Steam generators are classified based on the following:
- Water tube and fire tube: In a water tube boiler, water or steam flows through the tubes and heat is supplied to the external surface. Babcock and Wilcox and Stirling boilers fall under this category.In a fire tube boiler, hot gases flow through the tubes and water circulates outside the tubes. There may be one large tube surrounded by water as in a Cornish boiler, or two large tubes surrounded by water in a big tube of water as in the case of Lancashire boiler. There may be many smaller tubes through which hot gases pass and are surrounded by water as in the case of vertical Cochran or locomotive boiler.
- Natural/forced circulation: Natural circulation of water takes place by natural convection currents produced by application of heat as in the case of Lancashire boiler and Babcock and Wilcox boiler.In forced circulation, the fluid is forced ‘once through’ or controlled with partial recirculation as in the case of LaMont boiler, Velox boiler, and Benson boiler.
- Horizontal, vertical, or inclined: The disposition on the principal axis of the boiler decides whether the boiler is horizontal, vertical, or inclined.
- Single/multiple tube(s): A boiler may have only one fire/water tube or multiple tubes.
- Stationary/mobile: Boilers are called stationary (land) or mobile (marine and locomotive). Stationary boilers are used for power plant steam generation. Mobile boilers are portable boilers.
- Internally/externally fired: A boiler is said to be an external combustion boiler when combustion takes place outside the region of boiling water. A boiler is said to be an internal combustion boiler if the furnace region is completely surrounded by water-cooled surface as in the case of Lancashire boiler.
- Source of heat: Boilers may be classified according to the source from which heat is supplied to water for evaporation. For example, coal fired, oil fired, gas fired, electrical energy, or nuclear energy.
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