How Does A Boiler Work? | An-Overview

What Is A Boiler?

“A boiler is also known as a steam generator. A boiler is a closed vessel that is used to heat the water or other liquids. The boiled fluid or water will generate steams or vapors, and the temperature of that steam will be very high, either under pressure or under vacuum. In simple words, a boiler or steam generator is a device designed to convert fluid (mostly water) into steam. Electricity or nuclear energy directly uses in the boiler to burn the fuel.”

boiler, steam generator

In a conventional steam power plant, a boiler contains a furnace in which the fuel is burned to convert the heat from combustion products to the surface of the water from where the steam can be collected. A conventional boiler furnace burns fossil fuels or burns spent fuel in some facilities. In the steam generator or boiler, a nuclear reactor can also be a heat source that generates steam under pressure.

Water is an inexpensive and most valuable means of converting thermal energy to the process. As water turns into steam, its volume rises 1600 times and the resulting explosive force act as gunpowder. This process makes the boiler a very hazardous device and must be handled with care.


The heroes of Alexandria built boilers at the beginning of the first century, but they were only used as toys. In the seventeenth century, the potential for steam energy was seriously considered in practical work. The first boiler with a safety valve was designed in 1679 by French Denis Papin. The boiler was manufactured and used in England at the end of the eighteenth century. Early boilers were made of iron. When people realized the benefits of high pressure and high temperature, manufacturers began to use steel. Modern boilers are made of alloy steel and are resistant to high pressure and very high temperatures.

Most traditional steam boilers categorize as a type of fire-tube or water-tube. In the fire-tube type, steel pipe surrounds with the water, and hot air flows through the steel pipe. The cylindrical drum collects the steam formed on the surface of the water.

Fig: A moveable traditional boiler

Install a safety valve with the boiler so that it can let the steam to evaporate at a pressure higher than the working pressure. This device is essential for all boilers as the heat in the closed vessel’s water continues to increase, but it prevents steam from accumulating and causing the boiler to explode. The advantage of a fire tube steam generator or boiler is that it is easy to manufacture, install and easy to use. It is widely used in small-scale equipment for heating buildings and as a power source for power plant operations. Fire tube boilers are also used in steam engines.

Working of Steam Generator or Boiler

The boiler has a very simple working principle that is very easy to understand. Generally, a boiler is a closed container that is filled with water. Fuel (usually oil, natural gas or coal) is burned inside a furnace to produce hot gases. These hot gases make contact with the water vessel, where the heat from these hot gases is transported to the water and creates steam in the boiler. This generated steam is then directed to the turbines of the power plant. This steam will power the production equipment, disinfection, heat saving, steam purification, etc.

Boiler working

When 100% of the generated vapor is returned for reuse, the system is called a closed system. Specimens of closed systems are closed steam heating systems, hot water heating systems, and “single tube” system. Since some methods can taint the steam, it is not always necessary to drain the condensate back into the boiler.

In the power plant, different boiler types work for different operations, e.g., to warm up the surroundings, sterilize equipment, sanitize some area, or operate production units.

Types of Boiler

Different boiler types are given below according to different functions. 

1) According to the water and hot gases Positions

  1. Fire-tube boilers
  2. Water-tube boilers

I) Fire Tube Boiler

The Fire-tube Boiler is considered one of the simplest types of boiler, and this proposal is very out-of-date. It was common in the period of the eighteenth. It is mainly used in vapor locomotives.

Fire-Tube Boiler

The fire tube boiler or steam generator is intended to transfer heated air/gas to a heat resource and recycle. Gas flows over a tube equipped with a bucket. In this way, the hot gas’s heat is effectively transferred to the water and thus, steam is generated. The main qualities of a fire tube steam generator are that it has a clean layout, easy operation and low cost. These boiler types are particularly flexible in generating less than regular power, skill in configuration and power in a variety of designs.

Advantages of Fire-Tube Boilers:

  1. The construction of these boilers is relatively easy.
  2. Steam demand fluctuation can be met easily.
  3. The price of these boiler types is very low.
  4. Fire-tube boilers have an easy operation.

Disadvantages of Fire-Tube Boilers:

  1. Since the operation of this boiler requires a large amount of water, it takes a long time to bring the steam up to the required pressure.
  2. Since steam and water are in one container, the steam pressure cannot be converted to very high.
  3. It doesn’t provide very dry steam.

II) Water-Tube Boiler

Water-tube boilers are mainly used to produce high pressure and temperatures steam. The domestic composition of the boiler contains a small steam barrel and a tube of small width. The modules of this boiler produce large amounts of steam for high volume applications.

water tube boiler

These boilers are the most used, and for some reasons, many boilers have been replaced it hater tube boilers, such as fire tubes. For example, the water tubes are characterized by low weight, fast steam production process, custom design and high efficiency. The back faucet has a single-cylinder arranged by a single manifold, and the manifold is connected by a short, straight and attached spout. The hot gas will flow through the tube at the same time.

III) Combi Boiler

These boiler types work as a water heater and central heating boiler in the same compact unit. Combi boilers account for more than half of the domestic boilers sold in the UK. As they are very cost-effective and energy-efficient, which helps reduce energy costs. Unlike other boilers, a combi boiler or steam generator heats the water directly from the mains when the tap is opened. This means you don’t need an outdoor hot water tank or a cold water tank to save space. Using a combi boiler, you don’t have to deal with annoying pumps because you don’t constantly heat the stored water.

Combi Boilers

Combi boilers are very suitable for small home’s applications such as for bathroom and apartment applications. Because they only produce a certain amount of hot water at a time, which can be a problem in larger homes with multiple bathrooms where hot water requirements are too high. Larger homes strive to produce very hot water simultaneously, which usually reduces the water pressure.


  • Compact size is ideal for small features.
  • This boiler is a good solution if there isn’t enough space to store it well.
  • Get hot water right away – don’t wait for the water tank to heat up.
  • No cold water tank or hot water tank needed to free up living space.
  • Under the high pressure of the water supply, when you give hot water, it means you can have a more powerful shower.


  • Not ideal for large homes or homes with multiple baths.
  • Multiple water pressures can be affected if you use multiple stores simultaneously.
  • If there is a problem with the boiler, the stored hot water cannot be used.
  • Always be warmer.
  • The bath takes longer to fill.

In the latest Combi boilers, gas and oil can be used as fuel.

2) Boiler Types According to the Usage

I) Electric Boilers

Over 25M home boilers in the UK burn natural gas or oil to heat water, while electric boilers use an electronic component to heat the water. Nowadays, electric boilers are gaining more popularity because these can be installed in most small and medium-sized homes to produce enough hot water for central heating, bathrooms and ducts. If there is no gas, it is essential to consider the electric boilers, and people hope to reduce carbon emissions.

Electric Boiler, steam generator

The installed electric boiler (steam generator) looks just like a combi boiler, and it is the only unit that provides hot water and central heating at the same time. Combi Electric boilers use their own heating element to supply you hot water anytime. The installation kit of the electric boiler is the cheapest and most accessible, and since it does not require a water tank, it saves you space in the house. Keep in mind that due to the shortage of water, you will not be able to heat the water overnight at a cheaper rate, and therefore you will not be able to take advantage by saving the costs.

II) Burnham Boilers

The Burnham boiler is a multifunctional boiler designed for high efficiency and durability. The technology with that it has been equipped helps to save energy while maintaining comfort.

Burnham Burnham boiler has three major types and each type has different characteristics:

  1. Cast iron/steel oil boilers.
  2. Cast iron gas boilers
  3. Condensing gas boilers

III) Locomotive Steam Generator or Boiler

The locomotive steam generator is a fire-tube steam generator. It is a device that generates steam from water through thermal energy. It is a water boiler with a horizontal cylinder shaft. Its principal function is to produce high steam velocity. Thus this boiler has a large gravel area and is also equipped with a fire tube to increase steam generation. In addition, you can also notice an industrial steam jet stream in this boiler which can increase heat transfer. In this type of boiler, a chimney is used, and the chimney removes burning gas or liquefied gas.

Locomotive Steam Generator

As the name suggests, it is a locomotive boiler that means working; we don’t have to install a chimney in it. The airflow over the chimney eliminates the pressure difference between grinding and the outside. This type of boiler can produce high-speed steam and thus can be used for railway and seaway locomotives.

IV) Lancashire Boilers

A Lancashire boiler is similar to Cornish, but it has two high flues with fire. This boiler pressure range is about 0.7 MPa to 2 MPa, and the efficiency is from 65 to 70%. In these boiler types, fuel is added to the pumpkin to heat up the gas. Hot gas enters from the boiler’s front and leaves the boiler from its back, then enters the downstream and begins to flow to the front of the boiler. Let the hot gases flow to the next section and enter the sliding current, then go to the back of the boiler and enter the main outlet. If the hot gas is in the fire pipe, it transfers 85% of the heat, and if it is in the down and side ducts, it transfers only 15% of the heat.

Lancashire Boilers

V) Shell and tube Steam Generator

Shell and Tube boilers are also called fire-tube boilers or smoke boilers. A fire-tube boiler or steam generator consists of a long steel tube through which the hot gas and water passing; after that, these hot gas and water are converted into steam. Generally speaking, Shell and tube boilers that reduce upfront costs are more fuel-efficient and easier to use. Shell-tube steam generator production capacity reaches 25 tons/hour and 17.5 kg/cm.

Shell and tube Steam Generator

VI) Cornish Boilers

These are the oldest forms of high-pressure tube boilers. It consists of a long horizontal cylinder in which there are a large fire and one large flow. Fuel is added in the field area where the combustion results in hot gas. The hot gas then transfers heat to the water.

Cornish Boilr

The water is heated, and after a while, it starts to boil to produce steam. When they reach at the end of the fire tube, then the hot gas is divided into two, and each gas comes into one of the two-way ducts and then is brought to the front of the boiler, where it flows in and out. Turn them over to the stove.

The chimney releases these gases into the atmosphere outside the boiler. The most significant heat transfer occurs in the fire tube and shell section, then in the tributary and finally downstream.

VII) Multi-Tube Boilers

The design and manufacturing process of this type of boiler use various types of fuels, such as urban and agricultural waste, fuel oil, coal, natural gas, and other biomass, widely used in engineering-related industries, known for their corrosion resistance, ease of installation and robust structure.

Multi-Tube steam generator

3) Types According to the Position of Steam Generator or Boiler

I) Horizontal Boiler or Steam Generator

These have a long service life, sufficient steam, high thermal efficiency, and a large water capacity. It equips with a pressure feed pump and a high-quality branded combustion chamber. It has adequate combustion, low noise level and timely water supply. When the protective device is ready, the burner can be equipped with a cleaning device after high temperatures, which makes it reliable and safe.

Horizontal Boiler

II) Longitudinal Drum Boiler

The cylinder of this boiler locates above the heat source. The cold water flows into the inclined tube, and eventually, the water in the heat pipe gets hot. When the water boils, its density decreases, and hot water and steam circulate. The steam separates from the water in the steam drum and ejected. Long drum boilers range from 2,250 kg per hour to 3,600 kg per hour.

Longitudinal Drum boiler,

 III) Cross Drum Boiler

The drum of this steam generator is placed on the heat source. The temperature found in this type of composition is more consistent. If the steam charge is high, the overhead tubes can dry out, resulting in their failure. The test tubes are designed so that a large number of test tubes can be used—drum cross-section capacity from 700kg to 240,000kg per hour.

Cross Drum steam Boiler

IV) Stirling Boilers

Stirling boilers have nearly vertical and straight water tubes winding between different steam and water drums. Generally, four-cylinder layouts have three tubular edges. The feedwater enters the upper left bucket and falls from there into the lower bucket. The water is heated in the tube and the two barrels, and steam is generated in the upper barrel, separating the steam from it and leaving it outside.

Stirling Boilers

4) According to the Position of the Boilers

internal and external steam generator

I) Externally Fired Steam Generator or Boiler

In the case of an external combustion boiler, the fuel burns inside the burner that attaches to the boiler’s shell’s the outside area.

II) Internally Fired Boiler or Steam Generator

In the case of an internal combustion boiler, the burner attaches inside the boiler’s shell.

5) According to the Pressure 

I) High-Pressure Boilers

A high-pressure boiler is a shell and tube heat exchanger that generates high-pressure steam or hot water after boiling water. By using the metal tank and water injection tube, these boilers use thermal energy to convert the water into steam to generate electricity and to power the devices. They are used to improve factory efficiency. These products are designed due to an increase in fuel costs and to limit air pollution.

High pressure water Boiler

II) Low-Pressure Boilers

These boilers are hot water or steam boilers. The maximum steam pressure of the low-pressure boiler is 15 lbs./inch2, and the maximum hot water pressure is 160 lbs./inch2. Maximum steam pressure boilers have horizontal designed and have high steam supply efficiency.

Low-Pressure water Boiler

6) According to the Water Circulation Method

I) Forced Circulation Boilers

In a forced circulation boiler, the water in the water wall is pumped by a water pump. These pumps can only get the main water.

II) Natural circulation Boilers

In these boilers, the circulation is accomplished due to the alteration in the density during the heating process of the water in the boiler. In naturally circulating steam boilers, the water is circulated by a convection process which is created when the water is heated. It consists of two barrels placed horizontally above the boiler. These barrels are called water and steam drums.

7) According to the Boiler use

I) Fixed Boilers

Fixed boilers are large boilers that use in power plants to work under high pressure and are always of the water-tube type. The water-tube design can be designed for high-pressure boilers so that it can also be used for high pressures.

II) Mobile Boilers

Mobile boilers broadly use for trucks or trailer containers that can be legally transported on the motorway. These are designed and customized for “small” components of the steam powerplant.

8) According to Number of Drums

I) Multi-Drum Boilers

Multi-drum boilers are a type of water-tube boilers that use to produce steam and are typically used to power a ship. It is common due to its compact design and has a high evaporation capacity.

II) Single-Drum Boilers

These boilers mostly use to generate electricity. The advantage of this type of boiler is that it can be designed for very high temperatures, flow rates, and pressures.

9) According to the furnace

I) Single furnace Boiler

It has a single furnace.

II) Dual furnace Boiler

It has dual furnaces.

10) According to fuel

I) Gaseous fuel-fired Boilers

It uses gases such as LPG as burning fuel.

II) Liquid fuel-fired Boilers

It uses liquid such as petrol or diesel as burning fuel.

III) Solid fuel-fired Boilers

It uses solids such as wood and coal as burning fuel.

Boiler VS Furnace

The significant difference between boiler and furnace is given below in details.

boiler vs furnace, difference between boiler and furnace


It has a high cost compared to the furnace. The furnace has a low cost.
It requires large space for installation. It requires a low space for installation.
A boiler is very complicated to install. A furnace is easy to install.
It has a high weight than a furnace. It has a low weight.
Boilers require high time for installation. Furnaces require less time for installation as compared to boilers.
Boilers are less noisy compared to furnaces. Furnaces generate high noise.
A boiler is more energy efficient. A furnace is less energy efficient.
Ductwork is not needed. In the case of a furnace, ductwork is needed.
A boiler uses steam or hot water to heat your home. A furnace uses hot air.
Boiler powers with electricity, oil, natural gas or wood pallets. Furnace power by electricity, propane or natural gas.
It provides better air quality inside the home. Furnace can’t provide better air quality than the boiler.
It is more energy-efficient. A furnace is less energy efficient than a boiler.
Heat is much more comfortable. Generated heat by the furnace is worse for families with allergies.
The heated water can be reused. In this case, heated water can’t be reused.
It has high chances of freezing during winters. It has less chance of freezing during winters.

What Is Steam Boiler Efficiency?

The percentage of the total heat generated from the discharge steam divided by the total heat provided by the fuel (coal) is referred to as the steam boiler’s efficiency.

steam boiler efficiency

Applications of Boiler

There are many applications of the boilers, but significant applications are given below. They use in various industries like

  • Thermal Powerplants
  • Sugar Plants
  • FMCG
  • Food Processing Industries
  • Plywood
  • Textiles
  • Power Sector

Types of Boiler Fuels

The most famous fuels of the boilers are given below.

1) Gaseous Fuels

  • PNG can also use for specific applications.
  • LNG
  • LPG

2) Liquid Fuel

  • Furnace oil
  • LDO

3) Solid Fuels

  • Rice Husk
  • Pet Coke
  • Briquettes
  • Coal
  • Wood

What is the average life of a Boiler or Steam Generator?

The average service lifecycle of the industrialized steam generator or boiler is equal to fifteen years. When the steam generators or boilers reaches this era’s milestone, we will have to look for another steam generator system for thermal power plants. You may extend this duration if you want to maintain a timely annual maintenance service and keep the boiler in top condition at all times.

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