Water Tube Boiler

Water tube boiler is a form of boiler in which water rotates and circulates in tubes heated externally by the fire. Used as high pressure boilers, fuel is burned inside the furnace producing hot gas which heats up the water in steam generating tubes. Sometimes, water tube boilers are also regarded as steam generating units.

Operation
The heated water is sent into the steam drum where saturated steam is drawn off at the top of drum. Though water droplets can cause damage to the turbine blades, thus steam is superheated to about 730°F (390°C) or higher than that. From the bottom of steam drum, cool water is returned to the feed water drum through large bore down comer tubes. The exhaust gases are used to pre heat the air blown into the furnace and warm the feed water supply.

Water tube boiler vs fire tube boiler
Fire tube boiler has a much weaker structure, usually used for pressures above than 350 psi (2.4 MPa). Whereas water tube boiler has advantages over them. There are less chances of catastrophic failure in water tube boilers as compared with fire tube boilers.

Types of water tube boilers

• D-type Boiler:Applicable in both stationary and marine applications, this is the most common form of small-medium sized boiler. The boiler has large steam drum vertically attached to a smaller water drum via multiple steam-generating tubes.
• Low Water Content: Termed as "furnace-less" boiler, this has a lower and upper header linked by water tubes that are directly impinged upon from the burner. This boiler can generate steam and react to load changes quickly.
• Babcock & Wilcox Boiler:This boiler has a single drum, with feed water drawn from the source of the drum into a header that supplies inclined water-tubes. The water tubes produce steam back into the top of the drum.
• Stirling Boiler:Mainly used as stationery boiler, this boiler has three upper drums connected to two lower drums by water tubes.
• Thornycroft Boiler:This is a single steam boiler, comprised with two sets of water tubes at either side of the furnace.
• Yarrow Boiler: This boiler has three drums in a delta formation linked by water tubes, allowing for easy tube-cleaning.
• White-Forster: Similar to yarrow boiler, this boiler has tubes which are curved in shape.

Applications
Widely used in versatile applications like as follows:

• Land-based: Water tube boilers are vastly used for power generation and have the ability to work under high pressure and temperature conditions.
• Marine: High pressure water tube boilers are highly workable in marine boilers.
• Railway locomotives: They are exclusively used in varied railway locomotives.