Fire-tube boilers are the oldest and most common types of boilers in the industry. Due to their simple design, high reliability, and low maintenance costs, they have been in continuous use for many years. In a fire-tube boiler, the heat from the hot combustion gases is transferred to the water inside the boiler, generating steam or hot water. This steam is used to supply the energy required for various industrial processes, including electricity generation, heating, and chemical processing. This article examines fire-tube boilers, including their types, components, applications, and methods for enhancing efficiency.
Fire-tube boiler
Boilers are classified into two categories based on their structure: fire-tube boilers and water-tube boilers. Fire-tube boilers are one of the most commonly used types of boilers, which have been in operation across various industries for centuries. In this type of boiler, the hot gases from fuel combustion pass through several paths inside the smoke tubes, warming the water surrounding them. Ultimately, hot water or steam is generated. Meanwhile, in water-tube boilers, water flows through the tubes, and the combustion gases surround them. These boilers are designed to produce high-pressure steam or hot water and are primarily used in oil and gas, petro-refinery, and power plant industries.
The first fire-tube boilers were developed in the late 18th and early 19th centuries and were particularly used in industries such as electricity generation and transportation including steam locomotives. One of the first designs of these boilers was created in 1804 by Richard Trevithick in England, which was used to supply steam for steam locomotives.
As technology advanced over time, fire-tube boilers grew in size and efficiency. Their simple design, low maintenance and repair costs, and ability to generate large amounts of steam at medium pressures made them key heating equipment in various industries. These boilers play a crucial role in industries like building ventilation, food production, paper, chemicals, petrochemical refining, and small power plants.
Fire-Tube Boiler Components
Fire-tube boilers, whether used for hot water or steam, have several key components in common. The figure below shows the main parts of a fire-tube boiler, which include:
1- Boiler shell: This is a pressure vessel where the working fluid (water or oil) is heated.
2- Combustion chamber: This chamber is where the burner flame is formed. the heat from the flame is transferred to the boiler fluid (water or oil) through radiation, convection, and conduction. About 40 to 45 percent of the energy from fuel combustion is transferred to the boiler’s fluid.
3- reversal chamber: After passing through the combustion chamber, the combustion gases enter the reversal chamber. In this part, the gases release part of their energy to the surrounding water before moving into the smoke tubes (second pass).
4- smoke tubes: After passing the combustion chamber, the combustion gases enter these tubes. The thermal energy of the gases is transferred to the surrounding water through these tubes.
5- Front tube plate: This plate holds the beginning of the smoke tubes.
6- Rear tube plate: This plate holds the smoke tubes ends.
7- Reversal chamber tube plate: It holds the smoke tubes of second pass.
9- Wrapper plate:It is one of the components of fire-tube boilers, acting as the shell of the reversal chamber. This part is responsible for guiding and returning hot gases to the heat transfer tubes to enhance heat transfer and improve the boiler’s efficiency.
10- Burner: This device is responsible for the optimal combustion of fuel and air. In the burner, the energy from the fuel is converted into thermal energy by creating a flame.
11- Stack: The combustion gases are released into the atmosphere through the stack.
12- Inspection Openings: They are provided for inspection and periodic maintenance of the boiler.
13- Outlet flange: outlet flange is a crucial component in fire-tube boilers, positioned where hot gases or fluids leave the system. It facilitates a secure, detachable connection for directing fluids to tubes or additional equipment and is generally designed to meet specific pressure and temperature standards.
14- Safety valves: They are installed on the boiler to prevent excessive pressure and avoid the risk of explosion.
15- Pressure gauge and thermometer: Pressure gauges and thermometers are used to measure the temperature and pressure within the boiler.
16- Supports: These supports are responsible for securing the equipment in its intended location.
17- Boiler power and control system: This system includes all the electronic and control devices of the boiler, establishing the connection between the pumps, burners, pressure gauges, thermometers, and other equipment in the boiler room. These components may vary depending on the boiler type.
Different types of fire-tube boilers
Fire-tube boilers are categorized based on several factors, the most important of which are:
1- Wetback or dryback design of reversal chamber rear plate
Fire-tube boilers are divided into two types: wetback and dryback. When the rear plate of the reversal chamber is in contact with water, it is referred to as wet back; when it is insulated, it is called dry back. Wet back boilers have a greater heat exchange surface compared to the other types, which increases heat transfer in the boiler and consequently enhances its efficiency. Additionally, this also reduces thermal stresses in the boiler.
2- Number of combustion gas passages
Fire-tube boilers are classified based on the number of passes through which the hot gases travel from the combustion chamber to the stack, passing through the water. The most common fire-tube boilers are those with two, three, or four passes. The explanations of two-pass, three-pass, and reverse flame boilers are provided below.
In two-pass, three-pass, or four-pass boilers, the flame of the burner is formed in the combustion chamber, transferring 40 to 50 percent of its energy to the water. The passage of flame through the combustion chamber is counted as one pass. After the hot combustion gases reach the end of the combustion chamber, they enter the reversal chamber and then proceed to the second pass. After the second pass in two-pass boilers, the combustion products exit the stack. However, the combustion gases carry a large amount of energy to the stack, resulting in high heat losses from the stack.
In three-pass boilers, after the combustion gases travel through the second pass, they enter another reversal chamber located at the front of the boiler. In this chamber, the combustion gases are directed toward the third pass, and after passing through the smoke tubes of the third pass, they enter the stack. Three-pass boilers have higher thermal efficiency compared to two-pass boilers due to their greater heat exchange surface.
In four-pass boilers, after the combustion gases travel through the third pass, they enter another reversal chamber located at the rear of the boiler. The gases are then redirected to the fourth pass and subsequently exit the stack. These boilers offer higher thermal efficiency than three-pass boilers due to their greater heat transfer surface and more optimized tube arrangement.
In reverse flame boilers, when the flame reaches the end of the combustion chamber, the combustion products are redirected due to the blockage at the front of the flow. The hot gases then enter the smoke tubes surrounding the combustion chamber and eventually exit the stack.
3- Number of furnaces or combustion chambers
According to the energy required by the boiler, the number of combustion chambers can be one or two. If the boiler has two combustion chambers (furnaces), it is referred to as a twin furnace boiler. These boilers have two combustion chambers, where the combustion products from the furnaces enter a reversal chamber before passing through the second and third passes. These boilers are equipped with a single stack.
4- Condensing and non-condensing boilers
In the combustion products of fuel and air, a small amount of water exists in the form of steam. If the heat transfer surface and the boiler structure are designed in such a way that the temperature of the combustion products is lower than the dew point temperature, the steam will condense, and liquid water will be produced. In this case, the boiler is classified as a condensing boiler. In this scenario, the latent heat of the steam is also used and transferred to the water. These boilers are only found in the hot water boiler type.
At sea level pressure, the dew point temperature is around 55°C, and if the temperature of the inlet water to the boiler is below 55°C, condensation can occur. The efficiency of these boilers, based on the higher heating value (HHV) of the fuel, is close to 98%.
5- Boiler classification based on the phase of the output fluid
Fire tube boilers are generally used for heating water. The output of the boiler can be in the form of hot water, saturated steam, or superheated steam. If the output is hot water, the boiler is referred to as a hot water boiler. If the output is either saturated steam or superheated steam, it is called a steam boiler.
6- Classification based on horizontal or vertical orientation
Depending on the boiler’s capacity and the layout in the boiler room, boilers can be installed either horizontally or vertically. Generally, boilers with lower capacities can be designed and manufactured in both horizontal and vertical orientations. In a vertical boiler, the burner is placed at the top, and the flame direction is from top to bottom.
The operation of a fire-tube boiler
Fire tube boilers operate based on a simple heat transfer principle, where heat from the hot gases produced by combustion is transferred to the water in the boiler vessel. The operating process of these boilers is explained as follows:
1- Combustion in the boiler
In the burner, fuel and air flow together, where a chemical reaction occurs between them, resulting in the formation of a flame. The combustion process generates a large volume of hot gases within the combustion chamber, which contains a significant amount of thermal energy. The burner must be designed and adjusted to prevent direct contact between the flame and the chamber walls. If the flame comes into contact with these walls, it can cause serious damage to the combustion chamber and increase the emission of pollutants. One of the key factors in improving boiler performance and reducing environmental pollutants is the use of smart burners.
Raadman intelligent burners, using advanced and up-to-date controllers, are capable of precisely regulating the combustion process. These systems not only enhance combustion efficiency but also significantly reduce the emission of harmful pollutants such as nitrogen oxides (NOx) and carbon monoxide (CO).
2- Passage of hot gases through fire tubes
The hot gases by combustion pass through a series of metal tubes called smoke tubes. These tubes are put inside a large water chamber. As the hot gases move through these tubes, they transfer their heat to the surrounding water.
3- Heat absorption by water
The water in the tank surrounds the fire tubes and gradually heats up by absorbing heat from them. As the water temperature increases, hot water or steam is produced.
4- Disposal of Combustion Gases
After the hot gases transfer their heat to the water, they are directed outside through the chimney. This system helps in safely disposing of the combustion gases. To increase the boiler’s efficiency, the combustion gases can be passed through an economizer and air preheater before being released into the atmosphere, allowing the maximum energy to be extracted from the combustion products.
5- Pressure and temperature control
To ensure the safe operation of the boiler, the pressure and temperature of the water or steam inside the boiler are constantly monitored by control systems. These systems can automatically regulate the fuel flow and control the temperature and pressure of the output water or steam.
Advantages of fire-tube boilers
Fire-tube boilers are commonly used in many industries due to their simple structure and reliable operation. These boilers have gained significant popularity over the years due to their specific features and numerous benefits. Some of the most important benefits of fire-tube boilers are as follows:
1- Simple design and reliable structure
Fire-tube boilers, due to their simple design, offer an easy construction and installation process. This simplicity also ensures that the systems are easier to repair and maintain, minimizing breakdowns resulting from technical complications.
2- Low maintenance costs
Fire-tube boilers require less maintenance and repair compared to some water-tube boilers. Their simple design generally ensures fewer malfunctions over time, which leads to lower repair costs.
3- High durability and long lifespan
Fire-tube boilers are typically constructed using durable and high-quality materials, which enhance their lifespan and durability. Consequently, these boilers can continue operating for an extended period.
4- Appropriate for medium pressures
Fire-tube boilers typically operate at medium and low pressures, making them well-suited for common industrial applications. Since many industries do not require high-pressure steam, these boilers are the most efficient option for steam generation in these sectors.
5- Occupying Less Space
Some types of fire-tube boilers, such as vertical tube boilers, have a compact design that requires less space. This feature makes them highly suitable for use in areas with limited space, such as ships and small buildings.
6- Flexibility in fuel usage
Fire-tube boilers are capable of operating with various types of gaseous and liquid fuels. This fuel flexibility allows users to choose the most suitable fuel type based on availability and cost.
The advantages mentioned above have made fire-tube boilers a primary choice for producing hot water or steam in many industrial, power plant, and commercial applications.
Disadvantages of fire-tube boilers
Fire-tube boilers have limitations such as lower capacity and pressure, lower efficiency, and higher pressure drop in the gas passage. These disadvantages are discussed in detail below.
1- Capacity limitation
Fire-tube boilers generally have lower capacity compared to water-tube boilers.
2- Pressure limitation
Due to the structure and operation of fire-tube boilers, they cannot be used for high-pressure applications.
3- Lower efficiency compared to water-tube boilers
Fire-tube boilers have a smaller heat transfer surface, leading to lower efficiency compared to water-tube boilers.
4- Relatively high pressure drop on the gas side
The pressure drop along the air path to the combustion products outlet in fire-tube boilers is higher compared to water-tube boilers. As a result, supplying combustion air requires a more powerful fan and higher energy consumption.
5- Slower response to changes in heat load
A fire-tube boiler, compared to a water-tube boiler with the same capacity, contains a larger volume of water. Therefore, it requires more time to adjust the output temperature and respond to changes in heat load.
Technical standards
In the field of fire-tube boilers, various national and international standards have been established, the most significant of which are:
- The European standard of EN 12953
- The British standard of BS 2790
- The national Standard of 22156
The Applications of fire-tube boilers
Fire-tube boilers are commonly used for lower capacities compared to water-tube boilers. The key applications of fire-tube boilers are:
- Central heating systems for buildings
- Supplying hot water or steam to industrial facilities such as those in the chemical, pharmaceutical, food, textile industries, and more
Fire-tube boilers; A reliable choice for industrial applications
A fire-tube boiler is a type of boiler that produces hot water or steam by burning fuel. These boilers can also be used to heat other fluids, such as oil. The name “fire-tube boiler” comes from the fact that the flame from the burner is contained within a tube-shaped chamber, and the combustion products pass through a series of tubes. Depending on the design, these boilers can operate with different fuels such as gas, liquid, or solid fuels, and they have various applications. The most important of these applications include central heating systems and providing hot water or steam for industrial units.
Packman boilers, using advanced technologies and optimized designs, are recognized as a reliable and efficient solution in different industries. Raadman smart burners, with their advanced and up-to-date controllers, not only enhance boiler efficiency but also help reduce energy consumption and protect the environment.
