Steam Boilers, Engines, Nozzles and Turbines
Secondary air is the air used to

Convert CO (formed in lower zone of furnace) into CO₂ at higher zone
Air delivered by induced draft fan
Provide air around burners for obtaining optimum combustion
Transport and dry the coal

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Steam Boilers, Engines, Nozzles and Turbines
In fire tube boilers

Forced circulation takes place
Water passes through the tubes which are surrounded by flames and hot gases
None of these
The flames and hot gases pass through the tubes which are surrounded by water

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