A build unit for additively manufacturing three-dimensional objects may include an energy beam system, an irradiation chamber, an inertization system having a return manifold configured to evacuate a process gas from the irradiation chamber. The return manifold includes a plurality of return manifold bodies and return manifold inlet disposed at a bottom of the irradiation chamber. The plurality of return manifold bodies includes one or more return manifold pathways downstream of the return manifold inlet. The plurality of return manifold bodies also includes one or more recapture pathways positioned to receive the process gas into the one or more return manifold pathways that flows past the return manifold inlet.