Steel slab reheating (walking-beam type)—typical thermal efficiency For a modern steel slab reheating furnace of the walking-beam type, the overall thermal efficiency is approximately what percentage under good operating practice?

Introduction / Context:
Thermal efficiency expresses how effectively a furnace converts the chemical energy of fuel into useful heat in the load. Walking-beam reheating furnaces are common in rolling mills because they provide uniform heating and minimize skid marks. Knowing typical efficiency helps benchmark fuel usage and stack losses.

Given Data / Assumptions:

• Furnace type: walking-beam reheating furnace for slabs/billets.
• Normal operation with regenerative/recuperative heat recovery and good combustion control.
• Steady-state, well-maintained refractories and low air infiltration.

Concept / Approach:
Industrial references report that modern reheating furnaces achieve about 60–75% thermal efficiency with effective heat recovery, depending on charge temperature, exhaust temperature, and excess air. Values far below imply high stack losses; values much higher are rare in practice due to radiation, convection, and exhaust constraints.

Step-by-Step Solution:

Verification / Alternative check:
Heat balances of well-tuned reheaters commonly show 25–35% losses via exhaust and shell, which corresponds to ~65–75% efficiency, aligning with 70% as a representative figure.

Why Other Options Are Wrong:

• 15% and 40%: indicate severe inefficiencies; inconsistent with modern reheaters.
• 85%: unusually high for continuous furnaces due to inevitable exhaust/shell losses.

Common Pitfalls:
Confusing burner efficiency (combustion) with overall furnace thermal efficiency; the latter includes all heat losses and load recovery factors.

Final Answer:
70