Preheating the combustion air offers several advantages in furnace operation. Identify the item below that is <em>not</em> an advantage of air preheating.

Introduction / Context:
Air preheaters recover sensible heat from flue gases to elevate incoming combustion air temperature. This technique is central to furnace energy efficiency and productivity. However, some properties—like the intrinsic calorific value of the fuel—are fuel-dependent and do not change with air preheat.

Given Data / Assumptions:

• Fuel composition remains unchanged.
• Combustion air is heated via recuperation or regeneration.
• Operating goal is higher furnace efficiency and product quality.

Concept / Approach:
Preheated air reduces the sensible energy the flame must supply to raise oxidant temperature, effectively increasing adiabatic flame temperature and accelerating heat transfer to the load. As a result, fuel consumption typically drops for the same duty, and improved temperature control may reduce scaling of steels by limiting over-firing and shortening exposure times. But the chemical energy per unit mass or volume of fuel (its calorific value) is not altered by preheating the oxidant; it is defined by the fuel’s composition and heating value testing.

Step-by-Step Solution:
List common benefits: higher flame temperature, better heat flux, fuel saving, quality improvements.Identify the distractor: “increase in calorific value of the fuel.”Conclude it is not an advantage attributable to preheating air.

Verification / Alternative check:
Energy audits consistently report reduced specific fuel consumption with air preheating. Fuel heating value certificates remain unchanged regardless of oxidant temperature, confirming that calorific value is independent of air preheat.

Why Other Options Are Wrong:
Increase in flame temperature: True; less energy is spent heating the oxidant from ambient.Reduction in scale losses: Often observed due to better temperature control and shorter cycles.Saving in fuel consumption: Direct outcome of heat recovery.

Common Pitfalls:

• Equating higher flame temperature with higher fuel heating value.
• Ignoring dew-point and corrosion limits when pushing air-preheat too high.

Final Answer:
Increase in calorific value of the fuel.