Industrial furnaces – High-temperature air preheating: Which device is most suitable for preheating combustion air to temperatures above 650 °C in high-temperature furnace service?

Introduction / Context:
Efficient fuel use in high-temperature furnaces depends on recovering sensible heat from hot flue gases. Air preheaters raise combustion-air temperature, improving thermal efficiency and flame stability. Selecting the right heat-recovery device above 650 °C is critical for reliability and safety.

Given Data / Assumptions:

• Target preheat temperature > 650 °C.
• Combustion air and flue gas are corrosive/oxidizing at elevated temperature.
• Materials must withstand thermal shock and oxidation.

Concept / Approach:
Recuperators are continuous heat exchangers transferring heat from hot flue gas to incoming air. Metallic recuperators are limited by alloy oxidation and creep at high temperature, whereas ceramic recuperators use refractory ceramics with much higher temperature capability and resistance to corrosion and scaling.

Step-by-Step Solution:
Identify temperature regime: > 650 °C places service beyond typical metallic recuperator limits.Consider materials: high-temperature alloys suffer rapid oxidation and strength loss; ceramics tolerate higher skin temperatures.Evaluate alternatives: regenerators can also reach high preheats but are cyclic devices; the question asks for the most suitable recuperator type above 650 °C.Select ceramic recuperator as the appropriate continuous exchanger for > 650 °C preheating.

Verification / Alternative check:
Field practice shows metallic recuperators are commonly used up to roughly 500–600 °C air preheat, while ceramic designs extend to 800–1100 °C depending on geometry and duty, confirming suitability above 650 °C.

Why Other Options Are Wrong:

• Regenerator: viable at high temperature but is a cyclic store-and-release device; the question focuses on the most suitable recuperator type for > 650 °C.
• Metallic recuperator: temperature capability is generally too low for sustained > 650 °C air preheat.
• None of these: incorrect, ceramic recuperators are standard for this duty.

Common Pitfalls:
Confusing regenerators with recuperators; assuming exotic superalloys will remove oxidation limits for metallic units in typical furnace atmospheres.

Final Answer:
Ceramic recuperator