An additive manufacturing apparatus includes an environmentally sealed first chamber, a second chamber separated from the first chamber by a first valve, a platform positionable in the first chamber, a dispenser configured to deliver a plurality of successive layers of feed material onto the platform in the first chamber, at least one energy source to selectively fuse feed material in a layer on the platform in the first chamber, and an air knife assembly to direct a laminar flow of air across a layer of feed material on the platform in the first chamber. The air knife assembly includes an inlet module and an exhaust module that are movable through the first valve between the first chamber and the second chamber.