Steam Boilers, Engines, Nozzles and Turbines
When the circulation of water, in a boiler, is by a centrifugal pump, then the boiler is known as

Natural circulation boiler
Forced circulation boiler
Internally fired boiler
Externally fired boiler

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Steam Boilers, Engines, Nozzles and Turbines
In recuperative air preheaters, the heat is transferred

From a metal wall from one medium to another
By direct mixing
From heating an intermediate material and then heating the air from this material
Heat is transferred by bleeding some gases from furnace

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