A water tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire.
A water tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which heats 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 through a super heater to become superheated. Superheated steam is defined as steam that is heated above the boiling point at a given pressure. Superheated steam is a dry gas and therefore used to drive turbines, since water droplets can severely damage turbine blades.
Cool water at the bottom of the steam drum returns to the feed water drum via large-bore 'down comer tubes', where it pre-heats the feed water supply. (In 'large utility boilers', the feed water is supplied to the steam drum and the down comers supply water to the bottom of the water walls). To increase economy of the boiler, exhaust gases are also used to pre-heat the air blown into the furnace and warm the feed water 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.
1. 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.
2. 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.
1. D-type boiler: The 'D-type' is the most common type of small- to 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. M-Type Boilers Two fireboxes with a control super-heater.
2. Low Water Content: The 'Low Water Content' 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 quickly to changes in load.
3. Babcock & Wilcox boiler: Designed by the American firm of Babcock and Wilcox, this type has a single drum, with feed water 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.
• Combustion Engineering
4. Stirling boiler: The Stirling boiler has near-vertical, almost-straight water tubes that zigzag between a number of steams and water drums. Usually there are three banks of tubes in a "four drum" layout, but certain applications use variations designed with a different number of drums and banks. They are mainly used as stationary boilers, owing to their large size, although the large grate area does also encourage their ability to burn a wide range of fuels. Originally coal-fired in power stations, they also became widespread in industries that produced combustible waste and required process steam. Paper pulp mills could burn waste bark, sugar refineries their biogases waste.
5. Yarrow: Named after its designers, the then Poplar-based Yarrow Shipbuilders, 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 'down comers'. 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.
6. White-Forster: The White-Forster type is 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.
7. Thornycroft: Designed by the shipbuilder John I. Thornycroft & Company, the Thornycroft type features a single steam drum with 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 tube plate and creating a leak. There are two furnaces, venting into a common exhaust, giving the boiler a wide base tapering profile.