Which furnace typically exhibits the highest thermal efficiency? Considering common industrial units, select the furnace type that generally achieves the greatest thermal efficiency in practice.

Introduction / Context:
“Thermal efficiency” here refers to the ratio of useful energy captured (e.g., in steam) to fuel energy input. Different furnace classes serve different purposes—metallurgical heating vs steam generation—so their efficiencies vary by design and duty.

Given Data / Assumptions:

• Compare typical efficiencies for common units.
• Assume modern, well-operated equipment of each type.

Concept / Approach:
Boiler furnaces are optimized to transfer heat to water/steam circuits with extensive radiant and convective surfaces and effective heat recovery (economizers, air preheaters). As a result, modern boilers can achieve very high thermal efficiencies compared with metallurgical furnaces whose objective is heating massive workpieces, often with larger wall and opening losses and limited heat recovery.

Step-by-Step Solution:

Verification / Alternative check:
Plant benchmarks: well-tuned packaged boilers can exceed 85–90% fuel-to-steam efficiency (HHV basis) with economizers, while many high-temperature metallurgical furnaces operate at significantly lower thermal efficiencies.

Why Other Options Are Wrong:

Common Pitfalls:
Equating high flame temperature with high efficiency; efficiency depends on how much energy is captured as useful output, not the flame temperature itself.

Final Answer:
Boiler furnace