Boiler Efficiency — Identifying the Largest Source of Heat Loss In a typical steam boiler operating with proper combustion and reasonable excess air, which of the following accounts for the maximum heat loss from the boiler system?

Introduction:
Understanding where most of the energy escapes in a boiler helps engineers focus improvement efforts. In routine operations with correct firing and adequate air supply, the dominant loss pathway is usually sensible heat carried away by hot, dry flue gases exiting the chimney. Recognizing this is essential for optimizing economisers, air preheaters, and excess-air settings.

Given Data / Assumptions:

• Combustion is stable with moderate excess air (not grossly high or low).
• Fuel is conventional (coal, oil, gas) without extreme moisture content.
• Boiler surfaces are in good condition; blowdown is normal.

Concept / Approach:
The heat balance of a boiler distributes input chemical energy into useful steam generation and several losses. The sensible heat in stack gases rises with exit temperature and increases with the amount of excess air because higher mass of gas carries more energy. Losses due to moisture-in-fuel and hydrogen (latent plus sensible in water vapour) can be significant, but, for most plants, the single largest line-item remains the dry flue gas sensible heat.

Step-by-Step Solution:
Identify main losses: (1) dry flue gas sensible heat, (2) moisture/steam (latent and sensible), (3) unburnt carbon, (4) radiation and unaccounted.Relate to operating variables: stack temperature and excess air strongly influence dry flue gas loss.Compare magnitudes: in balanced-draft boilers, dry gas loss commonly exceeds other single categories unless fuel moisture is extreme.Conclude the maximum heat loss is via hot, dry flue gases leaving the stack.

Verification / Alternative check:
Typical heat-balance worksheets show dry flue gas losses in the range of 5–12% or more of fuel input depending on stack temperature and excess air, often surpassing other individual losses in well-tuned plants.

Why Other Options Are Wrong:

• Moisture in fuel: Important for very wet fuels, but usually smaller than dry gas sensible loss for standard fuels.
• Steam formation: Latent loss from hydrogen-in-fuel can be large for natural gas but still often not the single largest in many coal/oil units with high stack temperatures.
• Unburnt carbon: Indicates poor combustion; in a well-run boiler it is kept minimal.

Common Pitfalls:
Confusing latent losses (water vapour) with sensible dry gas losses, and ignoring the strong effect of excess air on mass flow and stack temperature.

Final Answer:
dry flue gases leaving the stack