Steam Boilers, Engines, Nozzles and Turbines
For the same diameter and thickness of tube, a water tube boiler compared to a fire tube boiler has

More heating surface
Less heating surface
Heating surface depends on other parameters
Equal heating surface

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

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
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
Feed pump is employed to supplement natural circulation in water wall type furnace
Water is converted into steam in one pass without any recirculation

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

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

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