Apparatus for thermally treating food products by impingement heating or cooling, comprising a plenum having a plurality of inwardly inclined front wall panels adapted to direct the flow of a temperature-controlled gas through at least one elongated plenum outlet port in fluid communication with a duct assembly comprising an elongated manifold coextensive with the elongated plenum outlet port and providing substantially unobstructed fluid communication between the elongated plenum outlet port and a plurality of tapered ducts having at least two inwardly inclined walls, including at least one inwardly inclined side wall.