Water Tube Boiler

A water-tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire. Water-tube boilers are generally used for high-pressure boilers. Fuel is burned inside the furnace, creating hot gas which heats up water in the steam-generating tubes. 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.

The heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. In some services, the steam will reenter the furnace in through a superheater in order to become superheated. Superheated steam is used in driving turbines. Since water droplets can severely damage turbine blades, steam is superheated in order to ensure that there is no water entrained in the steam.

Cool water at the bottom of the steam drum returns to the feedwater drum via large-bore 'downcomer tubes', where it helps pre-heat the feedwater supply. The feedwater is supplied to the steam drum and the downcomers supply water to the bottom of the waterwalls. To increase the economy of the boiler, the exhaust gases are also used to pre-heat the air blown into the furnace and warm the feedwater supply.

Water tube boiler is sub-classified as –
1. Bi-drum Boiler ( It has two drums )
2. Single drum Boiler ( It has only one drum )