a housing in the form of a hollow cylinder, and having a gas inlet and a gas outlet at opposite ends thereof; and

first and second cooling plates alternately disposed within the housing in the direction of the longitudinal axis of the housing, each of the first cooling plates having a first base and a plurality of concentric first grid members, the first base extending across the housing and defining a circular vent hole which extends through a central portion of the first base and is open to with the interior of the housing, and the plurality of concentric first grid members projecting from a surface of the first base at locations radially outwardly of the vent hole, and each of the second cooling plates having a second base and a plurality of concentric second grid members, the second base extending across the housing and defining a plurality of vent holes which extend through an outer peripheral portion of the second base and are open to the interior of the housing, and the plurality of concentric second grid members projecting from a surface of the second base at locations radially inwardly of the plurality of vent holes.