Indirectly heated furnace examples: Which of the following furnaces are examples of indirect heating (no direct contact of flame/flue gas with the charge)?

Introduction / Context:
In indirect heating, the charge is isolated from combustion products by a barrier, ensuring clean surfaces and controlled atmospheres during heat treatment. Understanding which furnace designs are indirect guides material selection and quality control.

Given Data / Assumptions:

• Muffle furnaces use a refractory muffle isolating the load.
• Hood (bell) annealing furnaces use an inner cover and protective atmosphere, heated by external burners.

Concept / Approach:
Both designs separate flames from the product and allow tight control of atmosphere composition, dew point, and carbon potential, minimizing scaling and decarburisation.

Step-by-Step Solution:
Check each option for isolation of charge from flue gases.Hood annealing uses an inner cover; muffle uses a refractory enclosure.Therefore, both are indirect heating examples.

Verification / Alternative check:
Process data sheets show nitrogen/hydrogen atmospheres circulated under the bell in hood annealing; flue gases remain external.

Why Other Options Are Wrong:

• Single selections omit the other valid example.
• Neither: incorrect because both are indirect types.

Common Pitfalls:
Assuming any annealing furnace is indirect; many continuous annealers are direct-fired. The presence of an inner cover is the key.

Final Answer:
Both (a) and (b)