NOVELTY - The nitrogen enriched air production system comprises a first gas separation module (13) comprising a first selectively gas permeable membrane (16) having an oxygen/nitrogen selectivity ( alpha 1) of greater than 1 and an oxygen permeance (F1), a second gas separation module (23) comprising a second selectively gas permeable membrane (26) having an oxygen/nitrogen selectivity ( alpha 2) of greater than 1 and an oxygen permeance (f2), and a transfer unit. Each gas separation module defines a feed-retentate chamber (4) placed on a side of the membrane, and a permeate chamber (5).    USE - The nitrogen enriched air production system is useful in an apparatus such as: a motor vehicle consisting of aircraft, automotive vehicle and marine vessel; an offshore natural resource recovery drilling platform; an onshore natural resource recovery facility; an oil production facility; a natural gas production facility; a food processing facility; a beverage processing facility; and an industrial chemical production facility, and for providing an inert atmosphere to a tank containing a volatile combustible liquid and a vapor in equilibrium with the liquid (all claimed).    ADVANTAGE - The compact system has light weight, is easy to operate, and ensures energy-efficient and reliable production of high concentration of nitrogen enriched air.    DETAILED DESCRIPTION - The nitrogen enriched air production system comprises a first gas separation module (13) comprising a first selectively gas permeable membrane (16) having an oxygen/nitrogen selectivity ( alpha 1) of greater than 1 and an oxygen permeance (F1), a second gas separation module (23) comprising a second selectively gas permeable membrane (26) having an oxygen/nitrogen selectivity ( alpha 2) of greater than 1 and an oxygen permeance (f2), and a transfer unit. Each gas separation module defines a feed-retentate chamber (4) placed on a side of the membrane, and a permeate chamber (5) that is placed on the side of the membrane and is opposite to the feed retentate chamber. One of the feed-retentate chamber and the permeate chamber of one gas separation module is present in gaseous communication with the permeate chamber of the other gas separation module. The oxygen/nitrogen selectivity and the oxygen permeance of the first membrane are different from oxygen/nitrogen selectivity and the oxygen permeance of the second membrane. The first and second selectively gas permeable membranes are selected such that alpha 1 is less than alpha 2 and f1 is greater than f2 or such that alpha 1 is greater than alpha 2 and f1 is less than f2. The feed-retentate chamber of the first gas separation module is present in gaseous communication with the feed-retentate chamber of the second gas separation module. The transfer unit is adapted for communicating gas between the feed-retentate chamber of the first gas separation module and the feed-retentate chamber of the second gas separation module, and is free of a compression unit for increasing pressure of a gas stream. The permeate chamber of the first gas separation module is present in gaseous communication with the permeate chamber of the second gas separation module. INDEPENDENT CLAIMS are included for:    (1) an apparatus such as a motor vehicle;    (2) a process for production of nitrogen enriched air; and    (3) a method of providing an inert atmosphere to a tank.    DESCRIPTION OF DRAWING(S) - The diagrams shows a schematic view of a membrane separation process to produce nitrogen-enriched air.    Feed-retentate chamber (4)    Permeate chamber (5)    First gas separation module (13)    First selectively gas permeable membrane (16)    Second gas separation module (23)    Second selectively gas permeable membrane. (26)