The present invention provides a mechanism device for preventing gas channeling between the sidewall and a heat exchange tube bundle located in the convection section of a horizontal tube fired heater while permitting in-situ repair of the sidewall refractory. The invention comprises a plurality of serially elongated plate members pivotably mounted on a tube and sized longitudinally to extend at a predetermined distance from the tube to the sidewall of the convection section of the heater, the number of plate members being sufficient to extend for substantially the length of the tube.
