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
Heat transfer area should be optimum
Approach temperature should be as low as possible
Handling and maintenance should be easier

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Steam Boilers, Engines, Nozzles and Turbines
In natural circulation type boiler,

Water is converted into steam in one. Pass without any recirculation
Water is supplied in drum and through down comers located in atmospheric condition it passes to the water wall and rises to drum in the form of mixture of water and steam
Feed pump is employed to supplement natural circulation in water wall type furnace
Heating takes place at bottom and the water supplied at bottom gets converted into the mixture of steam bubbles and hot water which rise to drum

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