Fuel savings with preheated combustion air: A soaking-pit furnace firing furnace oil uses preheated air at 400°C instead of ambient air, with flue gas leaving at 1200°C. Approximately what percentage saving in furnace oil is expected compared to using atmospheric air?

Introduction / Context:
Preheating combustion air recovers sensible heat from flue gases, reducing the additional fuel needed to reach required flame and furnace temperatures. Soaking pits and reheating furnaces commonly use recuperators or regenerators to preheat air, delivering noticeable fuel savings.

Given Data / Assumptions:

• Air preheat temperature: 400°C (significant above ambient).
• Flue gas exit temperature: 1200°C.
• Same production rate and final stock temperature.

Concept / Approach:
Fuel saving is roughly proportional to the fraction of the theoretical air and gas sensible heat supplied by preheat rather than by fuel. Typical industrial data for heavy oil firing indicate about 15–25% fuel reduction when air is preheated to several hundred degrees Celsius with high flue gas temperatures. The value of about 20% is a widely cited engineering estimate for this temperature pairing.

Step-by-Step Solution:

Verification / Alternative check:
Simplified heat balance comparing adiabatic flame temperatures with and without preheat supports a 15–25% band, placing 20% near the midrange.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming savings are linear with preheat at all ranges; diminishing returns occur as preheat approaches flue gas temperature.

Final Answer:
20