In this article we will discuss about:- 1. Introduction to Steam Boiler 2. Classification of Steam Boiler 3. Selection of Steam Boiler 4. Essentials 5. Accessories.

Introduction to Steam Boiler:

High-pressure steam is a working fluid of steam prime movers such as reciprocating steam engines and steam turbines. Various types of fossil fuels may be used to liberate heat after combustion. Fossil fuels are the main source of heat energy for the generation of steam in a boiler.

In general, a boiler is said to be a closed vessel in which steam is produced at high pressure and temperature from water by the combustion of fuel. According to IBR (Indian Boiler Regulations), “boiler is any closed vessel exceeding 22.75 liters in capacity which is expressly used for generating steam under pressure and includes mountings or other fittings attached to it, which is wholly or partly under pressure when steam is shut off.”

The steam generated is used for the following purposes:


(a) For generating power in thermal power plant

(b) For heating a building in cold weather

(c) For process work in industries

In steam power plants, steam is used to run steam turbine coupled with a generator to generate electrical energy. The high-temperature steam is used for the purpose of heating rooms in cold climate. As a process steam, it is used in textile industries. It is also used in paper mills and chemical industries.


The main function of a steam boiler is to supply steam at almost constant pressure as desired with its possibly in three qualities: wet steam or dry saturated steam or superheated steam. Superheated steam is the best amongst the three categories. The steam is essentially supplied at constant pressure by maintaining the rate of steam generation and supply of feed water. This way the water level inside the vessel is maintained constant.

Classification of Steam Boiler:

A boiler may be classified as follows:

(i) Fire tube and water tube boilers

(ii) High-, medium-, and low-pressure boilers


(iii) Natural and forced circulation boilers

(iv) Single tube and multi tube boilers

(v) Stationary and portable boilers

(vi) Coal-, oil-, and gas-fired boilers


(vii) Externally fired and internally fired boilers

(i) Fire Tube and Water Tube Boilers:

In the fire tube boilers, the hot gases produced after the combustion of fuel pass through the tubes and the water surrounds these tubes. In the water tube boilers, water flows inside tubes and gas remains outside these tubes (Fig. 3.1).

A comparison between fire tube and water tube boilers is as follows:


Fire Tube Boiler:

(a) Hot gases formed after the combustion of fuel flow through tubes and water surrounds these tubes.

(b) Internally fired.

(c) Working pressure limited to 20 bars.

(d) Steam generation rate is lower.

(e) For a given power, it occupies large floor area.

(f) Not suitable for large power plants.

(g) It carries lot of risk on less working pressure.

(h) Different parts cannot be separated easily, hence it becomes difficult to transport.

(i) Water treatment is not necessary.

(j) Parts are not so accessible for cleaning and inspection.

Water Tube Boiler:

(a) Water flows inside the tubes and gas surrounds these tubes.

(b) Externally fired.

(c) Working pressure may be as high as 150 bars.

(d) Steam generation rate is higher.

(e) For a given power, it occupies lesser floor area.

(f) Suitable for large power plants

(g) Less risk on explosion due to high working pressure.

(h) Each and every part can be separated easily, so transportation is easier.

(i) In this boiler, water treatment is necessary.

(j) Parts are easily accessible for inspection.

(ii) High-, Medium-, and Low-Pressure Boilers:

(a) High-Pressure Boilers:

The boiler which produces steam of more than 70 bar of pressure is known as high-pressure boiler. The efficiency of fire tube boilers has been found quite low. The generation of steam at high pressure and high temperature is not possible in a fire tube boiler.

The high rate of steam generation as demanded by the thermal power plants is also not possible in fire tube boiler. Only water tube boilers have been developed which can work as high-pressure boilers and have been found more suitable for thermal power plants.

The conventional water tube boilers such as Babcock and Wilcox water boilers even though can generate steam at high pressure with high steam generation rate, they have been found unsuitable. Babcock and Wilcox water boilers have undergone a vast change to generate high-pressure steam at a high rate of steam generation. The preheating of air by the incorporation of air preheater in the plant feed water heating by economizer by transferring the heat of flue gas.

These equipments are installed in the flow path of flue gases which are at high temperature. Thus, there is a recovery of waste heat. Further, regenerative feed heating is used by extracting some quantity of steam from the stages of turbine and getting this utilized for heating the feed water before fed to the boiler. There are other steps in this field.

Improvement in furnace design making furnace structure with water wall and replacing the conventional system of making furnace wall by fire bricks structures are another changes. The boiler tubes, or so-called water tubes, are embedded on cast iron plates in such a manner that these water tubes form the evaporative part of the boiler. In case of a pulverized fuel firing system, closely spaced water tubes furnace wall have been found very suitable and popular.

The use of super heater and pulverized coal firing; the supply of the powdered coal by pulverized coal burner; and the method to supply the same in a proper method are the three steps for improving the overall performance of the boiler.

(c) Medium-Pressure Boilers:

The boiler which produces steam between 20 and 70 bars is known as medium-pressure boiler.

(d) Low-Pressure Boilers:

The boiler which produces steam of less than 20 bars is known as low-pressure boiler.

(iii) Natural and Forced Circulation Boiler:

Natural Circulation Boiler:

All conventional boilers, generally low-pressure boilers, in which the circulation of water is done by thermo-symphonic method, are known as natural circulation boilers (Fig. 3.2). In natural circulation boiler, the circulation of water takes place due to natural convection current and density variation of fluid by the application of heat, i.e., thermo-siphon.

This is based on the principle that the density of water is more than steam and as such requires a boiler drum. The method is effective only up to a pressure less than 20 bar. After this pressure, the difference in density of water and steam becomes less and so this system fails to work.

Forced Circulation:

In forced circulation type boilers, the circulation of water inside the tube is done by a forced circulating pump. Water is forced inside the tube by mechanical means. This method is applicable in high-capacity water tube boilers.

(iv) Single Tube and Multi Tube Boilers:

The fire tube boilers may have single or multi tubes. Lancashire boiler is a fire tube boiler with a single tube in which hot gases move. Cochran boiler is a fire tube boiler called as multi tube boiler because it has large number of fire tubes.

(v) Stationary and Portable Boilers:

Stationary boilers are used for power plants where the boilers are installed permanently and cannot be moved. Locomotive boilers are called portable or mobile boilers.

(vi) Coal-, Oil-, and Gas-Fired Boilers:

Three types of fuel are used in steam boilers:

(a) Coal in solid form,

(b) Crude oil products in liquid form, and

(c) Gaseous form.

Coal is the most widely used fuels. This is the cheapest fuel and easy to handle. Coal in lumps is handled by a stoker which requires a grate at the bottom of the furnace to hold fuels. Most modern boilers use powdered coal known as pulverized coal that enters the furnace of the boiler through burners.

There are two types of liquid fuels used in the form of oils and are cheaper, (a) LDO, light diesel oil and (b) HSD, high stokes diesel. LDO is also called furnace oil. In using oil as a fuel, the oil burner breaks the fuel oil into minute droplets and mixes with air used for combustion. Gaseous fuels which are available in oil fields are used in boilers and are handled by burners. They have limited scope.

(vii) Externally Fired and Internally Fired Boilers:

Externally fired boiler is one in which the combustion of fuel takes place outside the shell and hot gases formed enter the space to heat up the tube surface such as Babcock and Wilcox water tube boilers.

Internally fired boiler is the one in which the furnace is located inside the boiler shell itself such as Cochran and Lancashire fire tube boilers.

Selection of Steam Boiler:

The following factors may be considered for selecting a boiler:

i. Working pressure and the quality of steam required, i.e., whether wet or dry, saturated or superheated steam

ii. Rate of steam generation

iii. Heating surface available

iv. Floor area availability

v. Ease in repair and inspection

vi. Initial cost of installation

vii. Facilities in erection

viii. Availability of fuel and water

ix. Operating cost

x. Maintenance costs

Essentials of a Good Steam Boiler:

A good boiler must possess the following features:

i. The boiler should be capable of producing large quantity of steam of the required quality.

ii. The steam must be generated at minimum cost.

iii. The rate of steam generation must match with the requirement.

iv. Boiler should be reliable in operation.

v. It must occupy less floor area.

vi. It must be compact.

vii. It must be started quickly.

viii. The transportation of boiler can be easily done without much difficulty.

ix. Construction should be so simple and well-designed that the repair and inspection can be done easily.

Accessories of Steam Boiler:

Boiler accessories are auxiliary systems in boiler installation for the proper functioning and increase of the thermal efficiency of the boiler.

The essential boiler accessories are as follows:

(a) Economizer

(b) Air preheater

(c) Superheater

(d) Feed pump

(e) Injector

(f) Steam separator

(g) Steam trap

(a) Economizer:

The purpose of economizer is to heat the feed water by the direct use of the heat of flue gas discharged to the atmosphere through chimney. The economizer reduces the temperature of flue gas. The feed water temperature is increased substantially.

Thus, there is a saving in heat. The boiler efficiency is increased substantially. The economizer is placed in the path of the exit gas nearer to the boiler. Next to it, another accessory, i.e., air preheater, will be installed in the passage. The economizer setup is shown in Fig. 3.13.

(b) Air Preheater:

The purpose of air preheater is to heat the air before it is supplied to the furnace of the boiler for combustion by the use of heat of flue gas. Supply of the preheated air into the furnace produces high furnace temperature and accelerates the combustion of fuel. Since the heat of the exit gases cannot be fully utilized through the economizer, the air preheater is employed to recover some of the heat escaping in these gases. The thermal efficiency of the plant will be increased. The air preheater is installed in between the economizer and the chimney.

Two major types of air preheaters are:

(a) Regenerative-type air preheater (Fig. 3.14) and

(b) Recuperative-type air preheater (Fig. 3.15).

(c) Superheaters:

Superheaters are used in the boiler installation to increase the temperature of steam above the saturation temperature. Superheated steam is characterized by the temperature of its vapors which is increased above the boiling point at that pressure. To achieve this, the steam is allowed to pass through a bank of tubes called superheater tubes exposed to the flue gas.

The heat absorbed from the furnace radiation or combustion of hot gases first evaporates the moisture carried over with the steam and then superheats the steam to the desired temperature. Superheating the steam raises the overall efficiency of the plant. Superheated steam is absolutely essential for steam power plant.

The following are the advantages of superheated steam:

(a) It reduces the losses due to condensation in pipes, steam engine cylinder, and steam turbine casing.

(b) It eliminates the possibilities of the turbine blades getting eroded due to the presence of condensate in the last stages of the turbine.

(d) Feed Pump:

A feed pump is a device to supply feed water with high pressure into the boiler for the generation of steam. The pressure created by the pump may be 6-10% more than the boiler steam pressure.

There are two types of pumps which are in common practice:

(a) Reciprocating pump and

(b) Centrifugal pump.

(e) Injector:

Injector is a device to supply feed water to the boiler by the direct use of steam already generated by the boiler. It can be used only when the boiler is in operation. It is installed in the feed lines. It is installed very close to the steam turbine in the main pipe lines.

(f) Steam Separator:

A steam separator is used to separate the water particle from the steam flowing in the steam pipe. The efficiency of steam power plant is higher with the use of superheated steam. This is to make steam dry by separating condensate.

(g) Steam Trap:

When the steam flows inside the steam pipe, the condensation of steam becomes inevitable due to the loss of heat. The purpose of the steam trap is to drain off the condensate collected in the steam pipe.