Classification of boilers

1. Horizontal, Vertical or Inclined Boiler.

-If the axis of the boiler is horizontal, the boiler is called horizontal, 

-If the axis is vertical, it is called vertical boiler and 

-If the axis is inclined it is called as inclined boiler.

2. Fire Tube and Water Tube

-In the fire boilers, the hot gases are inside the tubes and the water surrounds the tubes.
Examples: Cochran, Lancashire and Locomotive boilers.
-In the water tube boilers, the water is inside the tubes and hot gases surround them.
Examples: Babcock and Wilcox,
 Yarrow boiler etc.

3. Externally fired and internally fired

-The boiler is known as externally fired if the fire is outside the shell.
Examples: Babcock and Wilcox boiler boiler
-In case of internally fired boilers, the furnace is located inside the shell.
Examples: Cochran, Lancashire boiler etc.

4. Forced circulation and Natural Circulation

-In forced circulation type of boilers, the circulation of water is done by a
forced pump.
Examples: Lamont, Benson Boiler etc.
-In natural circulation type of boilers, circulation of water in the boiler takes
place due to natural convention currents produced by the application of
heat.
Examples: Lancashire, Babcock and Wilcox boiler etc.

5. Higher Pressure and Low Pressure Boilers
-The boiler which produce steam at pressures of 80 bar and above are
called high pressure boilers.
Examples: Babcock and Wilcox, Benson Boiler etc.
-The boilers which produce steam at pressure below 80 bar are called low
pressure boilers.
Examples: Cochran, Cornish, Lancashire and Locomotive boiler etc.

6. Stationary and Portable

-Stationary boilers are used for power plant steam, for central station utility
power plants, for plant process steam etc.
-Mobile boilers or portable boilers include locomotive type, and other small
units for temporary use at sites.

7. Single Tube and Multi Tube Boiler

The fire tube boilers are classified as 

-single tube and 

-multi-tube boilers,
depending upon whether the fire tube is one or more than one.
Examples: Cornish, simple vertical boiler are the single tube boiler and rest
of  the boilers are multi-tube boiler.


8. According to passes
-single pass
-Multi pass