Classification and working of boilers

Definition

A boiler is a closed pressure vessel in which water is heated and converted into steam at a required pressure and temperature by the application of heat from an external source.

Main Content

1. Classification of Boilers

According to the contents of tubes

Fire-tube boiler: Hot flue gases pass through tubes and water surrounds the tubes. Example: Cochran boiler, Cornish boiler, Lancashire boiler. These are simple in construction, suitable for low to medium pressure, and widely used in small industries.
Water-tube boiler: Water flows through tubes and hot gases surround them. Example: Babcock and Wilcox boiler, Stirling boiler, LaMont boiler. These are suitable for high pressure and high steam generation rates.

According to the axis of shell

Horizontal boiler: The boiler shell is placed horizontally. It occupies more floor space but is common in many industrial applications.
Vertical boiler: The boiler shell is vertical. It requires less floor space and is generally compact, used in small power plants and portable systems.
Inclined boiler: The shell is inclined at an angle, which may improve circulation and specific operational requirements in some designs.

According to location of furnace

Internally fired boiler: The furnace is inside the boiler shell, and combustion takes place within the boiler body. Example: Cochran boiler.
Externally fired boiler: The furnace is outside the boiler shell, and heat is transferred to the boiler through tubes or flue passages. Example: Babcock and Wilcox boiler.

2. Classification Based on Working Pressure and Circulation

According to steam pressure

Low-pressure boiler: Operates at relatively low steam pressure, usually used in heating and small industrial processes.
Medium-pressure boiler: Used where moderate pressure steam is needed for process and power applications.
High-pressure boiler: Operates at very high pressure and temperature, used in modern thermal power plants for better efficiency and higher output.

According to method of water circulation

Natural circulation boiler: Circulation of water occurs naturally due to density difference between hot and cold water. The movement does not require any external pump. Example: Babcock and Wilcox boiler.
Forced circulation boiler: A pump is used to circulate water through the boiler tubes. This improves heat transfer and is suitable for high-pressure operation. Example: LaMont boiler.
Once-through boiler: Water passes through the boiler only once and is converted into steam without recirculation. These are used in modern high-capacity units.

According to mobility

Stationary boiler: Fixed at one location, used in power stations and factories.
Portable boiler: Can be moved from one place to another, used in construction sites and temporary installations.
Locomotive boiler: Used in railway engines.
Marine boiler: Used in ships and marine propulsion systems.

3. Classification Based on Fuel, Firing, and Use

According to fuel used

Solid fuel boiler: Uses coal, wood, biomass, or coke.
Liquid fuel boiler: Uses furnace oil, diesel, or other petroleum fuels.
Gaseous fuel boiler: Uses natural gas, producer gas, or biogas.
Electric boiler: Uses electricity to heat water and generate steam.
Waste heat boiler: Uses waste heat from industrial processes such as exhaust gases from gas turbines or furnaces.

According to method of firing

Hand-fired boiler: Fuel is fed manually into the furnace.
Mechanical stoker-fired boiler: Fuel is supplied mechanically, improving efficiency and reducing labor.
Pulverized fuel-fired boiler: Coal is pulverized into fine powder and burnt in suspension, giving high combustion efficiency.
Fluidized bed boiler: Fuel is burnt in a bed of suspended particles, allowing efficient combustion and lower emissions.

According to application

Power boiler: Used to generate steam for electricity production.
Process boiler: Supplies steam for industrial processes like heating, drying, and chemical reactions.
Heating boiler: Used mainly for space heating and hot water supply in buildings.

Working / Process

1. Feed water supply and preheating

Water is first supplied to the boiler through a feed pump or feed system. In many plants, the feed water is treated and preheated before entering the boiler to remove impurities and improve efficiency.
Proper water treatment is important to prevent scale formation, corrosion, and tube damage.

2. Combustion and heat transfer

Fuel is burned in the furnace or combustion chamber, producing hot flue gases.
These hot gases transfer heat to the water through boiler surfaces such as tubes, shell walls, or heating surfaces. In fire-tube boilers, gases flow inside tubes; in water-tube boilers, water flows inside tubes.

3. Steam generation and steam delivery

Heat raises the water temperature to saturation point and converts it into steam.
The steam may be saturated or superheated depending on boiler design. It is then collected in the steam space and sent to turbines, engines, or process equipment through steam pipes.
The flue gases leave the boiler through the chimney or economizer path after giving up most of their heat.

Advantages / Applications

Boilers provide a continuous and reliable supply of steam for industrial and power applications.
They are used in thermal power plants, chemical industries, refineries, textile mills, paper industries, food processing, and heating systems.
Modern boilers can achieve high efficiency, better combustion control, and reduced fuel consumption, especially in water-tube and waste heat boilers.

Summary

Boilers are pressure vessels used to convert water into steam by applying heat.
They are classified based on tube contents, axis, furnace location, pressure, circulation, mobility, fuel, firing method, and application.
The working of a boiler involves feed water supply, combustion, heat transfer, and steam production.