THERMAL PROCESSING APPARATUS

A conveyor belt (36) is arranged in tiers at spiral stack conveyor unit (32) and/or (34). A ceiling or top sheet (58) is positioned over the spiral stack(s). A circulation fan (60, 62) draws thermal processing medium laterally from the tiers of the spiral stack, up the exterior of the stack and across the top of the stack above the ceiling or top sheet and through a heat exchanger (64). The treated thermal processing medium is then routed across the remainder of the diameter of the spiral stack and then down the side of the spiral stack diametrically opposite to the circulating fan thereby to enter the spiral stack in a lateral direction diametrically toward the circulating fan. At least one opening (70) is formed in the ceiling downstream of the heat exchanger and into the interior of the conveyor drive hub. The outer wall of the drive hub is partially open thereby to provide an alternative flow path for the thermal processing medium to enter the spiral stack from the interior of the drive hub.