Minimising stack (flue) heat losses: Which operational adjustment most directly reduces stack heat losses in a fired heater or furnace?

Introduction / Context:
Stack heat loss is the sensible heat carried away by hot, oxygen-rich flue gases. It is a major efficiency penalty in boilers, heaters, and furnaces. Operators can trim this loss through combustion control and heat recovery strategies.

Given Data / Assumptions:

• Normal air firing without recuperators/regenerators.
• Objective: reduce sensible heat leaving with the stack gas.
• Safe operation (avoid CO formation, soot) is maintained.

Concept / Approach:
Excess air above stoichiometric ensures complete combustion but also increases nitrogen and unused oxygen mass flow through the stack, raising both flue gas volume and temperature—hence more sensible heat loss. Tight control to the minimum necessary excess air reduces stack losses while preserving complete combustion.

Step-by-Step Solution:
Recognize stack loss ∝ flue gas flow * mean temperature rise.Excess air increases flue gas flow (more N2 and O2) → higher losses.Trim excess air with O2/CO analyzers to optimal setpoints.Therefore, controlling excess air most directly reduces stack loss.

Verification / Alternative check:
Efficiency curves show minimum stack loss near optimal excess O2 (e.g., ~2–3% O2 dry for natural gas, higher for solid fuels), validating the control strategy.

Why Other Options Are Wrong:
Oxygen enrichment often increases flame temperature and may raise stack temperature unless coupled with heat recovery. Low C.V. fuels do not inherently reduce stack losses and can require more flue volume. Proper draft is important for safety/operation but does not primarily target stack sensible heat.

Common Pitfalls:

• Over-trimming excess air leading to CO/smoke; balance is essential.
• Ignoring air preheat and economizers which also affect flue temperature.

Final Answer:
controlling the excess air