A thermal processing apparatus (20) includes a spiral conveyor system (22) configured into an ascending spiral stack (26). An inner mezzanine (40) is disposed within the circular interior of the stack (26) and an exterior mezzanine (42) encircles the spiral stack. The mezzanines divide the processing chamber (32) into a plurality of processing zones. A sealing system (90) seals the interior mezzanine (40) relative to the spiral stack (26). A sealing system (130) seals the outer mezzanine (42) relative to the exterior of the spiral stack, and a seal system (132) seals the outer mezzanine relative to the walls (56, 58) of the housing, thereby to limit the leakage of the processing fluid between the mezzanines and the spiral stack (26) and housing walls so that as much as the thermal processing medium is possible is forced through the spiral stack for processing work products carried on the conveyor system (22).