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

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

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Steam Boilers, Engines, Nozzles and Turbines
In water tube boilers

The flames and hot gases pass through the tubes which are surrounded by water
Water passes through the tubes which are surrounded by flames and hot gases
Forced circulation takes place
None of these

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