Described herein are glass forming apparatuses with cooled muffle assemblies and methods for using the same to form glass ribbons. According to one embodiment, a muffle assembly for a fusion forming apparatus may include a muffle frame comprising a back wall, a front wall opposite the back wall, and a pair of sidewalls joining the front wall to the back wall in a closed-loop. At least one first cooling tube may extend through the back wall and the front wall across the closed-loop. At least one second cooling tube may extend through the back wall and the front wall across the closed loop such that the at least one second cooling tube is spaced apart from and parallel with the at least one first cooling tube.

An apparatus for processing a substrate includes a reaction tube, a side cover, a heater, a first gas supplier, a second gas supplier and a controller. The reaction tube is configured to receive a substrate boat in which a plurality of the substrate is received to process the substrate. The side cover is configured to receive the reaction tube. The heater lines the interior of the side cover. The first gas supplier is provided to an upper portion of the side cover to supply a cooling gas at a first supplying rate to a space between the side cover and the reaction tube. The second gas supplier is provided to a lower portion of the side cover to supply the cooling gas at a second supplying rate different from the first supplying rate to the space between the side cover and the reaction tube. The controller controls the reaction tube.