| Corrosion and cracking of waterwall tubes at primary air ports in kraft recovery boilers have been reported frequently in recent years. A general consensus is that tube cracking is strongly related to the temperature excursions observed on the air port tubes. Laboratory and field experiments have been performed to investigate the tube temperature excursion phenomena, using test sections equipped with thermocouples. In conclusion, direct contact of the tube surface with hot molten smelt seems to be the main cause of high tube temperature excursions at primary air ports. In addition, the heat transfer results suggest that the tube enters a transition boiling regime when molten smelt contacts the tube surface. The results also confirm that the magnitude of the temperature spikes depend on thickness of the frozen smelt layer attached to the outer tube surface, the movement of the molten smelt, and coolant velocity inside the tube. |
