Water-tube boiler

Water-tube boiler

[waw-ter-toob, -tyoob, wot-er-]

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.

Uses of water tube boilers


Modern boilers for power generation are almost entirely water-tube designs, owing to their ability to operate at higher pressures. Where process steam is required, for heating or as a chemical component, then there is still a small niche for fire-tube boilers.


Again, their ability to work at higher pressures has led to marine boilers being almost entirely water-tube. This change began around 1900, and traced the adoption of turbines for propulsion rather than reciprocating (i.e. piston) engines - although water-tube boilers were also used with reciprocating engines.

Railway locomotives

There has been no significant adoption of water-tube boilers for railway locomotives. A handful of experimental designs were produced, but none of these were successful or led to their widespread use. Surprisingly none of these appear to have been steam turbine locomotives, the other great innovation of the period. Most were compounds, and a few uniflows. Most water-tube railway locomotives, especially in Europe, used the Schmidt system.

Rebuilt completely after a fatal accident

Using a Yarrow boiler, rather than Schmidt. Never successful and re-boilered with a conventional boiler.


A slightly more successful adoption was the use of hybrid water-tube / fire-tube systems. As the hottest part of a locomotive boiler is the firebox, it's an effective design to use a water-tube design here and a conventional fire-tube boiler as an economiser (i.e. pre-heater) in the usual position.

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.

Some 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.

Low Water Content

This boiler has a lower and upper header connected by water tubes that are directly impinged upon from the burner. This is a "furnace-less" boiler that can generate steam and react to load changes quickly.

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. This type of boiler was used by the Royal Navy's Leander class frigates. The Y160 variant used on the Batch 3 Leanders (eg HMS Jupiter) also incorporated steam atomisation equipment on the fuel supply so that the diesel fuel entering the boilers via the three main burners was atomised into a fine spray for better flame efficiency. The superheat temperature of the Y160 was controlled manually by the Boiler Room Petty Officer of the Watch between 7500F and 8500F and the steam supplied to the main turbines was at a pressure of 550 psi.

Stirling boiler

This type has three upper drums connected to two lower drums by water tubes. These are mainly used as stationary boilers. There are also Stirling's that are two drum, a steam drum and a mud drum. These are used mostly in industrial settings (Paper Mills, etc). These are also a type of boiler you will see that will burn multiple fuels, such as bark, bagasse, etc. .


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.


Similar to the Yarrow, but with tubes that are gradually curved. This makes their entry into the drums perpendicular, thus simpler to make a reliable seal.


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.

Other types

  • O-type
  • A-type
  • Flex-Tube Boiler

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