A water-tube boiler is a type of boiler in which water circulates in tubes 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.
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.
One famous example of this was the USA Baldwin 4-10-2 No. 60000, built in 1926. Operating as a compound at a boiler pressure of 350 psi it covered over 100,000 successful miles. After a year though, it became clear that any economies were overwhelmed by the extra costs and it was retired to become a stationary plant.
The only railway use of watertube boilers in any numbers was the Brotan boiler, invented in Austria in 1902 and found in rare examples throughout Europe. Hungary though was a keen user and had around 1,000 of them. Like the Baldwin, this combined a water-tube firebox with a fire-tube barrel. The original characteristic of the Brotan was a long steam drum running above the main barrel, making it resemble a Flaman boiler in appearance.
This type has three drums in a delta formation connected by water tubes. The drums are linked by straight water tubes, allowing easy tube-cleaning. This does however mean that the tubes enter the drums at varying angles, a more difficult joint to caulk. Outside the firebox, a pair of cold-leg pipes between each drum act as downcomers.
Due to its three drums, the Yarrow boiler has a 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.
A single steam drum has two sets of water tubes either side of the furnace. These tubes, especially the central set, have sharp curves. Apart from obvious difficulties in cleaning them, this may also give rise to bending forces as the tubes warm up, tending to pull them loose from the tubeplate and creating a leak. There are two furnaces, venting into a common exhaust, giving the boiler a wide base tapering profile.