AÂ fire-tube boiler Â differs from a water tube boiler, andÂ is a type ofÂ boilerÂ in which hot gases from a fire pass through one or more tubes running through a sealed container of water. TheÂ heatÂ of the gases is transferred through the walls of the tubes byÂ thermal conduction, heating the water and ultimately creatingÂ steam.
The fire-tube boiler developed as the third of the four major historical types of boilers: low-pressure tank or “haystack” boilers,Â flued boilersÂ with one or two large flues, fire-tube boilers with many small tubes, and high-pressureÂ water-tube boilers. Their advantage over flued boilers with a single large flue is that the many small tubes offer far greater heating surface area for the same overall boiler volume. The general construction is as a tank of water penetrated by tubes that carry the hotÂ flue gasesfrom the fire. The tank is usuallyÂ cylindricalÂ for the most partâbeing the strongest practical shape for aÂ pressurized containerâand this cylindrical tank may be either horizontal or vertical.
This type of boiler was used on virtually allÂ steam locomotivesÂ in the horizontal “locomotive” form. This has a cylindrical barrel containing the fire tubes, but also has an extension at one end to house the “firebox”. This firebox has an open base to provide a large grate area and often extends beyond the cylindrical barrel to form a rectangular or tapered enclosure. The horizontal fire-tube boiler is also typical of marine applications, using theÂ Scotch boiler.Â Vertical boilersÂ have also been built of the multiple fire-tube type, although these are comparatively rare: most vertical boilers were either flued, or with cross water-tubes.
AÂ water tube boilerÂ is a type ofÂ boilerÂ in which water circulates in tubes heated externally by the fire. Fuel is burned inside theÂ furnace, creating hot gas which heats water in the steam-generating tubes. In smaller water tubeÂ boilers, additional generating tubes are separate in the furnace of the Â water tubeÂ boiler, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generateÂ steam.
The heated water then rises into theÂ steam drum. Here, saturated steam is drawn off the top of the drum. In some services, the steam will reenter the furnace through aÂ superheaterÂ 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.
In the water tubeÂ boiler cool water at the bottom of the steam drum returns to the feedwater drum via large-bore ‘downcomer tubes’, where it pre-heats 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 economy of the boiler, exhaust gases are also used to pre-heat the air blown into the furnace and warm the feedwater supply. Such water tube boilers inÂ thermal power stationÂ are also calledÂ steam generating units.
The olderÂ fire-tube boilerÂ design â in which the water surrounds the heat source and the gases from combustion pass through tubes through the water space â is a much weaker structure and is rarely used for pressures above 350Â psi (2.4Â 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. Submit a request for a water tube boiler, fire tube boiler, or package boiler today.