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

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

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

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

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