29 Types of Boilers Explained


Denis Papin developed the first boiler in 1679 and since then, it has only increased in capacity and complexity.

Today, we have many different types of boilers for different applications. In this article, we simplify the various types of boilers for a better understanding of this indispensable piece of equipment.

But before we get there, let us start with the definition of a boiler.


What is a boiler?

A boiler is a pressurised shell and tube heat exchanger that heats up a fluid by using hot air or gases and converts it into steam for different uses.

The fluid is usually water but it can be oil or other media as well. The steam may provide propulsion, be a prime mover or a heating source for delivery agents downstream of steam pipelines.


The boiler is just one piece of the steam generation plant. For the plant to function as designed, it must be connected to other equipment such as condensers, cascade tanks, preheaters, fuel oil pumps, pressure reducing valves and so on.

Related: Boiler Water Treatment Basics

Different types of boilers

There are various ways of boiler classification. We use 12 different categories to explain 29 types of boilers in this section. These 12 categories are as follows.

  • Based on use
  • Based on the tube contents
  • Based on the steam pressure
  • Based on the heating medium
  • Based on the furnace location
  • Based on the number of tubes
  • Based on orientation
  • Based on mobility
  • Based on the energy source
  • Based on the circulation method
  • Based on the number of passes
  • Based on the reversing chamber design


Here is the tree diagram of the different categories and the types of boilers under them. The diagram is followed by a short description of each category and the boilers under it.


Types of boilers classification
Boiler classification

Based on use

One of the first ways to classify boilers is the function they serve in a setup. A boiler may be classified as a main or auxiliary boiler based on its use.

Main boiler

The main boiler refers to the boiler on a ship that produces steam for propulsion. The term may also refer to the primary boiler that is in use in industrial setups under normal circumstances.

Auxiliary boiler

Auxiliary boilers are boilers that do not provide propulsion steam but are capable of meeting all other demands. Auxiliary boilers on ships are used for cargo discharge, tank heating, accommodation heating, and machinery (purifiers, incinerators, etc).

The term may also refer to additional boilers that supplement main boilers in industrial processes when the requirement is high. An auxiliary boiler is also sometimes known as a donkey boiler.


Based on the tube contents

This is the most popular classification of boilers. Depending on the content of the tubes, we can classify boilers as water tube boilers or fire tube boilers. Let us learn a bit more about each of these types.

Water tube boiler

A water tube boiler is one that has water flowing inside the tubes within the shell. These tubes are surrounded by hot gases that heat the water up. The hot water flows upwards towards the steam drum where it is subsequently converted into steam.

The steam is then extracted through the top of the drum and sent to various applications. Generally, these boilers are used for high-pressure applications.

An advantage of water tube boiler is that there is very little possibility of an explosion. The tube diameters are kept within limits and the steam drum is protected from radiation and flame impingement.

In case of a tube leakage, the water escapes at a much lower rate. On the other hand, in a fire tube boiler, if the furnace overheats and ruptures, it will release all of the feed water.

Smoke or fire tube boiler

Contrary to a water tube boiler, a fire tube boiler holds flue gases within the tubes. These tubes are surrounded by water on the outside where the heat transfer to water takes place. The water heats up, converts into steam and travels to different consumers.


Based on the steam pressure

Not all applications require the same steam pressures. We can use this as a criterion to classify boilers. Based on pressure, we can classify boilers into the following three types.

Low-pressure boilers

When a steam boiler produces steam pressures up to 10 bar maximum, they are known as a low-pressure boiler.

Medium-pressure boilers

A medium-pressure boiler is one that produces steam pressures in the range of 10-25 bar.

High-pressure boilers

Any steam boiler that produces steam pressures above 25 bar is a high-pressure boiler.


Based on the heating medium

The heating medium absorbs the heat from the furnace and transmits it to the consumers in the boiler system. There are two main types of heating media. These are steam and thermal fluids.

Steam boiler

Steam is by far the most common boiler heating medium. Steam boilers can handle a wide range of applications as they are available in a range of pressures. Even in high-pressure boiler systems, we can provide low-pressure steam to select consumers by fitting a steam pressure-reducing valve.

Steam boilers split further into boilers that produce saturated steam and those that produce superheated steam. Superheated steam is steam that has no wetness or moisture in it.

This steam is mainly used for power generation in turbines as it does not cause any impact damage to the turbine blades. Saturated steam works well for all other uses.

Thermal fluid boiler

Thermal fluid boilers use liquid fluids instead of steam to absorb heat in the furnace and supply it to the consumers. The three principal types of thermal fluids in these systems are water, oil and water-glycol mixture.

Water provides the best heat capacity but is limited by the system temperatures it can accommodate. To increase the temperature range, we add glycol to the water. This increases the boiling point and decreases the freezing point but the water ends up losing some of its heat capacity.

Thermal oils can tolerate much higher temperatures (up to 425 deg C) than water without boiling or unreasonably increasing the system pressure uns. They also do not create any hard deposits or corrosion in the system which is a common disadvantage of water-based boilers.


Based on the furnace location

The furnace is responsible for providing the heat to the boiler system which is then transferred to the heating medium. Depending on its position in the system, we can classify a boiler as an externally fired or an internally fired boiler.

Externally fired

When the furnace is located outside the boiler shell, it is known as an externally fired boiler. In such a system, the water does not surround the boiler either completely or partially. Examples of externally fired boilers are Babcock and Wilcox boilers, Stirling boilers, etc.

Internally fired

When the combustion chamber exists within the boiler shell, it is known as an internally fired boiler. Examples of such boiler systems are Cochran, Lancashire and locomotive boilers.

Lancashire boiler
Lancashire boiler in operation


Based on the number of tubes

Tubes are present in virtually all boilers. We can use the number of tubes in the boiler shell to classify them as either single-tube boilers or multi-tube boilers.

Single-tube boiler

A single-tube boiler consists of only one fire tube or water tube. Examples of single-tube boilers are Cornish boilers and simple vertical boilers.

This type of boiler is more compact, affordable and easier to maintain. It, however, provides lower steam volumes compared to multi-tube boilers.

Multi-tube boiler

Boilers with two or more tubes are known as multi-tube boilers. These boilers may be fire or water tube boilers. Compared to single-tube boilers, these boilers have much higher steam generation capacity due to the high surface area for heat transfer.


Based on orientation

Depending on the orientation of the boiler shell, we can classify boilers as horizontal, vertical and inclined boilers.

Horizontal boiler

Boilers with a horizontal shell axis are known as horizontal boilers. These boilers take up more ground area but provide good access to all parts of the boiler for cleaning, maintenance and repair.

Common examples of horizontal boilers are the Lancashire boiler, Babcock and Wilcox boiler and locomotive boiler.

Vertical boiler

Vertical boilers are preferred where there is limited floor capacity. A vertical shell axis orients most of the boiler body vertically reducing the floor area covered by the equipment. As a result, we can have as much boiler capacity as we need

As such, vertical boilers find use in ships due to their limited real estate. Earlier, they would also be used in steam automobiles and locomotives due to their need for compactness and greater manoeuvrability.

Vertical boilers have greater water level tolerance compared to horizontal boilers due to the water level always staying above the firebox. Examples of vertical boilers are the simple vertical boiler and the Cochran boiler.

Inclined boilers

As the name suggests, these boilers have shells inclined at an angle to the ground.


Based on mobility

Another way to classify boilers is on the basis of their mobility. Boilers can either be affixed to a place or movable. Thus, they are classified as either stationary or portable.

Stationary boilers

When boilers are fixed in place and all the pipelines are permanent, it is known as a stationary boiler. Generally, these boilers are very large and find applications in industries/processes that require high steam volumes. Examples are power plants, oil tankers, rubber, cement, sugar and paper industries.

But these could also be small such as domestic boilers that are fixed in place.

Portable boilers

Portable boilers are those that have a movable nature. They may either have wheels or the capability to be carried manually. They are totally self-contained and are not fixed to any other equipment through hoses or pipes.

But these boilers may not always be small. Portable boilers can deliver capacities up to 125,000 lb/hr.

Based on the energy source

We can use a wide variety of energy sources to fire boilers. The five main types of boiler energy sources are as follows.

Solid fuel fired boiler

We can use solid fuels such as coal, wood, agro waste, tires, bagasse, biomass etc. to power these boilers. They have low running costs, wide fuel choice and can be carbon neutral (biomass boiler).

Solid fuel boiler burning bio pellets
Solid fuel boiler burning bio pellets

Oil fired boiler

These are the most common type of boilers and also come in a range of fuel choices such as diesel, heavy oil, hybrid fuel. Much work has been done on improving oil fired boilers in the last few decades. As a result, these boilers deliver high efficiency and durability than newer technologies.

Waste heat boiler

Wherever there is a waste heat source, we can use it to fire boilers and reclaim some of the energy. This waste heat could be in the form of flue gases that are a byproduct of combustion processes such as incineration or engine operation or hot waste air from industrial processes.

Waste heat boilers are commonly seen on ships as exhaust gas economizers. These boilers may also find use downstream of turbines to reduce energy waste and improve efficiency.

These boilers are generally used in conjunction with self-powered boilers as the waste heat source may not always be available.

A good example is when oil tankers are stopped in port and require steam to discharge oil through COPT, self-powered boilers must be used as exhaust gas economisers cannot produce any steam due to the stopped engine.

Electric boiler

In electric boilers, electricity generates heat instead of the combustion of a fuel source. These boilers are gaining popularity in recent years due to the increased awareness of the environmental impact of fossil fuel based systems.

Electric boilers are sustainable, save costs and energy, are easier to maintain and safer as there is no possibility of fuel leaks.

Gas fired boiler

Gas-fired boilers use natural gas or propane to heat water and produce hot water or steam. Natural gas is far more efficient and can potentially save hundreds of dollars in annual heating costs even in small setups.


Based on the circulation method

Water must circulate continuously in the boiler for the vapour pressure of the water to overcome that of steam. This process ultimately produces steam. But there are two ways to do it: natural circulation and forced circulation.

Natural circulation boiler

In a natural circulation boiler, water circulation occurs because of the density difference between the hot and cold water.

The hot water rises to the top of the water drum after absorbing heat from the tubes and the relatively colder water makes its way to the bottom of the water drum through a separate pipe known as the downcomer. The hot water receives no external impetus.

Forced circulation boiler

In this type, we force the flow of water instead of waiting for the density difference to occur.

A pump is placed along the downcomer where it pulls the water from the bottom of the steam drum and sends it onward through the heated area (furnace). This generates steam much faster than a natural circulation boiler.


Based on the number of passes

The number of times flue gases pass through the water drum is another way to classify fire tube boilers. This classification is only meant for fire tube boilers as the flue gasses pass through the tubes in these boilers. Thus fire tube boilers can either be single-pass or multi-pass boilers.

Single pass boiler

In single-pass boilers, exhaust gases flow through the boiler once before exiting the system.

Multi-pass boiler

In a multi-pass boiler, the hot gases travel through the boiler multiple times before leaving the system, These multiple passes are usually enabled by baffles.

A multi-pass fire tube boiler provides greater time and surface area for heat transfer. As a result, these boilers are more efficient than single-pass boilers.


Based on the reversing chamber design

The reversing chamber is where the flue gas change directions in a multi-pass fire tube boiler. There are two types of reversal chamber designs: dryback and wetback design. Let us take a brief look at each of them.

Dryback boiler

In the dryback fire tube boiler, the reversing chamber is only partially surrounded by water. The backside of the reversal chamber is open to the atmosphere which may cause heat loss due to radiation.

Dryback designs were more common in earlier boilers but they have not become extinct either. Some dryback designs such as the centerfire design require 10% less fuel than other designs because the burner can stay off longer as it develops a heat sink.

Wetback design

In the wetback fire tube boiler, the entire reversing chamber is integrated into the boiler shell such that it is surrounded by water on all sides.

This eliminates the radiation losses from the reversing chamber allowing the water to absorb greater heat and produce more steam.