Tunnel kilns serve for the thermal treatment of products in a continuous operation within a production process. The tunnel kilns are usually made up of a number of identical kiln segments, each segment having a blower, heating elements for heating up the fresh air and a common exhaust air line. For the treatment of the products, they are made to pass by either on the suction side or the pressure side of the blower. To reduce the overall volume of such kilns and to save energy, it is proposed to arrange the blower inside the kiln in such a way that it produces a circulatory flow transversely to the direction of continuous transport and to transport the products to be dried through the circulatory flow parallel to one another in the direction of continuous transport both on the pressure side and on the suction side of the blower. These kilns are preferably used in the production of catalytic converters for automotive exhaust for which a catalyst layer applied to monolithic honeycomb bodies has to be dried and calcined.