A conveyor belt is arranged in tiers at spiral stack conveyor unit. A ceiling is positioned over the spiral stack(s). A circulation fan draws thermal processing medium laterally from the tiers of the spiral stack, up the exterior of the stack and across the ceiling and through a heat exchanger. 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 to enter the spiral stack in a lateral direction diametrically toward the circulating fan. The thermal processing medium flows into an opening formed in the ceiling downstream of the heat exchanger and into the interior of the conveyor drive hub and then into the spiral stack through openings in the drive hub outer wall.