Which type of firebrick damage can be caused by lowering boiler furnace temperatures too rapidly?

The correct answer involves understanding the effects of rapid temperature changes on materials, particularly firebrick used in boiler furnaces. When boiler furnace temperatures are lowered too quickly, it can lead to deep fractures in the firebrick. This phenomenon occurs because the rapid decrease in temperature causes the material to contract unevenly. Firebrick, designed to withstand high temperatures, can experience thermal shock under such conditions, where the surface cools and contracts faster than the interior. This difference in contraction creates stress within the material, leading to deep fractures that compromise the integrity and performance of the firebrick.

In contrast, surface cracks are more likely to occur with gradual temperature changes, while surface spalling typically involves the peeling or chipping away of material due to prolonged exposure to extreme temperatures, rather than rapid temperature fluctuations. Thermal expansion refers to the material's tendency to expand when heated and does not directly relate to damage from cooling too quickly. Thus, the recognition of deep fractures being a consequence of rapid temperature drop is key to understanding the durability and maintenance of firebricks in boiler applications.