Example of an indirectly heated furnace Select the furnace in which the charge is heated indirectly—through refractory walls or a muffle—without direct contact with combustion gases.

Introduction / Context:
Furnaces can heat loads either directly (flames/hot gases contact the charge) or indirectly (heat passes through a barrier). Indirect heating is preferred when product purity must be protected from combustion products or when controlled atmospheres are required.

Given Data / Assumptions:

• Industrial furnaces commonly encountered in thermal processing.
• “Indirect” means no direct mixing of flue gases with the load.

Concept / Approach:
In a muffle furnace, the flame heats the enclosure (muffle), and heat transfers to the product by conduction, convection, and radiation through the muffle walls. Soaking pits and standard reheating furnaces are direct-fired, with combustion gases circulating around the stock. Therefore, the muffle furnace is the archetypal indirectly heated unit among the options.

Step-by-Step Solution:

Verification / Alternative check:
Process specifications for clean firing (e.g., certain metallurgical or ceramic operations) call for muffle furnaces to avoid contamination.

Why Other Options Are Wrong:

• Soaking pit/Reheating furnace: Direct-fired; gases contact the charge surfaces.
• Fluidised-bed calciner: Gas–solid contact is intrinsic, not indirect.
• None of these: Incorrect because a standard indirect example exists (muffle furnace).

Common Pitfalls:
Assuming any refractory-lined furnace is indirect; the key is whether combustion gases mix with the load.

Final Answer:
Muffle furnace