Furnace operations — effect of air infiltration: In industrial furnaces, cold air can infiltrate through cracks, charging doors, peepholes, or other openings. Which combined effect does this unwanted ingress typically produce on furnace operation and combustion atmosphere?

Introduction / Context:
Uncontrolled infiltration of cold ambient air into fuel-fired furnaces is a frequent operational problem. Leakage occurs through cracked refractories, poorly sealed charging doors, and expansion joints. Understanding its combined effects helps in energy auditing, combustion tuning, and emissions control.

Given Data / Assumptions:

• Fuel-fired furnace exhausting hot flue gases through a stack.
• Ambient air entering unintentionally (not part of measured combustion air).
• Steady fuel input and set draft conditions before leakage.

Concept / Approach:
Infiltrated air is cool and oxygen-rich. Mixing with hot flue gas reduces overall gas temperature via sensible heat dilution. Extra oxygen shifts the atmosphere toward oxidizing conditions, increasing scale formation risk on steel. The additional gas volume must be moved by the draft system, increasing induced draft (ID) fan load while simultaneously upsetting pressure balance and decreasing effective furnace draft across the chamber.

Step-by-Step Solution:

Verification / Alternative check:
Stack O2 and CO trends during leakage typically show increased O2, reduced CO, and lowered stack temperature. Fan current rises due to higher volumetric flow.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing deliberate excess air with uncontrolled infiltration; the latter bypasses flow measurement, complicating control and heat balance calculations.

Final Answer:
all (a), (b) and (c)