Why recirculate furnace gases? Forced recirculation of hot furnace gases is employed primarily for which objectives in high-temperature furnaces?

Introduction / Context:
Gas recirculation—via internal fans, eductors, or jet mixing—intensifies flow over the load and evens out hot/cold spots within the furnace chamber. This technique is widely used in heat-treatment furnaces, reheat furnaces, and some kilns to improve product uniformity and throughput.

Given Data / Assumptions:

• General high-temperature furnace operation (not limited to low temperature).
• Recirculation is “forced,” i.e., intentionally driven by devices or ducting.

Concept / Approach:
Increasing gas velocity over surfaces increases the convective heat-transfer coefficient h, raising the convective component of heat flux. Simultaneously, mixing reduces stratification and temperature gradients, improving temperature uniformity across the working zone. There is no inherent upper temperature limit like 750°C for the usefulness of recirculation; in fact, it is often applied well above that when equipment permits.

Step-by-Step Solution:

Verification / Alternative check:
Case studies of fan-assisted batch furnaces and jet-impingement zones in reheat furnaces show improved heat-up rates and tighter temperature bands due to recirculation.

Why Other Options Are Wrong:

• Only increase convection (a alone): Misses the equally important uniformity benefit.
• Only uniformity (b alone): Ignores the well-documented boost to convective coefficients.
• Only below 750°C / All including (c): Not generally true; applicability depends on materials/equipment limits, not a strict temperature threshold.

Common Pitfalls:
Believing recirculation is a low-temperature technique only; many modern high-temperature furnaces incorporate robust recirculation hardware.

Final Answer:
Both (a) and (b) only