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

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

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Steam Boilers, Engines, Nozzles and Turbines
In designing air preheaters, the important design consideration is that

Stack gases should not be cooled to the dew point
Approach temperature should be as low as possible
Handling and maintenance should be easier
Heat transfer area should be optimum

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