(i) Externally fired boilers, in which heat adding is complete outwardly that is furnace is outside the boiler unit for example, Lanchashire boiler, Locomotive boiler and so on.
(ii) Internally fired boilers, in which heat adding is done inside that is furnace is within boiler unit. Such as Bobcock Wilcox boiler, Cochran boiler and so on. (b) Based upon the direction/axis of shell: According to axis of shell boiler can be classify as vertical and horizontal boiler.
(i) Vertical boilers have its shell vertical.
(ii) Horizontal boilers have its shell horizontal.
(iii) Inclined boilers have its shell inclined. (c) Based upon function of boiler: Boilers can be classify as
(i) Portable boiler, such boilers are portable and are of little volume. These can be of follow kind, Locomotive boiler, which are exclusively use in locomotives. Marine boiler, which are use for marine applications.
(ii) Stationery boiler, such boilers are stationery and are widely use in power plants, industrial process, heating and so on. (d) Based upon tube content: Based on fluid inside the tubes, boilers can be,
(i) Fire tube boilers, such boilers contain the hot gases inside tube and water is exterior surrounding them. Examples for these boilers are Cornish boiler, Lancashire boiler, Cochran boiler, Locomotive boiler and so on.
(ii) Water tube boilers, such boilers contain water flowing inside tubes and hot gases encircle them. Examples for such boilers are Stirling boiler, La-Mont boiler, Babcock-Wilcox boiler, Benson boiler and so on. (e) Based on nature of fuel use: According to kind of fuel use the boilers can be,
(i) Solid fuel fired boilers, for example coal fired boilers and so on.
(ii) Liquid fuel fired boilers, for example oil fired boilers and so on.
(iii) Gas fired boilers, for example natural gas fired boilers and so on. (f) Based on transmission: According to flow of water and steam in boiler circuit boilers can be of follow types,
(i) Natural circulation boilers, in which flow of water/steam is cause by density dissimilarity which is due to temperature variation.
(ii) Forced circulation boilers, in which circulation of water/steam is caused by a pump that is outwardly assisted circulation. (g) Based on level of firing: According to level of firing boilers can be,
(i) Fired boilers, in which heat is provide through fuel firing.
(ii) Unfired boilers, in which heat is provide through some other source except fuel firing for example hot flue gases etc.
(iii) Supplementary fired boilers, in which a section of heat is give by fuel firing and remaining through some other cause.