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
Heat is transferred by bleeding some gases from furnace
From heating an intermediate material and then heating the air from this material

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Steam Boilers, Engines, Nozzles and Turbines
The efficiency of a boiler is defined as

Ratio of the heat liberated in the furnace to the heat actually used in producing steam
Ratio of the mass of steam produced to the mass of total water supplied in a given time
Ratio of heat actually used in producing steam to the heat liberated in the furnace
None of these

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