Furnace heat transfer to the charge — what does not directly control the rate? The rate of heat transfer to the stock/charge inside a furnace does not directly depend on which of the following?

Introduction / Context:
In high-temperature furnaces, the charge gains heat primarily by radiation and convection from hot gases and walls. Recognizing what governs the instantaneous transfer rate helps prioritize design and operating adjustments.

Given Data / Assumptions:

• Steady firing conditions at the burners with a given furnace temperature field.
• Heat transfer to the charge is evaluated at the process zone.
• Waste-heat recovery (e.g., regenerators, recuperators) acts outside the immediate transfer interface.

Concept / Approach:
Instantaneous heat flux to the charge depends strongly on gas/Wall temperatures, view factors, and surface emissivities. Refractory emissivity (option a) alters net radiation to the charge. Furnace size (option b) influences radiative geometry and view factors. Stock thickness (option d) affects internal conduction resistance and therefore the effective surface temperature driving force. Waste-heat recovery (option c) improves overall thermal efficiency by preheating combustion air or fuel but does not directly control the local heat flux at the charge surface at a given furnace setpoint.

Step-by-Step Solution:
Identify direct radiative–convective factors: wall emissivity, geometry (size/view factor), and stock conduction path.Recognize WHR shifts overall energy balance but, at fixed furnace temperatures, does not set the immediate heat-transfer rate to the charge.Therefore, choose the factor not directly controlling the rate: use of waste-heat recovery equipment.

Verification / Alternative check:
Rerun with/without an air preheater at the same furnace temperature: local heat flux at the charge surface is essentially unchanged; fuel savings come from lower firing rates to maintain temperature.

Why Other Options Are Wrong:

• Refractory emissivity: directly affects radiative exchange.
• Furnace size/geometry: changes view factors and flux distribution.
• Stock thickness: changes internal resistance and surface temperature.

Common Pitfalls:

• Confusing system efficiency improvements with local heat-transfer control.

Final Answer:
Use of waste-heat recovery equipment