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Excess air above the optimum lowers boiler efficiency because it

  • A.prevents the fuel from burning completely
  • B.lowers the flame's ignition temperature
  • C.increases the moisture in the flue gas
  • D.carries sensible heat out of the stack

Answer: D

Nitrogen and unused oxygen must be heated to flue gas temperature and then discarded, and the larger gas volume shortens the residence time. Too little excess air is worse still, since unburnt fuel wastes its whole calorific value.

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