pressure as in the mixing plenum 34 mixing of the internal air again occurs in the exhaust plenum 36 from the exhaust plenum 36 the air flows into the guide vanes 54 which are configured to guide the air into the fans 52 in a direction and at a speed that is optimal for the fans 52 in this manner the pressure loss and energy required to compensate for it associated with the recirculation of the internal air is minimized thus in general the internal air flows from the fans 52 through a heat exchanger in the return plenums 32 where it is cooled or warmed if desired the cool internal air then flows into the mixing plenum 34 and thence to the equipment 24 once in the equipment 24 the internal air cools the equipment 24 and returns to the fans 52 via the exhaust plenum 36 on the other side of the heat exchangers formed by the return plenums 32 the thermally conductive wall 43 and preferably the heat transfer fins 44 the ecs air absorbs heat which originated in the equipment 24 from the heat exchange surfaces 43 and 44 of course the ecs air enters the outer plenum 30 which forms the other side of the heat exchanger from the ecs.