Open-hearth furnace performance The typical overall thermal efficiency of an open-hearth furnace (with standard regenerative firing) may be approximately what percent under good operation?

Introduction / Context:
Open-hearth furnaces historically produced steel with large refractory chambers and regenerative checkers to preheat air and fuel. Despite regeneration, overall thermal efficiency (useful heat to process / fuel energy in) remained modest compared with modern basic-oxygen or electric arc processes, or with highly recuperated furnaces.

Given Data / Assumptions:

• Typical open-hearth operation using gaseous or liquid fuels with regenerators.
• Normal flue-gas temperatures and practical excess air levels.
• Measured efficiency includes stack, wall, and opening losses.

Concept / Approach:
Even with regenerative preheat, open-hearth furnaces exhibit significant radiant wall losses and high sensible heat in exhaust. Historical performance data place overall efficiency roughly in the 15–25% band for many installations, rising only with exceptional heat recovery and tight operation. Therefore, the representative value closest to standard practice is near 20%.

Step-by-Step Solution:

Verification / Alternative check:
Published heat balances and historical plant reports corroborate that open-hearth efficiency values trail those of modern processes by a wide margin, commonly around the 20% level.

Why Other Options Are Wrong:

• 5%: Too low for regeneratively fired open-hearths under good operation.
• 50% or 80%: Characteristic of highly recuperated/modern units; unrealistic for classic open-hearth practice.
• 35%: Higher than typical field averages.

Common Pitfalls:
Confusing instantaneous burner thermal efficiency with overall process efficiency; ignoring downtime and door-opening losses that depress the measured figure.

Final Answer:
20