Overview of Fire-Tube Boiler Types and Their Components

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.

What is a FireTube 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 these types of boilers, hot gases from combustion in the industrial burner pass through multiple pathways inside the smoke tubes, heating the surrounding water. Ultimately, hot water or steam is generated.

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.

8- Wet Back Rear Plate: This rear plate, depending on the boiler type, may either be in direct contact with water or insulated on the back side.

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.

This fire tube boiler diagram illustrates the internal structure and main components of a typical fire-tube boiler, including the combustion chamber, smoke tubes, reversal chamber, burner, and safety devices. The diagram clearly shows how hot combustion gases flow through the tubes while water surrounds them inside the boiler shell. This configuration enables efficient heat transfer from the flue gases to the water, resulting in reliable steam or hot water generation for industrial applications.

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

In this classification, fire-tube boilers are divided into two types—wetback and dryback—based on the design of the rear plate of the reversal chamber.

WetBack Fire-Tube Boiler: Here, the rear plate of the reversal chamber is directly exposed to the boiler water. This arrangement enhances heat transfer, minimizes thermal stress, and boosts boiler efficiency.

DryBack Fire-Tube Boiler: In this type, the rear plate is covered with refractory insulation and is not in direct contact with water. Although its efficiency is lower than that of the wet-back type, it offers easier maintenance and repair.

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.

• Two-Pass Fire-tube Boiler: In this design, the burner flame develops inside the combustion chamber, and the combustion gases flow through a second pass before exiting straight into the stack. Although structurally simple, this type tends to experience higher exhaust heat losses.
• Three-Pass Fire-Tube Boiler: In this design, the flue gases pass through the first and second passes, then enter the front reversal chamber and continue through the third pass before being discharged through the stack. Thanks to its greater heat exchange surface, this type offers better efficiency than the two-pass boiler and is widely used across industrial applications.
• Four-Pass Fire-Tube Boiler: In the four-pass configuration, the hot combustion gases, after completing the third flow path, are redirected through an additional reversal chamber into the fourth pass. Owing to the increased heat transfer surface and the more refined tube arrangement, this design provides higher efficiency than the three-pass boiler.
• Reverse Flame Fire-Tube Boiler: In this design, the burner flame travels to the back of the combustion chamber, where it is forced to reverse direction because the passage is closed. The flue gases subsequently pass through the surrounding smoke tubes and are vented via the stack. This arrangement ensures more complete combustion and achieves efficient operation in small- and medium-capacity boilers.

  

3- Number of Furnaces or Combustion Chambers

Depending on the required output capacity, Fire-tube boilers can be designed with one or two combustion chambers.

• Single Furnace Firetube Boiler: This design features one combustion chamber and is commonly employed in boilers with medium capacities.
• Twin Furnace Fire-tube Boiler: In the twin furnace configuration, the boiler contains two distinct combustion chambers. The flue gases from both chambers merge into a shared reversal chamber and then flow through the second and third passes to 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.

Non-Condensing Boilers: In conventional boilers, the temperature of the flue gases stays above the dew point, so no condensation occurs in the exhaust. Their efficiency is lower compared to condensing boilers.

Condensing Fire-Tube Boiler: In condensing boilers, the exhaust gas temperature falls below the dew point, allowing the water vapor to condense and transfer its latent heat to the water. This can raise the efficiency of the boiler to around 98% (based on HHV). These boilers are only available in hot water configurations.

For further details about these boilers, refer to the article on Condensing Boiler Introduction.

5- Boiler Classification Based on the Phase of the Output Fluid

Fire-tube boilers are classified based on the type of outlet fluid into hot water boilers or steam boilers (saturated and superheated).

Hot Water Fire-Tube Boiler: This boiler delivers hot water at a controlled temperature, commonly employed in central heating and various industrial processes.

Saturated Steam Fire-Tube Boiler: This boiler generates saturated steam, widely used in various industrial sectors and thermal processes.

Superheated Steam Fire-Tube Boiler: The boiler produces steam that, beyond saturation, is further heated to become dry, superheated steam, suitable for specific industrial applications and turbines.

6- Classification based on horizontal or vertical orientation

The installation orientation of boilers in a boiler room can be either horizontal or vertical, depending on the required capacity and available space.

Horizontal Fire-Tube Boiler: The horizontal firetube boiler is the most common installation type in industry. In this configuration, the boiler is positioned horizontally, and the burner is mounted at the back.

Vertical Fire-Tube Boiler: This type is suitable for low-capacity systems and limited spaces. In this configuration, the burner is installed at the top, and the flame travels from top to bottom.

Working Mechanism and 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 stack. 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.

The Importance of Economizers in Improving Fire-Tube Boilers Performance

Economizers are essential components for enhancing the thermal efficiency of boilers, significantly contributing to energy savings and operational cost reduction. These heat exchangers operate with hot exhaust gases flowing on one side and water flowing on the other, helping to enhance overall system efficiency by recovering heat from the exhaust gases, which would normally be wasted through the stack.

Economizer Operation

These systems use the heat from the exhaust combustion gases to preheat the water entering the boiler. This process leads to a reduction in the energy needed to heat the water. Consequently:

• Fuel consumption is reduced as the economizer provides part of the necessary heat to preheat the water before it enters the boiler.
• The temperature of the exhaust gases drops, which reduces heat losses and enhances overall efficiency.

Advantages of Using an Economizer

• Improved Thermal Efficiency: Economizers can enhance the efficiency of the boiler by 5 to 10 percent.
• Cost Savings: Reducing fuel consumption directly decreases operational costs.
• Decreased Greenhouse Gas Emissions: Optimizing fuel consumption leads to a reduction in greenhouse gases, such as carbon dioxide.

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

Guide for Selecting a Fire-Tube Boiler

Choosing the right fire-tube boiler directly affects system efficiency, the longevity of equipment, and operating expenses. Depending on the industrial application, required capacity, fuel accessibility, and other operational parameters, boilers must be carefully evaluated.

Thermal Capacity: The boiler should be able to produce steam or hot water that matches the needs of your process or heating system. If the capacity is too low, the system will not perform adequately; if it is too high, energy will be wasted and investment costs will rise. Accurate estimation of the thermal load of the building or industrial process is key to determining the correct boiler capacity.

Fuel Type: The selected fuel, whether natural gas, oil, or heavy oil, affects the burner design, combustion chamber layout, and safety considerations of the boiler. A boiler capable of burning multiple fuel types can be a more economical option in the long term and reduces dependency on a single fuel source.

Operating Pressure and Temperature: Fire-tube boilers are typically designed for low to medium pressure applications. As such, the steam or hot water pressure and temperature must be chosen to meet the needs of the system.

Thermal Efficiency: Efficiency is one of the most important criteria when selecting a boiler. Higher efficiency means lower fuel use and reduced operating costs. Three-pass, four-pass, and condensing boilers generally achieve better efficiency than two-pass models. Accurate evaluation of the boiler’s performance under real operational conditions is essential.

Water Quality & Feed System: The quality of feedwater is a crucial factor in fire-tube boiler operation. Elevated hardness, dissolved salts, or impurities can result in scale deposition on heat exchange surfaces and tube corrosion, thereby diminishing efficiency and increasing maintenance requirements. Hence, when choosing a boiler, it is important to consider access to water treatment and softening systems to guarantee reliable long-term operation.

Available Space & Layout: The size and placement of the boiler should fit the boiler room or installation area. While fire-tube boilers are usually installed horizontally, vertical designs exist for smaller-capacity units. Ensuring adequate access for maintenance and servicing is very important.

Maintenance & Spare Parts Availability: Choosing a boiler with easily accessible spare parts and straightforward periodic maintenance helps minimize system downtime and maintenance expenses. The simpler design of firetube boilers compared to water-tube boilers is a significant advantage in this regard.

Standards & Safety Requirements: The chosen boiler must adhere to established national and international standards. critical components, including safety valves, pressure and temperature sensors, and advanced control systems, are pivotal in ensuring operational safety and mitigating potential risks.

 Fire-Tube Boiler; Technical Tips, Installation, and Maintenance

For reliable and long-term operation of a firetube boiler, adhering to a few simple yet critical guidelines is necessary.

• Choosing an Appropriate Location: The boiler must be placed in a dry area with proper ventilation, ensuring convenient access for servicing and repairs.
• Inspection of Flue and Gas Paths: Stack and gas passages must be regularly kept clean and unobstructed to ensure the boiler operates efficiently.

• Water Quality: Using water with controlled hardness and impurities helps avoid scale buildup and corrosion.
• Safety System: Prior to starting the boiler, ensure that safety valves, pressure meters, and control sensors are functioning correctly.
• Periodic Inspection: Routine inspections of the tubes, combustion chamber, and connections are essential to prevent leakage or thermal cracks.

Suitable Burner for Fire-Tube Boilers

Choosing the right burner is crucial for the stable performance and thermal efficiency of a fire-tube boiler. The burner must be properly matched to the boiler’s thermal capacity, ensure optimal fuel combustion, and produce a uniform flame to maintain a consistent combustion chamber temperature and reduce thermal stresses. Additionally, easy access for servicing and cleaning, along with safety systems for flame and pressure control, are essential requirements in selecting a burner for fire-tube boilers.

As mentioned, selecting the appropriate burner directly affects boiler efficiency. Due to their cost-effectiveness and low emissions, gas burners have become widely used in fire-tube boilers today

Frequently Asked Questions about Fire-Tube Boilers

Question: What is a fire-tube boiler and how does it work?

A fire-tube boiler is a type of boiler in which hot combustion gases pass through metal tubes, heating the surrounding water or steam. The heat from the gases warms the water, allowing the boiler to generate hot water or steam efficiently.

Question: Can a fire-tube boiler operate under high pressure?

Fire-tube boilers are typically built for medium-pressure use. At higher pressures, the risk of thermal stress, tube cracking, and leakage increases. Therefore, for high-pressure operations, water-tube boilers are generally preferred.

Question: What is a multi-pass fire-tube boiler?

The number of passes refers to the path that hot gases take as they flow through the tubes. Increasing the number of passes improves heat transfer and boosts the boiler’s overall efficiency. Two-pass, three-pass, and four-pass firetube boilers are the most common types.

Question: What are the recommended service intervals for a fire-tube boiler?

Fire-tube boilers should be inspected and serviced at least twice a year. The service involves examining the flue passages, tubes, combustion chamber, safety valves, and control systems.

 Fire-Tube boilers; A reliable choice for industrial applications

Fire-tube boilers, depending on their design, can run on different types of fuel—gas, liquid, or solid—and are used in various applications, with the main ones being central heating systems and providing hot water or steam for industrial facilities.

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.