Power Plant Engineering

What is a Steam Boiler? How Does it Work?

A steam boiler or boiler represents a closed vessel designed to transform fluid, typically water, into steam through the utilization of heat energy, released from the burning of fuel. The term “steam generator” is attributed to its proficiency in producing steam. Generating high-temperature and superheated steam plays a crucial role in driving the turbine shaft, which is connected to the generator shaft, thus enabling the production of electricity. As a result, this substantial apparatus bears significant importance within a thermal power plant. In this discussion, we will briefly explore its functions, starting with its definition.

What is a Steam Boiler?

As per the Indian Boilers Act 1923, a boiler is defined as a closed vessel with a capacity exceeding 22.75 liters (equivalent to five gallons), specifically utilized for generating steam under pressure. It encompasses all mountings and fittings attached to the vessel, even if they are partially under pressure when steam is cut off.

In a thermal power plant, the steam turbine requires high temperature and superheated steam. To achieve these requirements, several heat exchangers including the economizer, primary superheater, secondary superheater, and In-bed superheater are integrated within the steam boiler’s furnace area. How does Energy conversion take place inside a steam boiler? Generally, the burning of fuel such as coal or High-Speed Diesel (HSD) Oil, transforms chemical energy into heat energy.

Typically, large-scale industry, such as thermal power plants, demands high-temperature and superheated steam for electricity generation, sterilization, and drying processes. And, small-scale industries often utilize low-pressure steam for comfort heating and dehumidification purposes respectively. Additionally, a low-pressure boiler is employed to heat domestic water and supply it to various schools, hospitals, and offices.

Diagram of Steam Boiler


Different Parts of Steam Boiler

Mountings of Steam Boiler

  • Burner: The burner is equipped with a gun that injects atomized air and High-Speed Diesel (HSD) fuel in the correct ratio. A lighter ignites the mixture, and for initial burning, LPG is supplied.
  • Heat Exchanger: A heat exchanger is a mechanical equipment where heat transfer takes place means heat energy is transferred from one system to another without any direct contact. Basically, heat exchangers are a series of multiple small tubes present inside a closed vessel. The heat generated by the burner inside the furnace is carried out by the feedwater and is supplied to the steam boiler.
  • Pressure Gauge: This instrument measures the pressure of a fluid (water or steam) inside a media-like vessel or tube in a system. High indication indicates an excess pressure, while low indication points to leakage. Generally, Bourdon-tube pressure gauges are used widely in thermal power plants. The unit is in Kg/cm2.
  • Supply Lines and Return Lines: The purpose of supply lines is to transport hot water or steam to the distribution header, including components like the steam drum and superheater. Conversely, return lines, like drain points and steam traps, are incorporated to redirect excess hot water and steam back into the system, channeling them toward the Blowdown Tank to minimize heat loss.
  • Combustion Chamber Or Furnace: It consists of numerous tubes, interconnected and joined together by a membrane bar through welding to form a closed chamber, which is also called a furnace. Generally burning of solid fuel i.e. coal is taking place here. Normally in a water tube boiler, the draft inside the furnace is negative.
  • Safety Valve: A safety valve is a mechanical safety device and also an important mounting of a steam boiler. When the pressure inside the steam drum or superheater line exceeds the maximum allowable working pressure (MAWP), it opens automatically and releases the excess pressure into the atmosphere. Similarly, it closes automatically after reaching the working pressure. It protects human life and the company’s assets of a thermal power plant.
  • Circulator Pump: It delivers hot water to the distribution channels through supply tubes and similarly brings it back through return lines. Generally in case of a forced circulation circulator pump is used. Due to free flow circulation, the water tube boiler doesn’t require any external pump.
  • Temperature Gauge: This instrument measures the temperature of a homogenous substance in a system. Generally, it consists of a capillary tube, a temperature sensor, and a Bourdon tube. The measuring system itself is filled with fluid.
  • Exhaust Stack or Chimney: This open chamber is connected to the Induced Draft Fan (ID Fan), which effectively draws out the flue gases from the boiler furnace through the Air Preheater and electrostatic precipitator (ESP). The ESP collects the remaining ash in a hopper, while the combustible gases continue through the system.
  • Gauge Glass: A gauge glass is a level indicator device that is used to measure the fluid level (water or steam) inside a steam drum. The upper portion of the gauge glass is linked to the steam side of the steam drum, while the lower part of the gauge glass is connected to the water side of the steam drum.
  • Start-up Vent: During startup conditions, the temperature and pressure inside the steam boiler are relatively low. So to remove the low-pressure and temperature moisturized steam and raise the steam pressure slowly, venting is required. It also dissipates the heat from the flue gases into the atmosphere, safeguarding the superheater tubes from overheating. During emergencies like island operations, the boiler load is reduced suddenly and the steam flow inside the steam boiler is more than the load generation. Excess steam could overheat the metal, so to protect it from overheating and maintain the steam flow startup vent released the excess steam into the atmosphere.
  • Thermocouple: A thermocouple is a thermoelectric instrument that is normally used to measure the temperature of a system. Because of this device, the temperature of a system could be measured through a Distributed Control System (DCS).
  • Electro-mechanical Relief Valve: This is a pilot-operated safety valve that uniformly maintains the main steam line pressure and balances the main steam pressure during peak and off-peak loading respectively. Generally, EMRV opens faster than the startup vent. It means EMRV can remove the entire steam in the main steam line into the atmosphere within a second.
  • Main Steam Stop Valve: A main steam stop valve is present at the discharge part of the main steam line of a steam boiler through which steam flows to the next system i.e. steam turbine for work done. This safety device is used to shut off or regulate the steam from the main line area. The main body of the steam stop valve is made up of cast steel and the valve and nuts are made up of brass with excellent resistance and limited dimensional tolerance.
  • Blowdown Valve: This mechanical device is utilized for periodic cleaning, effectively removing dissolved solids and sediments from the lower part of a steam boiler. It also minimizes the corrosion property of the material or shell of a boiler. Another use of this device is to drain the boiler whenever the boiler is not in operation.
  • Manhole: A manhole is a compact covered opening employed for inspection, cleaning, and maintenance tasks while the steam boiler is in shutdown condition. A person can enter the boiler furnace area through this hole and easily identify the integral defects such as water tube leakage, refractory work, bed material filling, etc.
  • Non-return Valve: It is also called a check valve that allows fluid to flow in one directional way and ensures the fluid flows in the right direction. If the fluid tries to flow in the reverse direction, the closing member is forced back over the entrance of the valve preventing any flow. The flow through this valve can create relatively a large pressure drop, which has to be taken into consideration when designing the system.

Accessories of Steam Boiler

The accessories are used to increase their efficiency and are listed below.

  • Economizer: An economizer is a closed chamber that is filled with multiple tubes. Feedwater that flows inside these tubes gains heat energy from the hot flue gases present inside the shell and supplies it to the steam drum to get saturated steam. Also, it reduces the firing rates which is needed for the boiler output calculation. Thus it increases the efficiency of a steam boiler.
  • Air preheater: An air preheater is a closed chamber that is filled with multiple vertical tubes where atmospheric air flows toward the boiler furnace to help in combustion. It recovers the heat energy from the boiler’s hot flue gases which are present outside the tubes (which means inside shell). To avoid heat loss from the combustion gases and increase the Steam boiler efficiency, this arrangement is provided.
  • Superheater: Superheaters like Primary, Secondary, Slinger, and In-bed Superheaters are used to increase the temperature of saturated steam without raising its pressure. Normally a boiler utilizes two types of heat energy i.e.(sensible heat and latent heat) to convert feedwater into superheated steam. A superheater utilizes only sensible heat to superheat the steam in order to increase its thermal efficiency and enthalpy of a boiler.
  • Deaerator: A deaerator is an open-type heat exchanger having a deaeration chamber and a storage tank. Generally in the deaeration chamber, the dissolved gases like oxygen and carbon dioxide are removed from the boiler feedwater to protect it from corrosion and finally stored in the storage tank which is then ready to supply to the boiler for generating steam.
  • FD Fan: A forced draft fan or FD fan is a centrifugal fan that is used to pull out atmospheric air at high pressure and force it into the steam boiler furnace through an air preheater. Inside the furnace, it mixes with the fuel to produce positive pressure. Normally it is located at the suction part of a boiler. In a thermal power plant FD fans are used as primary and secondary air fans which helps in the combustion process and to increase fuel efficiency.
  • ID Fan: An Induced draft fan or ID fan is a centrifugal fan that is used to pull out the flue gases from the furnace through an air preheater and Electrostatic precipitator (ESP) and extract them out into the atmosphere by the chimney. Normally it is located at the discharge part of a steam boiler between APH and ESP. This fan creates negative pressure to pull the hot flue gases out after burning from the furnace.
  • PA Fan: A Primary air fan or PA fan is a centrifugal fan that supplies air for conveying fuel i.e. pulverized coal from the coal bunker to the steam boiler furnace area. This fan produces a high pressure that allows the fuel to move into the furnace.
  • Ash Handling System: To maintain the temperature, storage, transportation, disposal, and reuse of the ash, an Ash handling system is required. In thermal power plants, two types of ash are found fly ash and bed ash. Flyash can be collected from the flue gases of the boiler through an economizer hopper, air-pre heater hopper, and Electrostatic precipitator which is then finally stored in a silo. This can be used for making bricks. Similarly, bed ash can be collected from the furnace and bed ash cooler drains. This ash can be reused in the boiler.
  • Dosing System: Two types of dosing systems are there in a thermal power plant. HP dosing is used in the steam boiler and LP dosing is used in the Deaerator. Hp dosing is used in the boiler and Lp dosing is in Deaerator. In the HP dosing system, a high-pressure dosing pump is used to supply a chemical like Tri-sodium Phosphate (TSP) which delivers this chemical to the pressurized parts like tubes and pressure gauge lines, etc. To protect the inner surface of the pressurized tube from corrosion this chemical maintains the p.H, TDS, and conductivity of the feedwater respectively.
  • Coal Feeding System: To supply pulverized coal to the furnace area coal feeding system is required. It includes a Bunker, surge hopper, plate gate, rod gate, Drag chain feeder or Coal feeder, Distribution chute, and Mixing chamber. The pulverized coal is then supplied by the coal feeder and mixed with the primary air which pushes it into the furnace of the steam boiler for combustion.
  • Boiler Feed Pump (BFP): Boiler Feed Pump is a centrifugal pump that supplies feedwater at high pressure to the steam boiler (through an economizer) for generating steam. Normally the pump has its suction from the Deaerator storage tank and then discharge the feedwater to a High-pressure Heater for reheating purpose. In case of any disturbances in the HP heater line, the feedwater can be bypassed to the economizer before proceeding to the boiler. That’s why this feed pump is considered the heart of a thermal power plant

Application of Steam Boiler

Generally, these are used in the following industries that are given below.

  • Thermal Power Plant
  • Textile Industries
  • Paper Mills
  • Pharmaceutical Industry
  • Refineries and Distilleries
  • Fast Moving Consumer Goods (FMCG)
  • Health Care Industries

Types of Steam Boiler

These are classified into the following types that are given below.

According to the position of water and hot gases

  • Fire Tube boiler:- The fire tube boiler is designed with multiple tubes containing hot flue gases, while the water surrounds these tubes. Due to the presence of fire inside these small tubes, it is called a Fire tube boiler.
  • Examples:- Cochran boiler, Lancashire boiler, locomotive boiler, scotch marine boiler, Simple vertical boiler, etc.
  • Water Tube Boiler:- It is a boiler in which the fluid i.e. water is present inside multiple tubes and the hot flue gases surround these tubes. Due to the presence of water inside these small tubes, it is called a water tube boiler.
  • Examples:- Benson boiler, La-mont boiler, Babcock boiler, Stirling boiler, and Wilcox boiler.

According to the axis of the boiler shell

  • Vertical boiler:- In the vertical boiler type, the axis of the cylinder is vertical and is mostly used in ships.
  • Example:- Scotch marine boiler and Cochran boiler etc.
  • Horizontal boiler:- In the horizontal boiler type, the axis of the cylinder is horizontal and is mostly used in steam-powered vehicles.
  • Example:- Cornish boiler and Lancashire boiler, etc.

According to the position of the combustion chamber

  • Externally fired boiler:- Externally fired boilers have a separate combustion chamber outside the boiler.
  • Example:- Wilcox boiler and Babcock boiler etc.
  • Internally fired boiler:- Internally fired boilers have a combustion chamber at an integral part of the boiler.
  • Example:- Lancashire boiler and Cochran boiler etc.

According to the use of a Boiler

  • Stationary boiler:- This type of boiler is fixed (fixed boiler) means transportation from one place to another is impossible. These boilers are mostly used in power plants because of their bulky and large structure.
  • Mobile boiler:- This type of boiler is not fixed in one place (portable boiler) means transportation from one place to another is possible. Generally, marine boilers are mobile boilers.
  • Example:- Locomotive boiler.

According to the method of circulation

  • Natural circulation boiler:- In this type of boiler water is circulated by natural processes that are produced by the application of heat.
  • Example-: Locomotive boiler, Babcock boiler, Wilcox boiler, Lancashire boiler, etc.
  • Forced circulation boiler:- In this type of boiler water is circulated forcefully by a centrifugal pump.
  • Example:- Benson boiler.

According to the number of drums

  • Single drum boiler:- In this type of boiler water and steam are present in a single shell. Normally moisture content is present in the steam.
  • Multi-drum boiler:- In a multi-drum boiler, water, and steam are present at the different shells. We can see one drum is present on the top section and another is just below it called mud drums and they are interlinked by a tube through which the process of heat transfer takes place. In this way, we could get dry steam.

According to the number of furnaces

  • Single furnace boiler:- Generally these types of boilers contain a single furnace.
  • Example:- Single-ended scotch marine boiler.
  • Dual furnace boiler:- In this case, the boiler contains a double furnace. Here the operation process is quite lengthy but consistent and it reflects better efficiency.
  • Example:- Double-ended scotch marine boiler.

According to the fuel-firing

  • Solid fuel-fired boiler:- In this type of boiler, solid substances are used in combustion.
  • Example:- rice husk, briquettes, pet coke, coal, and wood, etc.
  • Liquid fuel-fired boiler:- In this type of boiler, liquid substances are used in combustion.
  • Example:- Furnace Oil and LDO (Light Diesel Oil) etc.
  • Gaseous fuel-fired boiler:- In this type of boiler, gaseous substances are used in combustion.
  • Example:- LNG (Liquefied Natural Gas) and LPG (Liquified Petroleum Gas).

How does a steam boiler work?

The working principle of a Steam Boiler or steam generator is very simple. If we study the working principle of a steam boiler, then some of its functions are similar to the working principle of a pressure cooker that we use in our day-to-day lives. Both produce steam at different pressures. In a Boiler, we get steam at the required pressure (normally high pressure) while a pressure cooker produces steam at a fixed pressure.


It is a closed vessel, typically made up of steel to prevent corrosion. This vessel contains fluids, usually feedwater, and undergoes heating through a heat energy source generated by the combustion of fuel. The feedwater gets converted into steam by the convection process of heat transfer. An economizer is an accessory of a boiler and also a heat exchanger that receives the feedwater from the boiler feed pump. During this process, the feedwater captures the waste heat from the flue gases (originated in the combustion chamber inside the boiler), and by this, its temperature increases. The hot flue gases are then released into the atmosphere through the chimney.

Subsequently, the saturated or wet steam undergoes further heating by multiple superheaters, resulting in high-pressure, superheated steam, or dry steam. This superheated steam drives the steam turbine to generate electricity within a thermal power plant.

Steam Boiler Efficiency

It is defined as the ratio of the heat actually employed in the steam production to the heat supplied by the fuel in the same period. It is also known as the thermal efficiency of a steam boiler.

Boiler Efficiency Calculation Formula

Boiler efficiency (η)= Qs(Hs-Hf) / 100(Qf * GCV)


  • “Qs“ stands for Quantity of steam generation per hour (kg/hr)
  • “Qf“ stands for the quantity of fuel used per hour (kg/hr)
  • “Hs“ stands for Enthalpy of steam (kCal/kg)
  • “Hf“ stands for Enthalpy of feed water (kCal/kg)
  • “GCV” stands for Gross Calorific Value of fuel (kCal/kg)

Note: If the boiler is attached to an economizer and superheater, then the efficiency is known as the overall efficiency of the steam boiler.

Advantages of Steam Boiler

  • Boilers are portable (They can easily move from one place to another).
  • It creates comfortable heat and doesn’t make any noise.
  • It also requires less floor area.

Disadvantages of Steam Boiler

  • It requires high headspace because of its vertical shape.
  • The chances of leakage in the joints section are high.
  • The purchasing cost is also very high.

This is all about Steam boilers or Steam Generators. Thanks for giving your valuable time. If you have any doubts please don’t hesitate, to ask in the comments section below. If you have any prior experience related to this boiler and know more about it, please share it with us. We promise that we will solve your doubts as soon as possible. If you find this article helpful, consider sharing it with colleagues and friends. Best of luck!

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