Fuel economy practices in industrial furnaces — identify the exception Which practice below does <em>not</em> contribute to fuel economy in furnace operation?

Introduction / Context:
Furnace fuel economy is achieved by optimizing air supply, improving heat recovery, and reducing losses. Some practices clearly save fuel, while others increase consumption for the same duty.

Given Data / Assumptions:

• We compare standard operational choices.
• “Fuel economy” means lower fuel per unit of useful heat to stock.

Concept / Approach:
Using preheated combustion air reduces fuel demand or increases flame temperature for the same fuel. Recuperators recover heat from hot flue gases to warm incoming air, directly saving fuel. Operating at or near stoichiometric air (or judicious excess air) also minimizes stack losses. In contrast, non-preheated air throws away potential heat-recovery gains and leads to higher fuel usage.

Step-by-Step Solution:

Verification / Alternative check:
Energy balances show lower specific fuel consumption with air preheaters; stack losses drop as more heat is recycled to incoming air.

Why Other Options Are Wrong:

Common Pitfalls:
Chasing oxygen enrichment before fixing basic losses such as lack of air preheat, infiltration, and poor insulation.

Final Answer:
non-preheated combustion air.