Steam Boilers, Engines, Nozzles and Turbines
The function of a piston in a steam engine is

To convert heat energy of the steam into mechanical work id) to exhaust steam from the cylinder at proper moment
To transfer motion from the piston to the cross-head
To guide motion of the piston rod and to prevent it from bending
None of these

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

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
From a metal wall from one medium to another

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