Classification of Boiler

Boilers are a tool for creating steam. The existence of boilers has been important since its development in the range of the 18th and 19th centuries. Boilers also took an important role in the era of the Industrial Revolution and encouraged various other important discoveries. In subsequent developments many studies have succeeded in bringing out a variety of new boiler designs.

To classify a boiler, we can only do it by looking at it from various points of view. These various perspectives depend on the design of the three constituent components of the boiler, the elements of water, steam, and combustion chamber. For more details, let’s discuss one by one.

Classification of boiler based on the relative position of steam with the combustion chamber

  1. Fire-tube boiler

    Fire-tube boilers are the simplest type of boiler. This boiler allows it to be applied from low to medium steam requirements. This is possible because the design is not complicated than a water-tube boiler.

    schematic diagram of a Fire tube boiler

    As the name implies, fire-tube boilers deliver hot gas from the combustion to the pipes which are covered by water. Hot gas from the combustion of fuel in the combustion chamber (furnace) is passed to these special pipes before being discharged into the atmosphere.

    Fire-tube boilers have a very simple design that only requires less space. In fact, many of these boiler designs allow it to be moved to one place to another. However, fire-tube boilers have limited steam production which is only a maximum of 9000 kg/hour with a maximum pressure of 17 bar.

    Fire-tube boilers themselves can still be classified into several types:

    • Haystack Boiler
    • This boiler is the simplest design boiler. Only composed of a giant stove carrying a large pan. This pan-shaped boiler was once inspired by a cooking pot. Whether in the century, how many boilers have started to be developed, but nowadays boilers that are only capable of working at a maximum pressure of 5 psi are rarely encountered. However, the Haystack boiler became the forerunner to the development of a variety of new boiler designs until the discovery of a modern fire-pipe boiler design.

      Boiler Haystack

      (Credit: Science Museum Group)

    • Center-flue Boiler
      In the next development, boiler began to be designed more complex. The boiler center-flue is the beginning of the birth of a fire-tube boiler, because the combustion gas is flowed into the water tank through a large pipe before being discharged into the outside air. The flue gas pipe only has one direction away from the furnace.

      Centre-flue Boiler

      This boiler is popular after being used as the first locomotive engine. This boiler is quite good at the side of the exhaust gas flow because of the use of the chimney. However, it is not very efficient if it is used to burn too much fuel such as wood or coal.


    • Return-flue Boiler
      Boiler return-flue is a further development of the center-flue type. If the center-flue uses one exhaust gas pipeline, then the exhaust gas pipe in the return-flue boiler is made to have a U-shaped backflow. The purpose of this design is to further improve boiler efficiency. Boilers that developed in the early 19th century were used as locomotive machines to replace boiler center-flue that were not very efficient.

      Return-flue Boiler

      (Credit: Wikipedia: Flued Boiler)

    • Huber Boiler
      The Huber boiler became the first fire-tube boiler to be more complex than several types of boilers before. This boiler has not used one large pipe as a return channel for exhaust gas, but has used several small pipes or tubes in order to maximize heat transfer from the flue gas to the water in the tank. The shape of the exhaust gas channel after exiting the combustion chamber also has a better design. The design makes gas distribution to be maximized to all pipelines.

      Huber Boiler

    • Cornish boiler
      Another development of a fire-tube boiler design is the Cornish Boiler. This boiler is a horizontal boiler with a natural draft system, so it requires a high chimney shape to ensure adequate oxygen supply.

      This boiler is made from a large water tank with the combustion chamber right in the middle. Flanked by a brick building, such that the flow of combustion gases coming out of the combustion chamber in the middle of the tank will flow back along the outer edge of the tank. Next the brick building will direct the exhaust gas to traverse a passageway under the tank, before finally passing through the chimney and out into the atmosphere.

      For more details, let’s look at the picture on the side, top and front of this Cornish boiler.
      Boiler Cornish

    • Butterley Boiler

      Butterley boilers are the development of the Cornish boiler, which initially aimed to accommodate the needs of boilers in the northern United States which are rich in coal with lower calorific value than the southern mainland. This boiler is similar to the Cornish boiler design but by removing the exhaust gas lines under the water tank.
      Butterley Boiler

    • Lancashire boiler
      The Cornish boiler also has another fire-tube boiler design derivative called the Lancashire Boiler. If the Cornish boiler has only one combustion chamber and at the same time one large fire pipe in the middle of a water tank, then the Lancashire boiler has two combustion chambers that are simultaneously two fire-pipes in the middle of a water tank. The boiler developed by William Fairbairn in 1844 tried to adjust the Cornish boiler design when using coal fuel in the Lancashire area on the English plain, which tends to be difficult to burn in small boilers.
      Lancashire Boiler
      Lancashire Boiler

    • Locomotive Boiler
      The Locomotive Boiler becomes the first complex fire-tube boiler. Even this boiler is still often encountered today. Boilers that are named according to their use as a train driving machine are designed to produce superheater steam. The water vapor will be directly used as a piston drive on a steam engine that is designed to blend into the Locomotive boiler system.

      This boiler has also been designed to have a lot of medium sized fire pipes that are smaller than fire pipes in the Center-Flue and Return-Flue Boilers, so that it will increase the transfer of heat energy from the combustion gas to the water.

      An important component of the Locomotive Boiler is the presence of a superheater steam valve that is inside a section called the dome. This one-way valve will only open by superheater steam when it reaches a certain pressure. Furthermore, the superheater steam will enter into a steam piston driving medium.
      Boiler Lokomotif

    • Scotch Marine Boiler
      The Scotch Marine boiler is the most popular fire-tube boiler design used even today. This boiler was originally made to meet the needs of steam in marine engines. Even the legendary Titanic ship uses a total of 29 Scotch Marine boilers.
      Scotch Marine Boiler

      Scotch Marine boilers have high efficiency. This is obtained because the design of the fire pipe in the water tank is very much. Hot gas from the combustion process comes out of the combustion chamber in the middle of the water tank, towards the fire pipes which are next to the combustion chamber with the opposite direction of flow. Then the exhaust gas flows back to the fire pipes on the upper side with the direction of the direction of the direction of combustion in the combustion chamber. In short, the flow of combustion gases inside the fire pipes seemed to form the letter S.

      Scotch Marine Boiler

    • Vertical Fire-tube Boiler
      Fire-tube boilers that are vertically areanged, known as vertical fire-tube boilers. This type of boiler has design advantages and the manufacturing process is not too complicated. The combustion chamber is under a water tank, with pipes for exhaust gas lines arranged vertically in the water tank.

      Vertical Fire-Tube Boiler

    • Horizontal Tubular Return Boiler

      The Horizontal Return Tubular Boiler is similar to other fire-tube boilers that we have discussed. Has a flat arrangement of fire pipes. What is slightly different is the design of the placement of a combustion chamber that is not in a water tank, but is under the tank. The fire pipes in the tank will only be passed by hot exhaust gas from the combustion of fuel in the combustion chamber.
      Horizontal Return Tubular Boiler

    • Admiralty-type direct tube boiler

      This fire-tube boiler is unpopular and has not been used by many since its appearance in the Ironclad warship era in the mid-19th century. One thing that made it unpopular was the design of the fire pipe that was connected directly to the combustion chamber so that over-heat was often occured on the pipe.

    • Immersion Fired Boiler
      This last fire-tube boiler has one characteristic that is not haved by other fire-tube boilers. The boiler developed by the Sellers Manufacturing manufacturer is designed so that each fire pipe in the water tank functions as a combustion chamber as well as a hot exhaust gas from the combustion process. So that this boiler has a lot of burners (burners) with an amount equal to the number of existing fire pipes. With an automatic boiler design that is only suitable for using liquid or gas fuels, it is claimed to have a relatively low temperature voltage. This boiler is still marketed to date by the manufacturer Sellers Manufacturing as the owner of the patent design.
      Immersion Fired Boiler

  2. Water-Tube Boiler
    Water-tube boilers have a reverse design with fire-tube boilers. This boiler circulates water through the pipelines with a heat source coming from the furnace. A water tank commonly called a steam drum is one of the characteristics of a water-tube boiler. Steam drum serves as a water tank that is maintained at a certain level to ensure there is always circulating water to the water pipes. Besides that, the steam drum is also to separate steam from water-steam mixture inside the drum. Wet steam coming out of the steam drum will be heated further to produce superheated steam.

    Water-tube Boiler

    The popular water-tube boiler design is using the water pipes as the combustion chamber wall (wall tube). The water from the steam drum drops through a downcomer pipe to a header pipe connected to all the lower ends of the wall-tube pipe. The other wall-tube end that is at the top of the combustion chamber is directly connected to the steam drum. In this part of the wall tube, the phase changes from water to water vapor. This water-tube system produces a closed-loop circulation between the steam drum-downcomer-wall tube-and returns to the steam drum. From the steam drum, only saturated steam will come out.

    Even water-tube boilers have slightly more complex designs than fire-tube boilers, but water-tube boilers tend to be able to produce higher quality steam (more superheated). Therefore, water-tube boilers are more suitable to be applied in large industries that are more demanding of high quality steam such as steam power plants.

    Based on different designs, water-pipe boilers can be classified as follows:

    • John Blakey Boiler (1766)

      This boiler designed by John Blakey became the forerunner of the water tube boiler. This boiler is composed of a vertical funace with several connected pipes inside, which are made tilted to form a certain angle. The two ends of the pipe are connected to a smaller pipe.

      This boiler was patented by John Blakey in 1766, but not too popular at the time.

    • James Rumsey Boiler (1788)

      The first functional water-tube boiler was created by a mechanical engineer from the United States, James Rumsey. He is known to patent several water-tube boiler designs, making James Rumsey touted as the inventor of the water-tube boiler. One of the most famous designs is a steam-powered boat.

      The boat, which was then made to cross the Potomac River, was equipped with a water-tube boiler. The water tube in the boiler are twisted horizontally, inside a large enough furnace. The steam produced is used to drive a steam piston.

      The steam piston, using a single shaft, is connected to another piston underneath it. The second piston serves as a water pump, with water from the river as the media where the boat operates.

      The piston shaft, also connected to a large pendulum. The pendulum is connected to an injector pump and an air pump on the condenser.

      Steam that enters the steam piston will lift the shaft, so that the water piston also lifted, and sucks the water of the river into the piston cylinder. When the piston reaches the top dead centre, a knob on the shaft will touch a stick mechanism, so the control valve will change its position.

      When the control valve changes its position, the steam inside the cylinder will be pushed out and enter the condenser. The water piston will also be pushed down, so the water comes out of the cylinder, passing through the nozzle on the back of the ship, thus creating a force for the ship.

      When the shaft position down, the injector pump pushes the river water into the boiler, while the air pump pushes the water inside the condenser to exit.

    • Julius Griffith Boiler (1821)

      This boiler has a fairly simple design, but has a significant impact on the development of the next water-tube boiler design.

      This boiler was designed by Julius Griffith in 1821, which is composed of several horizontal pipes in several levels and placed inside the heat source. The horizontal pipes are connected to the twin vertical pipes on either side. At the very end, there is a last horizontal pipe as a gathering place for steam produced, which then comes out of the boiler. This top horizontal pipeline design will next to be the forerunner of the design of the steam drum in modern water-tube boilers.

    • Joseph Eve Boiler (1825)

      The first sectional water-tube boiler with well-defined circulation was designed by Joseph Eve in 1825. This boiler composed of several vertical pipes with curve variations in the center of the pipe, two larger horizontal pipes as a water reservoir and a steam reservoir, as well as two large external vertical pipes as a circulating pipe between the steam reservoir on the upper side and the water reservoir on the lower side. These two vertical pipes have a function to ensure good natural circulation of water and steam inside the boiler system between the water pipes, reservoirs and external pipes.

    • Goldsworthy Gurney Boiler (1826)

      The Gurney water-tube boiler design was patented in 1825, first made in 1826, and tested in 1827 by Simon Goodrich.

      Model, scale 1:6, of Gurney’s Water Tube Boiler, patented 1825, built 1826, tested 1827 by Simon Goodrich.

      This boiler is composed of several U shaped pipes laid with one side in the top position. Each end of the U pipe is interconnected with a horizontal pipe with a larger diameter, both top and bottom. Then these two horizontal pipes are connected with vertical pipes to ensure the occurrence of water-steam circulation. There is also a long and large diameter cylindrical tube, vertically standing connected to the upper and lower horizontal pipes. This cylindrical pipe serve as a water reservoir and water vapor.

    • Stephen Wilcox Boiler (1856)

      This Wilcox design boiler became the first water-tube boiler to use inclined pipe design. These tilted pipes connect the water spaces on the front and back, with the steam room at the top.

      This boiler design later developed into Babcock & Wilcox boiler design, and dominated the water-tube boiler market in the late 19th to early 20th centuries.

    • Spiral Water-Tube Boiler

      If the fire-tube boiler develops along with the train development, the water-tube boiler design was developed in tandem with car technology.

      The birth of car technology in 1770 created by Nicolas-Joseph Cugnot, encouraged the development of cars in the 1800s that still use steam engines. Most of the car’s engines use spiral water tube boilers with different designs. Since then, spiral water tube boilers have developed into various uses.

      Spiral water tube boiler designs include Climax Boilers, Lune Valley Boilers, Monotube Boilers, The Baker Boiler, Ofeldt Boilers, and many others.

      Lune Valley Boiler

      Ofeltd Boiler

      Climax Boiler
    • D-Type Boiler
      The first water-tube boiler type that we will discuss is called the D-type because the shape of the boiler is similar to the letter D. This boiler is equipped with two tanks namely the steam drum on the upper side and the mud drum (water tank) on the lower side. These two tanks are connected to many water pipes which are partially arranged vertically, and some are arranged in the shape of the letter D. In the middle of the D-shaped pipes has a functions as a combustion chamber.
      D-Type Boiler
      D-Type Boiler
    • Type-A Boiler
      Still because the design is similar to the form of one of the Latin letters, the A-type boiler is named so indeed because its design is similar to the letter A. This boiler has one steam drum but with two water tanks below. The purpose of using these two water tanks is to further extend the life of the boiler because the water pipe will be longer than the D-type design. This boiler has a slimmer design than a D-type boiler, however an A-type boiler cannot produce higher energy-containing steam than the D-type for the same dimensions.
      Type-A Boiler

      (Credit: Wikipedia: Package Boiler)

    • O-Type Boiler
      The O-type boiler is the last type of water-tube boiler whose design is similar to one of the letters. This O-shaped boiler has a symmetrical shape with the position of the above steam drum and water tank below. Both are connected with symmetrical water pipes so that in the middle become boiler combustion chambers. This O-type boiler is claimed to be able to produce water vapor faster than the D-type. The low maintenance requirements are also another advantage of this boiler.
      O-Type Boiler

    • Babcock & Wilcox Boiler
      As the name implies, the Babcock & Wilcox boiler was developed by a firm with the same name as the boiler. This boiler design was developed and patented in the mid-nineteenth century. This boiler has only one tank, the steam drum positioned at the top of the boiler. The steam drum is partly filled with water and the other part contains wet water vapor. The typical design of this boiler is water pipes that are designed to be tilted to form a 15 ° angle. This slope serves to ensure the occurrence of natural circulation of water-water vapor in the boiler. On top of the water pipes there is also a further hot steam pipe which functions to further heat water vapor that has been sufficiently hot and escaped from the steam drum to be further heated to achieve superheated quality. For the flow of combustion gases in the boiler is made tortuous so as to maximize absorption of heat from the flue gas to the water fluid.

      Babcock & Wilcox Boiler

      (Credit: Mech4Study)

    • Stirling Boiler
      The Stirling boiler is one of the predecessors of the water-tube boiler. These boilers were popularly used in the early 1900s, and are very difficult to find at this time. This boiler has the characteristics of using two kinds of water tanks, steam drum at the top with an amount that is always more than the second tank, the water tank at the bottom of the boiler. Characteristics of the design make the Stirling Boiler can be classified based on the number of water tanks, there are three tanks with two steam drums and one water tank, four tanks with three steam drums and one water tank, and five tanks in the form of three steam drums at the top and two water tanks at the bottom of the boiler. The more number of tanks, the higher ability to produce steam. However, this boiler is old-fashioned and is no longer used because it has relatively lower efficiency values ​​than modern boilers.

      Three Drum Stirling Boiler

      Boiler Stirling Tiga Tanki
      (Credit: Wikipedia: Stirling Boiler)

      Four Drum Stirling Boiler

      Boiler Stirling Empat Tanki

      Five Drum Stirling Boiler

      Boiler Stirling Lima Tanki

    • Yarrow Boiler
      The Yarrow boiler is an important type of high pressure water-tube boiler. They were developed by Yarrow & Co. (London), and is widely used on ships, especially warships.
      Yarrow boiler design has the characteristics of a boiler with three water tanks: two tubes of straight water are arranged in a row of triangles with a single furnace between the two. A single steam drum is installed at the top between them, with a smaller water drum at the base of each bank. Circulation, both up and down, occurs in this same bank tube. Yarrow’s specialty is the use of straight tubes and also the circulation in both directions that occur entirely in the tube bank, and does not use external energy or we are familiar with natural circulation.

      (Credit: Wikipedia: Yarrow Boiler)

      Because of the characteristics of the three drums, Yarrow boilers have greater water capacity. Therefore, this type is commonly used in boiler applications for old warships. Its compact size makes it attractive for use in power generation units that can be transported during World War II. In order to be transported in its time, boilers and auxiliary equipment (fuel oil heaters, pump units, fans etc.), turbines and condensers are installed in their own carriages to be carried through the railroad tracks.


    • Thornycroft boiler
      This boiler was designed by the ship manufacturer John I. Thornycroft & Company. The special design of this boiler is to use just one steam drum on the upper side, with three downcomers so that it is arranged similar to the M formation boiler. However, due to the design of several pipes that have sharp bending, it risks leaking quickly not only because of the possibility of thermal stress, but also because of its own difficulties when needing to be cleaned. Because of these weaknesses, this boiler is not as popular as Yarrow Boiler.

    • Tube-Walled Boilers

      In the early days of its development, water-tube boilers were not as fast developed as fire-pipe boilers. This is because water-tube boilers require more complex design calculations and manufacturing techniques. But the main advantages of fire-tube boilers that have almost no maximum capacity limit, making the development in the next period, only need to wait for the birth of modern welding and materials technologies.

      After electricity was discovered, then the construction of steam power plants began intensively in the early 20th century, the Stirling Boiler type still dominated. To meet this need, numbers of Stirling Boilers are built at once in parallel so they can produce more steam.

      Susunan Boiler Stirling pada sebuah pembangkit listrik di awal abad 20
      Stirling Boiler by Babcock & Wilcox

      Why isn’t the Stirling Boiler made larger, and bigger, so it can produce more steam?

      The main reason was the use of fire bricks as the boiler wall. Fire brick walls would certainly be troublesome if you have to be arranged too high, as well as widened, following the boiler design if you want to be enlarged. Beside that, this large-sized wall must be able to isolate the heat energy of the combustion chamber, to ensure maximum heat absorption in the boiler.

      Gradually, new innovations were born to replace fire brick walls. The advancement of pipeline material and welding technology are also driving the advancement of boiler wall technology.

      Tube & Tile Boiler Wall

      Tube and tile boiler walls, became the initial innovation of the boiler wall design revolution. Found in the 1920s, this boiler wall combines a 6 inch diameter pipe with 2.5 inch thick tiles or 4.5 inch thick fire brick. Tube and tiles are arranged alternately, and the outer side of the wall is insulated to maintain boiler efficiency.

      The existence of the tube in the boiler wall, has a function to cool the wall so that the thickness of the fire bricks can be reduced from the previous thickness which can reach 22 inches. Since that time, the boiler design continue to grow in both size and capacity.

      In the late 1920s and early 1930s, the appearance of the flat studded tube and the loose tube wall constructed boilers. This design was able to increase the absorption of boiler heat. So in those days, boilers that used those two designs were able to receive the highest heat generated from coal combustion.

      Flat studded tube and the loose tube

      Large changes in the water-tube boiler wall design occurred in the late of 1950s and early 1960s. Since then, and are still used today, water-tube boiler walls made from long steel tube which arranged in a row and welded each other with certain widths of steel membrane bar between every tube.

      Modern boiler wall design

      This design is much easier to fabricate because making every wall panels can be done at the workshop. Then the wall panels can be much easier to assemble when build the boiler. The process of building boilers has become much more practical, time-saving, and certainly effective in cost.

      Not only that, the main advantage of this boiler wall-tube design makes the water-tube boilers could be built even larger and bigger. Known this day, huge water-tube boilers are capable to produce superheated steam of more than 4,000 tons per hour. That’s the same as more than a thousand kilograms of steam produced by this boiler, every second.

      Again, in every second!

    • Once-Through Boiler

      Once-through boiler is a concept of water tube boiler that does not occur a non-vaporizing water circulation. That is, each water molecule only passes through the boiler pipes once. This concept greatly increases boiler efficiency because it is no longer requires a steam drum as a water and steam separator, so there is no need for additional boiler circulation pumps.

      This boiler concept is actually not new. The boiler design was once patented in 1824. But the first commercial application of this boiler could only be done in 1923, by a Czechoslovakia inventor, Mark Benson. At that time, Benson only could build 1.3 kg per second boiler capacity. The boilers built to fulfill orders from English Electric Co.. The boiler was originally designed to operate at critical steam pressures. However, due to the frequently damage of the pipeline, boiler operating pressure was then forced to be lowered.

      Modern Once-Through Boiler

      The once-through boiler continues to grow until now. Supercritical and ultra-supercritical boilers have used this concept. So even though this boiler is used in power plants with a capacity of 1000 MW, the efficiency can reach 46%.

Classification of boilers based on water circulation method

In water-tube boilers, the circulation of water in boiler pipes is important to pay attention. In addition to good boiler water circulation, it will increase boiler efficiency, water circulation is also important to maintain boiler durability. This is because the water in the boiler also as a cooling medium, delaying the water circulation, resulting high thermal stress on the pipe. Of course this is very avoided.

Against this background, there are known two types of boilers based on the way of water circulation. Here are both:

  1. Natural Circulation Boiler
    Boilers with natural water circulation do not use external energy to circulate water in boiler pipes. The water in this boiler is naturally circulated due to the pressure difference between low temperature water and high temperature. Naturally high temperature water will have a relatively lower density. Therefore, the water is getting hotter and the phase changes to steam, the more it will be pushed upwards. Because of this process, the water in the boiler pipes will be circulated. 

    Boilers with natural circulation include Babcock & Wilcox boilers, Lancashire, Cochran boilers, locomotive boilers, and so on.

    Perbedaan Boiler Sirkulasi Natural dan Paksa
    (Credit: Wikipedia: Forced Circulation Boiler)

  2. Forced Circulation Boiler
    Boilers with forced circulation, use additional pumps to help the circulation of water in the boiler. This type of boiler does not need to wait for water phase differentiation to be able to circulate water. With the help of external energy for the process of water circulation, the process of generating steam will not be limited by the size of the boiler. When compared, forced circulation boilers can produce twenty times more steam than natural circulation boilers that have the same volume size.

    Examples of forced circulation boilers include Benson boilers, La Mont boilers, Velox boilers, and so on.

Classification of boilers based on their working pressure

In accordance with technology advancements, the quality of boiler steam also continues to improve. The boiler designers believe that the higher steam pressure can be achieved, the boiler efficiency will be higher to. So the following are the classification of boilers based on the steam pressure produced:

  1. Low-pressure boiler: This boiler produces 15-20 bar of steam.
  2. Medium-pressure boiler: This boiler produces steam from 20 to 80 bars.
  3. High-pressure boiler: This boiler produces steam pressure above 80 bar.
  4. Sub-critical boiler: The critical point of a boiler is a condition where boiler steam reaches a temperature of 560 ° C at a pressure of 221 bar. If a boiler works below these conditions, the boiler is called a subcritical boiler. Typically subcritical boilers are designed to work at 160 bar and steam temperature of 540 ° C.
  5. Supercritical boiler: If a boiler works above its critical point, the boiler is called a supercritical boiler. Supercritical boilers have better fuel efficiency than subcritical boilers. Supercritical boilers have a design efficiency value of around 45%. While subcritical boilers can only reach 38%.

    This is due to the impossibility of forming bubbles in the supercritical boiler cycle. As a result of the work pressure and temperature above the critical point, the water will not experiencing the nucleate boiling phase (the transition phase from liquid to vapor) and immediately changes the phase immediately to steam. One characteristic of supercritical boilers is that they do not use steam drum components which to separate water from wet steam at sub-critical boiler.

  6. Ultra Supercritical boiler:
    The working point of the boiler which is high above the critical point, the boiler will be more efficient. To achieve this, more sophisticated and expensive boiler pipe material technology is needed. The last few decades have made possible the manufacture of the material in question, so that at present the boiler design has been able to reach the point of work very far above its critical point. The boiler that we know as the Ultra Supercritical (abbreviated as USC) has an operational point of around 260 bar and a temperature of 700 ° C. This modern boiler has a theoretical efficiency value of up to 50%.

Classification of boilers based on their energy sources
In accordance with technological advances as well, now there are so many sources of energy that can be used as a source of boiler heat. Boilers developed at the beginning of its history only used fossil fuels, now there are several boiler technologies that can use renewable energy.

The following are among them:

  1. Coal-Fired Boiler
    Coal is the most commonly used fuel in large capacity boilers, including in Indonesia. The price is cheap, abundant (especially in coal-producing countries including Indonesia), high heat values, a number of reasons for the use of coal as boiler fuel to date.

    The use of coal as boiler fuel requires special treatment not carried out on other types of boilers. Characteristic of solid coal, the average size of your fist, requires a grinding process before it is burned in the boiler combustion chamber. Of course this is the main objective to facilitate the burning of the coal.

    Not only that, the processing of coal boiler exhaust gas is also different from other boilers. This boiler exhaust gas contains ash, carbon dioxide, sulfur, to NOx. Some binding processes for these wastes also need to be considered. Like the use of Electrostatic Precipitator to bind ash, then use Flue-Gas Desulphurization to bind sulfur, to the use of staggered combustion technology to minimize the formation of NOx.

    The complexity of coal boiler design, makes the economics of these boilers not as good as oil-fired boilers if used on a small scale. Therefore, coal boilers are more widely used for subcritical to ultra-supercritical production scales.

  2. Oil-Fired Boiler
    Oil-fired boilers are quite popular for small scale use only. This is due to a much simpler design than a coal-fired boiler. These boilers are generally fire-tube boiler, which only require the main component of the burner and the pipe network for the flow of fire (hot gas) which is made inside the water furnace.

    These boilers generally use diesel fuel, or commonly known as High Speed ​​Diesel (HSD). The simple design makes this boiler very suitable for the production of low pressure steam with low steam production capacity.

  3. Nuclear Boiler 
    As the name implies, nuclear boilers use nuclear technology as a source of heat energy. This boiler is very popular for use in nuclear power plants. In nuclear power boilers, the heat energy from the fission reaction inside a nuclear reactor is absorbed by the coolant material which can be gas, liquid, or even liquid metal, depending on the type of reactor. This cooling material then flows into the boiler and is used to heat water so that it changes phase to further steam. The steam that is produced is channeled into turbines to generate electricity in nuclear power plants.

    The popular raw material for nuclear reactors is Uranium. Uranium is a type of heavy metal that is not very useful on Earth and is easily found in the oceans and rocks. There are two types of uranium with different isotopes that we know, namely uranium-238 (U-238) and uranium-235 (U-235). These two types of uranium have a major difference in their reactive age. U-238 has a longer reactive life than U-235, which also shows that U-235 is less radioactive than U-238.

    One major risk of using a nuclear reactor is of course the radioactive hazard. Therefore, nuclear reactors are always made in a dome that serves to prevent radioactive reactor leakage. Generally the outside of a nuclear reactor is made in a dome shape with strong concrete material which not only serves to prevent radioactive leakage, but also to resist natural disturbances from the outside.

  4. Solar Concentrated Powered Boiler
    The boiler we will discuss next is very new technology. This boiler uses a very renewable source of energy from sunlight. Although sunlight is only available during the day, this boiler can operate 24 hours a day. Thanks to the special fluid called molten salt, that able to stored heat from the sunlight.

    Solar concentrated boilers use main component of large number of mirrors, arranged around a heat-receiver tower. The mirrors are positioned in such a way that the reflection of the sunlight captured by each mirror is reflected centrally to the heat-receiver tower. Each mirror component is equipped with an automatic mechanism, so that it can move following the sun, so that the direction of the sunlight reflection always leads to the heat-receiver tower.

    A mechanism is used to circulate the molten salt into a heat-receiver tower. It is estimated that the heat caught in this tower can reach 1500 times hotter than what we normally feel. The heat is absorbed by the molten salt and stored in a special thermal storage tank.

    Then, through a heat exchanger, the water absorbs the heat from the molten salt so that the water boils and reaches the superheat temperature. In concentrated solar power plants, this steam is then used to rotate steam turbines and produce electricity.

    Many power plants that have used this technology are built in Spain, the United States, South Africa, India, and a little in China.

  5. Waste-to-Energy Boiler

    Waste power boilers or also known as waste-to-energy boiler are the most environmentally friendly solution to two problems at once: garbage and fossil fuel crisis. Waste production that continues to increase every time becomes one of the energy sources that can be used as boiler fuel.

    Waste-to-energy boilers are not much different from other biomass boilers. First, the waste is brought to the facility. Then, the waste is sorted to remove recyclable and hazardous materials. The waste is then stored until it is time for burning. A few plants use gasification process, but most combust the waste directly because it is more efficient. The waste can be added to the boiler continuously or in batches, depending on the design of the plant.

    It is known that waste-to-energy boilers have a friendlier emission level than coal-fired boilers. This is due to the absence of sulfur pollutants as contained in coal.

    Biggest Waste-to-Energy Power Plant will be built in Shenzhen, Cina

    Waste-to-energy power plants have been used for more than two decades in Sweden. And now many have been built in China, the United States, and many other countries.

What is Fire-Tube Boiler?

What is Fire-Tube Boiler? – Do you remember what boiler mean? Boiler is a vessel that serves to heat water. In principle, a pan is also a boiler, but it’s not a boiler of this kind that we will discuss. Long ago, mechanical engineers created boilers by simply increasing the size of the ‘pan’. Then gradually they design the boiler with more complex again. One important principle is that larger surface contact between the heat source and the heated water, the more steam produced by the boiler in equal size. Then came the idea of ​​making pipelines in the gigantic ‘pan’, so the pipes flowed the fire – or at least the hot gases – through the amount of water housed by the giant ‘pan’. This is the forerunner of the fire-tube boiler. So the definition of a fire-tube boiler is one type of boiler that flows the heat of the combustion process into one or more pipes, which were inside a sealed container filled with water.

What is Fire-Tube Boiler?
Haystack Boiler: ‘Giant Pans’ as the pioneer of fire-tube boiler
What is Fire-Tube Boiler?
Vertical Fire-Tube Boiler: Developed from Haystack Boiler

Fire-tube boiler is the simplest type of boiler. This boiler can be applied from low to medium water steam requirements. This is possible because the design is not complicated as the water-tube boiler. The size of the fire-pipe boiler is also relatively smaller, and allows it to be moved very easily. That advantages make this boiler is very popular when developed in conjunction with the steam engine. In the 19th century until the beginning of 20th century, fire-tube boilers were developed massively to meet the transportation needs of the time. Steam locomotive, naval ships, and early models of cars, became the most sophisticated models of fire-tube boiler transportation in that period.

What is Fire-Tube Boiler?

As the name, the fire-tube boiler flows the hot gas to the water-coated tubes. Different pipeline designs from the various fire-tube boilers, are to maximize the heat absorption of the combustion gases. The water level inside the boiler tank, must be maintained to avoid overheating. On the other hand, this boiler is also equipped with a safety relief valve avoid the explosion from over pressure.

Many types of fire-tube boilers are also equipped with advanced steam heating systems to produce superheated steam. Nevertheless, the fire-tube boiler has limited steam production of only 2500 kg/h with a maximum pressure of 10 bars only.

Types of Superheater Boiler

Types of Superheater Boiler – Superheater is a subcritical boiler component that serves to heat the saturated steam, at constant working pressure, so it becomes superheated steam. In its development since the beginning of the 20th century, along with various boiler design races, some engineering experts patented the design of different superheater. Here are the types of superheater boiler, according to the patents:

1. Radiant Superheater

Radiant superheater is a superheater positioned in the boiler’s combustion chamber, so the superheater pipes instantly absorb the radiant heat from the combustion inside the furnace. In modern water-tube boilers, these superheater radiant pipes are placed hanging over the top of the boiler furnace. These pipes will absorb the second greatest heat energy, after the wall tubes (raiser/evaporator tube).

Types of Superheater Boiler
Radiant Superheater

At radiant superheater, the more steam flows in the superheater pipes of radians, the steam temperature output are decreasing.

2. Convective Superheater

As the name implies, the convective superheater is the superheater pipes of the boiler which placed in the flow of the flue gases that still contain heat. These convective superheater pipes will absorb heat from combustion exhaust gases convectionally. This concept aims primarily to maximize heat absorption from combustion.

In contrast to the radian superheater, the characteristic of the convection superheater is that the more steam flows in the convection superheater pipes, the superheater steam temperature output increased.

3. Separately Fired Superheater

Separately fired superheater is a superheater that is placed separately from the main boiler, which has own separate combustion system with the main boiler. This superheater design is not like the radians or convection types that still use the combustion heat inside the furnace, but instead put additional burners in the area of ​​superheater pipes. This type of superheater is not popularly used, and even tends to be extinct due to efficiency of combustion ratio with steam quality that is not better than other superheater types.

Separately Fired Superheater
One of the patented designs of Separately Fired Superheater

4. Combination Radiant and Convection Superheater

The last superheater type is the most popular, and still applied today. This superheater simultaneously combines two opposite characteristics between radiant and convection superheater, resulting in more homogeneous superheated steam temperature output in various steam flow. The graph below will explain these characteristics.

Types of Superheater Boiler
Variation of Steam Flow vs. Superheated Steam Final Temperature

In modern subcritical boilers, the superheater component of radians is subdivided into several stages. As in the subcritical boiler diagram below for example, after passing the Primary Superheater which is a convection superheater, steam is streamed sequentially to the Platent Secondary Superheater, Intermediate Secondary Superheater, then the last is the Final Secondary Superheater. This design aims to maximize the absorption of radiant heat from combustion inside the furnace.

Subcritical Boiler

What is Reheater on Boiler?

What is Reheater on Boiler? – Reheater is a part of boiler which to reheat steam output from the first level of steam turbine. Reheated steam will again absorb the heat energy from boiler to be used in the next level steam turbine. Reheater is one way to improve the thermal efficiency of the Rankine Cycle. Visit the following article to find out how to improve the thermal efficiency of the Rankine Cycle.

Let us consider the following Rankine T-s diagram.

What is Reheater on Boiler?

The T-s diagram above is the ideal, standardized Rankine Cycle diagram, without the use of the reheater concept. Superheated steam produced by the boiler only once flows the turbine blades, and ends with a condensation process in the condenser. Next let’s compare it with the Rankine Cycle with the reheater system.

What is Reheater on Boiler?

The Rankine Cycle equipped with a reheater, will have at least two levels of steam turbine: high pressure turbine and low pressure turbine. High pressure steam turbine output steam, commonly known as cold reheat steam, has lower pressure and temperature of main steam, before entering the turbine. Although the cold reheat pressure is drop significantly, it still has not reached its saturation point. So, if this steam reheated, it will return to superheated steam. Therefore the cold reheat steam that exits the high pressure turbine, will be reheated back into the boiler.

Modern boilers equipped with reheater pipes, specially designed so that the reheated steam absorb heat just like the main steam. The reheated steam, also called hot heat reheat steam, will reach the same temperature as the main steam vapor. To achieve this, the reheater pipes will be placed not far from the final pipes of the superheater. More specifically, let us refer to the following supercritical boiler diagram.

Boiler Supercritical

Seen in above boiler diagram, reheater pipes are divided into two heating stages. First the cold reheat steam passes through the Low Temperature Reheater pipes, which shown by the name “LTRH” at the diagram. Furthermore, LTRH output steam flows to the pipes called Final Reheater. The steam produced by the Final Reheater is then called Hot Reheat steam. Hot Reheat steam then goes to the low pressure steam turbine, so that the heat energy contained in it is converted into mechanical energy of the turbine shaft rotation.

Theoretically, the addition of one stage reheater utilization will increase the thermal efficiency of the rankine cycle by 3-4%, the addition of two stages of the reheater increases the efficiency by 1.5-2%, the addition of the three-stage reheater increases efficiency by 0.75-1%, and so on. Commonly, modern boiler only use one or two stage reheater.

How to Calculate Thermal Efficiency of Rankine Cycle

How to Calculate Thermal Efficiency of Rankine Cycle – The thermal efficiency of the rankine cycle is the ratio between the work produced by the steam turbine that has been reduced by the pump work, with the incoming heat energy from the boiler. Before further discussing the thermal efficiency of the rankine cycle, it is easier to understand by discuss the processes first.

How to Calculate Thermal Efficiency of Rankine Cycle

The rankine cycle is one form of energy conservation laws. The source of energy that exists on earth is converted into another beneficial form of energy to humans. Heat energy is used as the energy source of the rankine cycle process. This energy can be taken from the burning of fossil fuels, geothermal usage, or from nuclear reactions.

The heat energy from the above sources is transferred to the working fluid, such as water. When the fuel used is coal then this process occurs in the boiler. Through the diagram above D-E-A-F, the D-E line, water is still in liquid form, on the E-A line boiling process occurs and the water phase is liquid and steam mixture, while on the A-F line the water working fluid has water-vapor phase and got further heating process to obtain superheated point. The calorific value absorbed by water vapor can be calculated using the following formula:

Qin = m (hF – hD)

The superheated steam produced by the boiler then goes to the steam turbine. Heat energy from water vapor is then converted into motion energy, shown by the F-G line in the image above. The reduction of the enthalpy can be used to calculate the magnitude of the motion energy produced by the steam turbine using the following formula:

Wout = m (hF – hG)

The steam coming out from the steam turbine enters the condenser to be condensed back into liquid phase. Here we can see there is amount of heat energy not converted into motion energy, because the energy is used to convert the water into steam (latent heat). The decreases of the enthalpy (G-C line) can be used to calculate the thermal energy of condensed water using the following formula:

Qout = m (hG – hC)

The next process is pumping the condensate water to increase its pressure before entering the boiler. Shown by the C-D line, water does not experience much increase in enthalpy. This means that the energy given to the air is not too significant. Incoming energy values ​​can be calculated using the following formula:

Win = m (hD – hC)

So now we can calculate the thermal efficiency by using the formula below:

\eta _{thermal}=\dfrac {W_{out}-W_{in}}{Q_{in}}

eBook Rankine Efficiency Cycle Calculations:

Working Principle of Supercritical Steam Generators

Working Principles of Supercritical Steam Generator – Supercritical boilers have been popular since the mid-20th century. Since then until now, this boiler is very popularly applied to power plants, replacing subcritical boilers. Supercritical boilers have several advantages not possessed by subcritical boilers:

  1. The efficiency of the supercritical boiler is higher because to generate the same heat energy, it takes less fuel than the subcritical boiler.
  2. The exhaust emissions, especially carbon dioxide, are relatively lower than those of subcritical boilers.
  3. The size of the boiler is relatively smaller than the subcritical boiler with the same output.

Working Principle of Supercritical Steam Generators
T-s diagram of Supercritical Steam Generators

Supercritical Steam Generators

One basic concept of supercritical boilers is the working pressure and temperature above the critical point of water. If you already understand what is a critical point, then you should be know that above the critical point, water has a very different character than the other phase. Known as supercritical water, this phase has an indistinguishable characteristic of whether liquid water, or gas water (read the following article about supercritical steam). Supercritical water no longer has a boiling point, no longer having a saturation vapor phase, so that the pressurized water above 221 MPa if kept heated to above 374°C will immediately turn the phase into a supercritical steam, without passing the mixed phase between water and steam as in the boiler subcritical. This concept makes supercritical boiler components slightly different from subcritical boilers. Also because of this phenomenon, the term of boiler is not appropriate to use, since the water never actually boiled at supercritical boiler. We use supercritical steam generators term instead of supercritical boilers.

From those concept, there is already a fundamental difference between subcritical boilers and supercritical steam generators. In a subcritical boiler before the water completely changes phase to superheated steam, water passes through the saturation phase. Therefore, the subcritical boiler required a steam drum component as a separator between liquid water with saturation vapor that can be heated further into superheated steam. While the formation of water vapor in the supercritical steam generators does not pass through the saturated steam phase, it can be ascertained that supercritical steam generators do not require steam drum. It is also one of the economic advantages of supercritical steam generators.

h-P Diagram of Supercritical Steam Generators Sliding Pressure
h-P Diagram of Supercritical Steam Generators Sliding Pressure

However, supercritical steam generators do not completely negate the separation system of liquid water with steam. In the initial condition of boiler ignition, the boiler is still working at the pressure below the critical point. In this condition, water heating will surely pass the phase of saturation, so the steam separator component is needed. To ensure continuous flow of water inside the boiler evaporator as long as the working pressure is below the critical point, a boiler circulation pump is occupied. Gradually, the working pressure will be increased (sliding pressure) until it reaches the ideal pressure above the critical point. If the working pressure of the water is above the critical point (generally designed boiler load is more than 30%), the separator will automatically direct the fluid to the low temperature superheater, and no longer recirculated to the evaporator via boiler circulating pump. Boiler circulating pump stop working at this state. Under these conditions, the supercritical steam generators fully enters a once-through boiler flow process. Thus, the amount of water flow into the boiler via economizer is fully controlled by the boiler feed water pump.

Working Principle of Supercritical Steam Generators
Working Principle of Supercritical Steam Generators

If we look again, supercritical steam generators will experience two kinds of circulatory systems:

  1. Wet mode. Wet mode occurs when the boiler load is still below 30%, or in other words the water pressure is still below the critical point. In this condition, because water will still experience saturation phase, there will be circulation process inside the boiler so it is similar to that happened in subcritical boiler where the amount of water circulation passing boiler evaporator will be more than the amount of superheated water vapor produced. If referring to the example of the above boiler illustration, then the water flow will be like the scheme below.
    Wet Mode Supercritical Steam Generators Flow Chart
  2. Dry Mode. Dry mode occurs when the boiler load is above 30%, and the fluid pressure is above the critical point. In this condition the boiler will no longer pass through the saturation phase, so the separator and boiler circulation pump will stop working. Supercritical boilers will experience a single fluid flow (once-through) in the sense that all water entering the boiler will only pass once through the boiler pipes in the absence of recirculating porosity through the evaporator again. Supercritical boiler flow scheme will be as below.
    Dry Mode Supercritical Steam Generators Flow Chart
    Supercritical Boiler Circulation during High Load Dry Mode

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What is Rankine Cycle?

What is Rankine Cycle? Rankine cycle is a theoritical cycle which converts heat energy into work. Developed by William John Macquorn Rankine in the 19th century, it has been widely used to steam engines. Currently, the rankine cycle is used in power plants and produces 90% of the world’s electricity.

What is Rankine Cycle
Basic System of Rankine Cycle
What is Rankine Cycle?
Rankine Cycle T-s Diagram

Water becomes the closed-loop rankine cycle’s working fluid. It means that water at the end of the cycle process, back to use into the first step of the cycle. In the rankine cycle, this water undergoes four processes according to the image above:

  1. C-D Process: The working fluid is pumped from low to high pressure, and in this process the working fluid is still at liquid phase. This process is called isentropic-compression because when pumped, ideally there is no entropy change.
  2. D-F Process: High pressure water from C-D process, pumped into the boiler. Water is isobaric heated (constant pressure) at the boiler. Boiler use various heat sources from outside such as coal, diesel, or nuclear reactions. In the boiler, water undergoes a phase change from the liquid, a mixture of liquid and steam (saturated steam), and 100% dry vapor (superheated steam).
  3. F-G Process: This process occurs in a steam turbine. The steam from the boiler enters the turbine and undergoes an isentropic-expansion process. The energy stored in water vapor is converted to motion energy in the turbine.
  4. G-C Process: The steam coming out from the steam turbine enters the condenser to condensed at isobaric pressure. The steam changed its phase back to a liquid so it can be reused in the cycle process.

The cycle described through the T-S diagram above is the most basic and simple Rankine Cycle. In its practical use, there are several process modifications to obtain higher thermal efficiency. Such as the use of preheated water prior to entering the boiler, as well as the use of reheated steam from the first turbine (so called high-pressure turbine) so that it can be used again to enter the second steam turbine (intermediate pressure turbine).

Rankine Cycle with Water Preheater and Reheater System
Rankine Cycle with Water Preheater and Reheater System

In the illustration above, the condensate water pumped by the condensate extraction pump from the condenser, through the low pressure preheater system, before flowing to the deaerator/feedwater tank. As well as the water pumped by feedwater pump from the feedwater tank, also passing the high pressure preheater system before going to the boiler. The heat source used by the preheater is derived from the steam extraction taken from the steam turbine at certain stages.

Temperature-Entropy Diagram For Rankine Cycles with Water Preheater and Reheater System
Temperature-Entropy Diagram For Rankine Cycles with Water Preheater and Reheater System

Another difference with the conventional rankine cycle is the reheating of steam coming out from the high-pressure turbine by the reheater boiler to re-gain the superheater phase. The reheated steam than use as the energy source of the intermediate-pressure turbine.

There is also a bypass system, so steam not to be passed to the steam turbine. The superheater steam from the boiler does not enter the turbine and bypasses back into the reheater boiler. The steam coming out of the reheater boiler is bypassed for direct entry to the condenser. The function of this bypass system is as a protection system of the rankine cycle so that it can avoid severe damage. And also used at the beginning of the start-up process of the cycle system and even the process of turning it off.

Superheater Working Principle

Superheater Working Principle – Superheater is a subcritical boiler’s component that heat the saturated vapor, at constant pressure, so it becomes superheated steam. Superheater technology has been used since the use of steam engines early 20th century. The main purpose is to increase the heat energy contained by the steam, so that increasing the thermal efficiency of the engine. Until now the use of superheater is still very popular, especially in large water-tube boiler steam power plant.

Superheater Working Principle

Picture above is a simplified of a subcritical water-tube boiler. This water-tube boiler is composed by two water tanks on the bottom and top. Both tanks are connected with pipes that we know as the raiser tube. The heat from the combustion will first pass through the raiser tube, heat the water inside the pipe. Water than reaches its saturation point and turns the phase into saturated steam.

Saturated steam is still mixed with liquid water so it needs a mechanism to separate the saturated steam with water. This is the function of the top side tank. This tank is commonly known as steam drum. The liquid water will remain in the steam drum and will be recirculated by the raiser tube. While the saturated steam will exit the steam drum and go to the superheater pipes. The superheater pendant will absorb heat by convection and radiation from the flue gas of combustion, until saturated steam dried and become superheated steam. Superheated steam have a greater heat energy content than saturated vapor.

Above is a much more complex and modern subcritical boiler scheme. This boiler is very popular used in steam power plants. The concept is not much different from the previous subcritical boiler principle. The superheater components in modern subcritical boilers made into several levels to fulfill the needs of the quality and quantity of superheated steam produced. In the diagram the superheater is shown by red pipes.

The subcritical boiler’s combustion chamber is composed of vertical raiser tubes that will circulate the water from and to the steam drum. In modern subcritical boilers, only one water tank is used as a steam drum on the upper side of the boiler.

The water in the raiser tube will absorb the heat directly from the combustion process. The water from the raiser tube goes back to the steam drum, and will be separated between the saturated steam phase and the liquid water. Liquid water will be re-circulated through the raiser tube, while the saturated vapor go out to the first stage superheater pipe (primary superheater). Primary superheater is also commonly known as Low Temperature Superheater (LTSH). LTSH pipes absorb heat conventionally from combustion exhaust gases.

From LTSH, the steam will pass consecutively the Secondary Superheater Platform, Intermediate Secondary Superheater, and the Final Secondary Superheater. This steam produced by Final Secondary Superheater is called superheated steam or dry vapor. One phase of water that actually gas phase. It contains no moisture at all, and stores very high heat energy, much higher than the saturated vapor.

Understanding Superheater and Supercritical Boiler

Superheater Boiler

The superheater boiler produces superheated or dry water vapor. This steam stores more heat energy than saturated water steam, characterized by higher enthalpy values. The steam produced by conventional boilers generally only reaches the saturated phase, but in this superheater boiler, saturated steam will be heated further to reach the superheated phase. In addition to storing greater heat energy, superheater boiler removes the dampness of saturated steam.

The main advantage of using a superheater boiler is to increase the boiler efficiency, so it can reduce fuel and water consumption. But on the other hand, there are additional costs required for the erection and maintenance. Since the superheater boiler works at high pressure and temperature, it is using higher pipe quality than conventional boilers.

Superheater Boiler

The superheater boiler at the beginning of its emergence is used on steam trains. And then more widely used for the needs of steam power plants. The size also depends on the needs of steam consumption, 640 megawatts of power plants for example using a superheater boiler with steam production of about 1800 tons per hour.

Supercritical Boilers

Supercritical boilers produce supercritical steam. This boiler is called supercritical because it operates above the critical pressure and temperatures, which is 3,200 psi and 647 Kelvin. In contrast to a superheater boiler that requires a device to separate water vapor with a mixture of steam and water (usually called steam drum), supercritical boilers do not need it. During the process of supercritical steam formation there will be no transition phase from liquid water to steam. This leads to less fuel consumption, and further reduces CO2 gas emission. Actually the term boiler is not appropriately used in supercritical boilers, because in the process does not occur boiling process in it. So that supercritical boiler better known as supercritical steam generator.

Supercritical Boiler

What is Supercritical Steam?

What is supercritical steam? While a supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. So, supercritical steam is water at a temperature and pressure above its critical point (>647.096 K, >22.064 MPa), where distinct liquid water and steam phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid. In addition, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a supercritical fluid to be ‘fine-tuned’.

Above the critical point there is a state of matter that is continuously connected with (can be transformed without phase transition into) both the liquid and the gaseous state. It is called supercritical fluid. All differences between liquid and vapor disappear beyond the critical point.

What is Supercritical Steam?

This can be seen in the water phase diagram above, beyond the critical point (>647.096 K, >22.064 MPa) in the liquid-vapor space on the right. At the critical point there is about 30% free monomeric H2O molecules and about 17% hydrogen bonding. As with other supercritical fluids. supercritical water has no surface tension with its gas or liquid phase or any other supercritical phase as no such interfaces exist. Above 647.096 K, water vapor cannot be liquefied by increasing pressure.

The properties of supercritical water are very different from ambient liquid water. For example, supercritical water is a poor solvent for electrolytes. However, it is such an excellent solvent for non-polar organic molecules, due to its low relative permittivity (dielectric constant) and poor hydrogen bonding, that many are completely miscible. Viscosity and dielectric both decrease substantially whereas auto-dissociation increases substantially. The physical properties of water close to the critical point (near-critical) are particularly strongly affected by density. An extreme density fluctuation around the critical point causes opalescent turbidity. At the critical point, only one phase exists. The heat of vaporization is zero.

As pointed out in numerous studies, water’s properties exhibit dramatic changes under supercritical conditions: the fraction of hydrogen bound (HB) molecules greatly decreases with respect to ambient P and T, and there appears to be a consensus on the persistence of some hydrogen bounds up to at least 600 °C and 134 MPa (3). The hydrogen bound network, where present, is substantially distorted. Whether supercritical water is homogeneous (or composed of patches of clearly defined HB regions and non-HB ones), and over which length-scale possible density heterogeneities might appear, are still matters of debate. The presence of inhomogeneous patterns in the density has long been a contentious topic for the liquid at ambient conditions as well. Again, a tenuous consensus is forming in the scientific community around the idea that possible heterogeneities represent transient rather than equilibrium states of the liquid or supercritical fluid.