Furnace fuel economy vs. stock thickness — when is it insensitive? The thickness of the workpiece (stock) does not significantly affect furnace fuel economy when the material being heated has which property?

Introduction / Context:
Fuel economy in furnaces depends on how rapidly heat penetrates the workpiece and how much heat is lost to exhaust and walls. The stock thickness often matters because thicker sections require more time for the core to reach temperature. However, material properties can make thickness less critical.

Given Data / Assumptions:

• Comparing materials of differing thermal properties under similar furnace conditions.
• Focus on how stock thickness impacts heat-up time and fuel used.
• Emissivity matters for surface absorption; conductivity matters for internal heat conduction.

Concept / Approach:
High thermal conductivity materials (e.g., copper, aluminum) quickly conduct surface heat into the core, minimizing internal temperature gradients. As a result, even thicker sections heat relatively uniformly and quickly compared to low-conductivity materials, making fuel consumption less sensitive to thickness. Low emissivity reduces surface absorption of radiant heat and can worsen fuel economy, not improve insensitivity to thickness.

Step-by-Step Solution:
Identify the rate-limiting step: internal conduction for thick parts.High thermal conductivity reduces internal resistance, so thickness has a smaller penalty.Therefore, fuel economy becomes less dependent on thickness for high-conductivity materials.

Verification / Alternative check:
Heating curves for copper bars of varying thickness show much smaller differences than for low-conductivity steels or refractories under similar conditions.

Why Other Options Are Wrong:

• Low emissivity: decreases radiative coupling and typically increases heating time and fuel usage.
• Both / Neither: Only high conductivity provides the stated insensitivity.

Common Pitfalls:

• Overemphasizing emissivity when the dominant resistance is internal conduction for thick parts.
• Assuming the same conclusion applies to low-conductivity alloys.

Final Answer:
High thermal conductivity