Ceramic (SiC) recuperators: above what approximate combustion-air preheat temperature do such recuperators become economically justified in furnace service?

Introduction / Context:
Ceramic recuperators made from silicon carbide or similar materials withstand very hot and corrosive flue gases. They are more expensive than metallic units, so economics favor their use when very high air preheat is required.

Given Data / Assumptions:

• Device: SiC ceramic recuperator.
• Goal: high combustion-air preheat temperature.
• Economic threshold depends on durability and heat-recovery benefit.

Concept / Approach:
Metallic recuperators suffer from oxidation and creep at elevated temperatures; practical air-preheat limits for metals are lower. Ceramic units justify their cost when target air-preheat is very high (on the order of 800–900 °C or more), improving thermal efficiency and fuel savings significantly.

Step-by-Step Solution:
1) Determine typical metallic limit: several hundred °C, often < 650–700 °C.2) Ceramic units allow higher approaches, commonly ≥ 850 °C.3) Compare options: 850 °C matches the common economic crossover.

Verification / Alternative check:
Industry references show ceramic recuperators selected when very high preheat is required and flue gas temperatures permit large recovery, typically above ~850 °C air preheat.

Why Other Options Are Wrong:
250 and 650 °C: Within the range where metallic recuperators often suffice.1000 °C: Possible in special designs, but the threshold of economic advantage is reached earlier around ~850 °C.

Common Pitfalls:
Selecting ceramics at too low preheat, where added cost is not justified.

Final Answer:
850