Classification of Steam Boilers :

Though three are many classifications of steam boilers, yet the following are important from the subject point of view : 

1. According to the contents in the tube:

(a) Fire tube or smoke tube, and

(b)Water tube.

Fire tube or smoke tube :

In fire tube steam boilers, the flames and hot gases, produce by combustion of fuel, pass through the tubes(called multi-tubes) which are surrounded by water. Examples, of fire tube steam boilers are : Simple vertical boiler, Cochran boiler, Lancashire boiler, Cornish boiler, Scotch marine boiler, Locomotive boiler, and velcon boiler.

Water tube :

In water tube steam boilers, the water is contains inside the tube (called water tubes) which are surrounded by the flames and hot gases from outside. Example of water tube boiler are : Babcock and Wilcox boiler, La-mont boiler, Benson boiler, Yarrow boiler and Loeffler boiler.

2. According to the position of the furnace : 

(a)Internally fired, and

(b)Externally fired.

Internally fired :

In internally fired steam boilers, the furnaces is located inside the boiler shell. Most of the fire tube steam boilers are internally fired.

Externally fired :

In externally fired steam boilers, the furnace is arranged underneath in brick-work setting.water tube steam boilers are always externally fired.

3. According to the axis of the shell:

(a)Vertical, and

(b)Horizontal.

In “vertical steam boilers”, the axis of shell is vertical, whereas it is horizontal in case of “horizontal steam boilers”.

4. According to the number of tube:

(a)Single tube, and

(b)Multitubular.

In “single tube steam boilers”, there is only one fire tube or water tube. Simple vertical boiler and Cornish boilers are single tube boilers. In “multi tubular steam boilers”, there are two or more fire tubes or water tubes.

5. According to the method of circulation of water and steam:

(a)Natural circulation, and

(b)Forced circulation.

In natural circulation steam boilers, the circulation of water is by natural convection currents which are set up during the heating of water. In most of the steam boilers, there is a natural circulation of water. In “forced circulation steam boilers”, there is a forced circulation of water by a centrifugal pump driven by some external power. Use of forced circulation is made in high pressure boilers such as Lamont boiler, Benson boiler, Loeffler boiler and velcon boiler.

6. According to the use:

(a)Stationary, and

(b) Mobile.

The “stationary steam boilers” are used in power plants, and in industrial process work. These are called stationary because they do not move from one place to another. The “mobile steam boilers” are those which move one place to another. These boilers are locomotive and marine boilers.

7. According to the source of heat:

The steam boilers may also be classified according to the source of heat supplied for producing steam. These sources may be the combustion of solid, liquid or gaseous fuel, hot waste gases as by products of other chemical processes, electrical energy or nuclear energy, etc.