The reasons for adopting a water-tube boiler instead of a smoke tube are as follows :
Water-Tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which heats water in the steam-generating tubes. The working principle of a water tube boiler is thermal siphoning (circulation of natural water). In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate steam.
Due to large water, the required steam pressure rising time is quite high. Output steam pressure cannot be very high since the water and steam are kept in the same vessel. The steam received from the fire tube boiler is not very dry
