Water-tube boiler

  A water-tube boiler is a type of boiler in which water circulates in tubes which are heated externally by the fire. Water-tube boilers are 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.

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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 to 730°F (390°C) or higher 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. (In 'large utility boilers', 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 gasses are also used to pre-heat the air blown into the furnace and warm the feedwater supply. Such water-tube boilers in thermal power station are also called steam generating units.

The older fire-tube boiler design—in which the water surrounds the heat source and the gases from combustion pass through tubes through the water space—is a much weaker structure and is rarely used for pressures above 350 psi (2.4 MPa). A significant advantage of the water tube boiler is that there is less chance of a catastrophic failure: There is not a large volume of water in the boiler nor are there large mechanical elements subject to failure.

Types of water-tube boiler

• D-type boiler - This is the most common type of small-medium sized boilers, similar to the one shown in the schematic diagram. It is used in both stationary and marine applications. It consists of a large steam drum vertically connected to a smaller water drum (a.k.a. mud drum) via multiple steam-generating tubes. These are surrounded by walls made up of larger water filled tubes, which make up the furnace.
• O-type
• A-type
• Flex-Tube Boiler
• Babcock & Wilcox boiler - this has a single drum, with feedwater drawn from the bottom of the drum into a header that supplies inclined water-tubes. The water tubes supply steam back into the top of the drum. Furnaces are located below the tubes and drum.
• Stirling boiler - This type has three upper drums connected to two lower drums by water tubes. These are mainly used as stationary boilers.
• Thornycroft boiler - A single steam drum is supplied by a single water drum via two sets of water tubes that arc around the boiler. Furnaces on either side of the water drum vent into a common exhaust, giving the boiler a wide base tapering profile.
• Yarrow boiler - This type has three drums in a delta formation connected by water tubes and is generally fuel oil-fired. Due to its three drums, the Yarrow boiler a has greater water capacity. Hence, this type is usually used in older marine boiler applications. Its compact size made it attractive for use in transportable power generation units during World War II. In order to make it transportable, the boiler and its auxiliary equipment (fuel oil heating, pumping units, fans etc.), turbines, and condensers were mounted on wagons to be transported by rail.
• Miura boiler - The Miura boiler company is the largest boiler manufacturer in the world by sales, with over 60% of the market in Asia. The Miura design has effectively replaced fire-tube technology outside of North America and Europe, with copy designs making up another 38% of the boiler market. Miura boilers are now produced in North America, and the Japanese-engineered compact super-efficient and low NOx boilers will likely become an industry prefered standard here as well. The Miura boiler is designated as a "once-through force-flow steam generator" under ASME code, typically requiring less than half the space of a fire-tube boiler of the same HP output and maintaining 81-85% efficiency under all operating conditions.