use of the oxidant in the electrochemical conversion assembly. The oxidant delivery assembly has a compressor, a heat exchanger, a water vapor transfer unit, and a housing defining a chamber. The compressor pressurizes the chamber with oxidant from the oxidant source. The heat exchanger is positioned within the chamber and transfers heat from oxidant within the chamber to an exhaust fluid produced by the electrochemical conversion assembly. The water vapor transfer unit is positioned within the chamber and transfers water vapor from the exhaust fluid to the oxidant in the chamber. A controller monitors and varies the extent to which the oxidant delivery assembly modifies the oxidant.    INDEPENDENT CLAIMS are also included for the following:    (1) operating the above fuel cell system; and    (2) an integrated air delivery device comprising a housing defining an air flowpath including a chamber, a filter in the air flowpath, the compressor in the flowpath, the heat exchanger in the flowpath, the water vapor transfer unit in the flowpath, and the controller.    The water vapor transfer unit comprises porous microtubes in connection with the electrochemical conversion assembly so that a portion of an exhaust fluid flowing from the electrochemical conversion assembly and through the porous microtubes can diffuse through the conversion assembly and into the oxidant in the chamber to increase the humidity of the oxidant. The heat exchanger reduces the temperature of the oxidant exiting the compressor while increasing the temperature of the exhaust fluid entering the porous microtubes.    DESCRIPTION OF DRAWING(S) - The figure shows an elevation cutaway view of an integrated air delivery system.    Fuel cell system (700)    Housing (702)    Chamber (705, 706)    Compressor (722)    Heat exchanger (730)    Water vapor transfer unit (732)