Steam Boilers, Engines, Nozzles and Turbines
The difference between Cornish boiler and Lancashire boiler is that

None of the listed here
Former contains one fire tube and latter contains two fire tubes
Former is water tube type and latter is fire tube type
Former is fire tube type and latter is water tube type boiler

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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
Heat transfer area should be optimum
Handling and maintenance should be easier

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