In stack observations, a persistent “white smoke” or white plume from a furnace chimney generally indicates which combustion condition?

Introduction / Context:
Stack plume appearance provides quick diagnostic clues about combustion quality. White plumes are common and often misunderstood compared to black or blue smoke. Understanding excess air and condensation phenomena helps operators tune burners appropriately.

Given Data / Assumptions:

• Observation: persistent white plume visible from the chimney.
• Ambient conditions allow water-vapour condensation in the exhaust plume.
• Fuel/air ratio can be varied (excess air high or low).

Concept / Approach:
A high excess-air setting introduces surplus cool air, reducing flue-gas temperature. When the humid exhaust mixes with ambient air, water vapour condenses into fine droplets visible as a white plume. This is distinct from black smoke (soot from insufficient air) and blue smoke (unburned oil aerosols). White plumes can appear for gaseous or liquid fuels if conditions favour condensation; thus the diagnostic points to excess-air/temperature rather than fuel type per se.

Step-by-Step Solution:

Verification / Alternative check:
Flue-gas analysers show higher O2 and lower CO with high excess air; stack temperature drops, and visible white plumes are common in cool, humid weather.

Why Other Options Are Wrong:

Common Pitfalls:
Equating white plume with “clean combustion”; although not soot, it may signal efficiency loss due to over-ventilation.

Final Answer:
Very high excess air (cool plume with water-vapour condensation)