Intermittent furnace operation: which refractory brick type should be avoided because of poor thermal-shock tolerance during frequent heat-up and cool-down cycles?

Introduction / Context:
Furnaces that are cycled on and off impose severe thermal shocks on linings. Some refractories tolerate rapid temperature changes better than others. Selecting bricks with good shock resistance is essential to avoid spalling and premature failure.

Given Data / Assumptions:

• Intermittent operation leads to steep thermal gradients.
• Materials with inversion-related expansion or brittle behavior are disadvantaged.
• Silica bricks have characteristic phase inversions.

Concept / Approach:
Silica bricks undergo volume changes associated with transformations among silica polymorphs. Under rapid cycling, these changes concentrate stresses and can cause spalling. In contrast, many high-alumina, sillimanite, or SiC refractories show better thermal-shock resistance, making silica bricks ill-suited for frequently cycled furnaces.

Step-by-Step Solution:
Assess thermal-shock risk of cycled operation.Recall silica’s phase-change-related expansion behavior.Compare with shock-tolerant materials (e.g., SiC, certain alumino-silicates).Select silica bricks as the type to avoid.

Verification / Alternative check:
Thermal shock indices (like quench tests) consistently rate silica poor for cycling compared with SiC or selected alumina bricks.

Why Other Options Are Wrong:
Fireclay/High-alumina/Sillimanite/SiC: generally better shock resistance than silica for cyclic duty.

Common Pitfalls:
Assuming high refractoriness automatically means good thermal-shock resistance; it does not.

Final Answer:
Silica bricks