In industrial furnace engineering, what is the highest practically achievable thermal efficiency for an air and fuel-gas preheating recuperator under good design and operation conditions? Provide the closest percentage value.

Introduction / Context:
Recuperators are continuous heat exchangers that recover sensible heat from hot flue gases to preheat combustion air and sometimes fuel gas. Their thermal efficiency indicates how much of the available heat in the exhaust is transferred to the incoming streams. Understanding realistic efficiency ranges is crucial for furnace design, fuel economy, and emissions control.

Given Data / Assumptions:

• Device: air/fuel-gas preheating recuperator.
• Application: high-temperature industrial furnaces with clean, steady flue gas.
• Well-designed metallic or ceramic units with proper surface area and flow arrangement.

Concept / Approach:
Thermal efficiency (η_recup) is commonly defined as: heat gained by the cold stream / maximum recoverable heat from flue gas at the stack exit reference, expressed in percent. Practical limits arise from approach temperature differences, fouling, heat loss, material constraints, and pressure drop. Recuperators typically reach high, but not perfect, effectiveness; values in the 70–85% range are achievable for well-engineered systems, whereas numbers approaching 100% are not practical.

Step-by-Step Solution:
1) Identify practical upper bound for recuperator effectiveness from industry practice: often 0.75 to 0.85.2) Recognize diminishing returns beyond ~85% due to larger surface, higher cost, more pressure drop, and closer approach temperatures.3) Compare options: 50 and 65% are feasible but not the highest; 99% is unrealistic for recuperators.4) Therefore, the closest realistic maximum is 85%.

Verification / Alternative check:
Cross-check against typical vendor data and furnace energy audits, which commonly cite ~80–85% for high-performance recuperators, with regenerators sometimes higher but by a different mechanism (periodic heat storage).

Why Other Options Are Wrong:
50%: Too conservative for modern designs.65%: Achievable but not the highest practical figure.99%: Physically unrealistic for continuous recuperators due to finite approach temperatures and losses.

Common Pitfalls:
Confusing recuperators with regenerators (which can show higher apparent effectiveness).Ignoring fouling and pressure drop penalties that limit efficiency gains.

Final Answer:
85