Water tube boilers essential for high-pressure boilers, have water circulated in tubes which are heated externally, while the fuel is burned inside the furnace. Thus, hot gas is produced heating up the water in the steam-generating tubes. For steam generation, the larger utility boilers depend upon the water filled tubes functioning as furnace walls. Through the steam drum, the saturated steam is drawn off. For super heating process, the steam reenters the furnace, which is then used in driving turbines.

To make steam devoid of water, considered dangerous for turbine blades, it is superheated to 730°F (390°C) or higher. The feed water in the feed water drum is pre-heated by the already stored cold water at the bottom of the steam drum. For warming the feed water supply, exhaust gases are used for heating the air pumped into the furnace. This increases the economy of the boilers. In thermal power station, they are also popularly known as steam generating units. Water tube boiler's another positive feature is that there is less chance of a catastrophic failure. Neither is there the presence of a large volume of water nor are there large mechanical elements subject to failure.

Types of Water Tube Boiler:

• D-type Boiler
• Babcock & Wilcox Boiler
• Stirling Boiler
• Thornycroft Boiler
• Yarrow Boiler