Types of industrial furnaces – Indirectly heated design: Which one of the following is an indirectly heated furnace, where products do not contact the flame or flue gases?

Introduction / Context:
Furnaces can be direct-fired or indirectly heated. In direct firing, hot products of combustion contact the workload; in indirect designs, a barrier separates flame/flue gas from the charge, protecting sensitive materials and preventing contamination.

Given Data / Assumptions:

• Muffle furnaces use a refractory enclosure (muffle) between flame and product.
• Open-hearth, soaking pits, and most reheating furnaces are direct-fired for steelmaking/rolling.

Concept / Approach:
An indirectly heated furnace transfers heat by conduction through refractory walls and by radiation from hot refractory to the charge. This allows atmosphere control and avoids oxidation or contamination from combustion gases.

Step-by-Step Solution:
Evaluate each choice for contact between combustion products and workload.Only the muffle furnace includes a sealed chamber (muffle) isolating the charge.Therefore, select muffle furnace.

Verification / Alternative check:
Application examples include firing of ceramics, heat treatment of parts requiring clean surfaces, and laboratory ashing where contamination must be avoided.

Why Other Options Are Wrong:

• Open-hearth: direct contact with combustion gases in steelmaking.
• Soaking pit: direct-fired for ingot/bloom soaking.
• Reheating furnace: typically direct-fired to heat billets/slabs prior to rolling.

Common Pitfalls:
Confusing the presence of a roof or checkerwork with a muffle; the defining feature is a separate chamber fully isolating the charge.

Final Answer:
Muffle furnace