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

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Steam Boilers, Engines, Nozzles and Turbines
Spontaneous combustion is a phenomenon in which

All the fuel burns instantaneously producing high energy release
Coal bursts into flame without any external ignition source but by itself due to gradual increase in temperature as a result of heat released by combination of oxygen with coal
Fuel burns with less air
Explosion in furnace

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