What is the effect of running a burner with far more excess air than it needs?
- More heat is carried up the stack
- More soot forms in the tubes
- The flame becomes long and smoky
- The fuel flow rises to compensate
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A. More heat is carried up the stack
Air that takes no part in combustion is heated in the furnace and carries that heat out of the stack, so efficiency drops. Soot and a smoky flame are signs of too little air, the opposite fault.
Reference: Combustion: excess air and stack loss
An oil burner is producing black smoke at the stack. What is the most likely cause?
- Too much excess air for the fuel
- The feedwater temperature is much too high
- Not enough combustion air for the fuel
- The steam pressure setting is too low
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C. Not enough combustion air for the fuel
Black smoke is unburned carbon, which means the fuel is not getting enough air or is not mixing with it properly. Too much air cools the flame and wastes heat, but it does not make black smoke.
Reference: Combustion: smoke and air-fuel ratio
As the amount of excess air supplied to a burner is increased, what happens to the flue gas readings?
- Oxygen rises and carbon dioxide falls
- Oxygen falls and carbon dioxide rises
- Oxygen and carbon dioxide both rise
- Oxygen and carbon dioxide both fall
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A. Oxygen rises and carbon dioxide falls
Extra air adds unused oxygen and dilutes the carbon dioxide, so the O2 percentage climbs while the CO2 percentage drops. That is why either reading can be used to set excess air.
Reference: Combustion testing: O2 and CO2 in flue gas