Overall thermal efficiency of a lumpy-coal-fired heating furnace For a suitably designed, well-operated and well-maintained furnace burning lumpy coal, the typical overall thermal efficiency may be approximately what percent?

Introduction / Context:
Overall thermal efficiency for fuel-fired furnaces accounts for useful heat delivered to the load divided by the chemical energy in the fuel. For conventional lumpy-coal-fired heating furnaces (e.g., older reheat or small industrial units), stack losses, wall losses, and incomplete heat recovery limit achievable efficiency compared to modern regenerative/recuperative designs.

Given Data / Assumptions:

• Fuel: lumpy coal, hand- or mechanically-fired.
• Furnace: suitably designed and properly maintained.
• Operating: reasonable draft and air-fuel control but without advanced heat recovery.

Concept / Approach:
Typical literature values place plain, non-regenerative solid-fuel heating furnaces in the low-to-mid 20–30% range when averaged over a shift, because of high excess air, intermittent firing, radiation through walls/doors, and hot-flue losses. Higher values require extensive waste-heat recovery or continuous high-load operation with tight control.

Step-by-Step Solution:

Verification / Alternative check:
Benchmarking of traditional shop furnaces shows fuel-to-load efficiencies in the mid-20s without strong heat recovery or continuous operation at design capacity.

Why Other Options Are Wrong:

• 10–15: Too pessimistic for a properly designed and maintained furnace.
• 45–50, 65–70: Generally require recuperators/regenerators or modern burners with superior controls.
• 35–40: Above typical for lumpy coal without significant heat recovery.

Common Pitfalls:
Confusing boiler thermal efficiency with furnace efficiency; furnaces heating solids often have lower efficiency than steam generators.

Final Answer:
25–30