Mechanical Basics MECHANICAL THEORY Thermal Engineering

What is a Boiler? Classification of Boiler, Selection, Boiler Mountings & Accessories

What is a Boiler? Boiler Mountings & Accessories: In this article, I will explain about What is a Boiler? Selection of a boiler, Essentials of Good boiler, Classification of a boiler, Diagram of the boiler, Boiler Mountings and Boiler Accessories in a detailed way.

Now let’s discuss about the Introduction of a boiler.

What is a Boiler?

 The equipment used for producing steam is called Boiler or Steam Generator.


It is also defined as “ A closed vessel in which steam is produced from water by the combustion of fuel.”

The steam generated is used for

  • Power generation
  • Heating
  • Utilization in industries like chemical industries, sugar mills, etc.

Considering, the above factors, I had written an article on Boiler-Definition, Classification, Selection, Essentials of a good boiler, Boiler Mountings & Accessories in a detailed way.

Classification, Good Boiler, Boiler Mountings & Boiler Accessories:

As the definition of the boiler machine was stated above, therefore, I am giving another detail such as

  1. Classification of boiler
  2. Diagram of the Boiler
  3. Selection of Boiler
  4. Essentials of good boiler
  5. Boiler mountings
  6. Boiler Accessories.

1.7 Classification of Boiler:

The boilers may be classified as follows.

  1. Horizontal, Vertical and Inclined Boilers
  2. Externally Fired and Internally Fired
  3. Fire-tube boiler and Water-tube boiler.
  4. Natural circulation and Forced Circulation
  5. High Pressure and Low-Pressure boilers
  6. Stationary and Portable boilers
  7. Single tube and Multitube boilers.

The explanation of the above types of boilers is as follows.

1.Horizontal, Vertical and Inclined Boilers:

  • If the axis of the boiler is horizontal, then the boiler is of horizontal type.
  • If the axis of the boiler is vertical, then it is called a vertical boiler.
  • If the axis of the boiler is inclined, then it is called an inclined boiler.

The parts of a horizontal boiler can be inspected and repaired easily. But, it occupies more space and the vertical boiler occupies less floor area.

2. Externally Fired and Internally Fired Boilers

  • The boiler is known as externally fired boiler if the fire is outside the shell.

Ex: Babcock and Wilcox Boiler.

  • In case of internally fired boilers, the furnace is located inside the boiler shell.

Ex: Cochran, Lancashire boiler, etc.

3.Fire-tube boiler and Water-tube boiler:

  • In the fire tube boilers, the hot gases are inside the tubes and the water surrounds the tubes.

Ex: Cochran, Lancashire boiler, etc.

  • In the water tube boilers, the water is inside the tubes and hot gases around them

Ex: Babcock and Wilcox Boiler.

4.Natural circulation and Forced Circulation:

  • In forced Circulation type of boilers, the circulation of water is done by a forced pump.

Ex: Lamont Boiler, Benson Boiler, etc.

  • In natural circulation type of boilers, circulation of water in the boiler takes place due to the natural convection currents produced by the application of heat.

Ex: Lanarkshire, Babcock and Wilcox boiler.

5.High Pressure and Low-Pressure boilers:

High-Pressure Boilers:

The boilers which produce steam at pressures of 80 bar and above are called high-pressure boilers.

Ex: Babcock and Wilcox, Lamont, Benson boilers.

Low-Pressure Boilers:

The boilers which produce steam at a pressure below 80 bar are called low-pressure boilers.

Ex: Cochran, Lancashire and locomotive boiler.

6.Stationary and Portable Boilers:

  • Primarily, the boilers are classified as either stationary or mobile(marine and Locomotives)

Stationary boilers are used for power plant steam for Central station utility power plants.

Portable Boilers:

Portable or mobile boilers include the locomotive type and other small units for temporary usage at sites(Just as in small coal-field pits).

7.Single tube and Multi-tube Boilers:

  • If a fire tube is just one, then it is called a Single tube boiler.

Ex: Cornish, Simple Vertical Boiler.

  • If fire tubes are more than one, they are called as Multi-tube boilers.

Difference between Water Tube Boiler and Fire Tube Boiler:

Criterion Fire Tube Water tube
Position of hot gases and water Hot Gases in tubes surrounded by water outside Water in tubes and  Hot gases flow outside
Mode of firing Internal fired External fired
Floor area For a given power, it occupies more floor area. For a given power, it occupies less floor area.
Capacity 50000kg/hr
Evaporation Slow Fast
Operating Pressure range 15 to 20 bar 170 to 200 bar
Suitability Not suitable for large power plants. Suitable for large power plants. 
Risk on burning Involves Lesser risk of explosion due to lower Pressure Involves more risk on bursting due to High Pressure
Application Not suitable for large power plant Suitable
Requirement of Skill Requires Less skill for efficient and economic working Requires more skill for efficient and economic working
Water Treatment Not so necessary More necessary.
Construction Difficult Simple
Shell Diameter More Less
Transportation Difficult Simple
Accessibility of various parts Not easily accessible for cleaning, repairing and inspection. Easily accessible

2.Diagram of Boiler:

The Boiler Diagram is shown below in the form of an image.

what is a Boiler-Def,Classification,selection, Essentials,Boiler Mountings & Accessories
what is a Boiler-Def,Classification,selection, Essentials,Boiler Mountings & Accessories

3.Selection of Boiler:

The selection of Boilers is based on below conditions.

  • Floor area available
  • Portable load factor
  • Erection Facilities
  • The fuel and water availability
  • Accessibility for repair and inspection
  • Comparative initial cost
  • Steam generation rate
  • Operating and maintenance cost

4.Essentials of Good Boiler:

• The boiler should produce the maximum weight of steam of required quality at minimum expenses.
• Should have easy access to various parts of boilers for inspection and repairs.
• Installation of the boiler should be simple.
• Boiler components should be such that they would be transportable from one region to another without any difficulty.
• Steam production rate should be as per requirements
• Reliable.
• It should have to occupy in a minimum space.
• Tubes should not accumulate soot and water deposits.

5.Boiler Mountings:

The Boiler Mountings are presented below.

  • Water Level Indicator
  • Blow off cock
  • Feed check valve
  • Safety Valves
  • Pressure Gauge
  • Fusible Plug

6.Boiler Accessories:

The Boiler Accessories are presented below.

  • Feed Pumps
  • Injector
  • Steam Separator
  • Steam Trap
  • Economizer
  • Air Preheater
  • Superheater

This is the complete explanation of Boiler Machine-Definition, Classification, etc. in a detailed way. If you have any doubts feel free to ask from the comment section.


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