Steam Boilers, Engines, Nozzles and Turbines
The object of producing draught in a boiler is

To discharge the gases of combustion to the atmosphere through the chimney
To provide an adequate supply of air for the fuel combustion
All of the listed here
To exhaust the gases of combustion from the combustion chamber

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

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

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