In most modern industrial sectors, steam is an indispensable source of thermal energy. It plays a crucial role in various processes such as heating, sterilization, drying, distillation, and serving as an indirect heat transfer medium. Industries like textiles, food processing, chemicals, pulp and paper, building materials, and renewable energy all require a reliable supply of steam at appropriate pressure and temperature levels.
At the heart of this thermal energy supply chain is the boiler – the key equipment that converts chemical energy from fuel into thermal energy in the form of steam. The performance and reliability of the boiler directly impact product quality, production efficiency, operating costs, and overall industrial safety.
Among various types of boilers used in practice, the fire-tube boiler stands out as one of the most commonly used configurations in small to medium-sized industrial facilities. This boiler type is not only simple in structure but also easy to operate and maintain. It effectively meets the demand for steam at medium pressures and temperatures. Notably, thanks to its stable operation and compatibility with various fuel types, the fire-tube boiler has become the preferred choice across many industries in Vietnam and the region.
1. What is a Fire-Tube Boiler? Basic operating principle
Technically, a fire-tube boiler is a type of boiler where hot combustion gases pass through tubes that are surrounded by water inside a horizontal cylindrical shell. The basic structure consists of a horizontally mounted pressure vessel filled with water, with multiple fire tubes running through it. Hot gases generated from fuel combustion flow through these tubes, transferring heat through the tube walls into the surrounding water. This is in contrast to water-tube boilers, where water flows inside the tubes and is heated from the outside.
Operating principle:
The operation of a fire-tube boiler is based on forced convective heat transfer from the combustion gases to the water:
- Combustion occurs in the primary furnace chamber located at the front end of the boiler. Fuel is introduced into the chamber and ignited either by a burner (for liquid/gaseous fuels) or mechanical stoker (for solid fuels).
- The resulting hot gases, typically ranging from 800°C to 1200°C, travel through the fire tubes arranged parallel inside the boiler shell. Some designs utilize two or three-pass tube arrangements to maximize heat exchange time.
- As the hot gases flow through these tubes, heat is transferred through carbon steel or alloy steel tube walls to the surrounding water, causing it to boil and generate steam.
- Steam accumulates at the top of the shell and is routed through steam valves to the end-use process.
- After releasing most of their heat energy, the flue gases are discharged through the stack, possibly passing through an economizer or heat recovery system to maximize thermal efficiency.
The primary modes of heat transfer in a fire-tube boiler are radiant heat transfer in the furnace and convective heat transfer in the fire tubes. These characteristics contribute to the boiler’s relatively stable thermal efficiency, typically ranging from 75% to 85% under standard designs, and potentially higher when integrated with waste heat recovery systems.
2. Detailed construction of a Fire-tube Boiler
While the structure of a fire-tube boiler is relatively simple, it requires high precision in design, fabrication, and installation to ensure pressure integrity and optimal heat exchange efficiency. Below are the main components of a standard fire-tube boiler system commonly used in industrial settings today.
2.1. Shell
The boiler shell is a horizontally oriented cylindrical vessel, typically fabricated from pressure-grade steel plates—commonly ASTM A516 Gr.70 or equivalent. Circumferential and longitudinal welds are performed using the Submerged Arc Welding (SAW) process for structural integrity.
The shell thickness is calculated based on ASME BPVC or Vietnamese Standard TCVN 7704, taking into account design pressure, internal diameter, and safety factors.
The interior of the shell is filled with water, while the upper section acts as a steam space.
2.2. Furnace (or Combustion chamber)
The furnace is where fuel combustion occurs. In fire-tube boiler designs, the furnace is usually located at the front end of the shell and directly connected to the fire tube system.
It is typically constructed from heat-resistant or alloy steel to withstand high temperatures.
Depending on the configuration, the furnace may be of the wet-back or dry-back type:
Wet-back: The rear end of the furnace is surrounded by water, improving thermal efficiency and reducing damage due to radiant heat.
Dry-back: The rear end is lined with refractory materials. While easier to maintain, it typically offers lower thermal efficiency.
2.3. Fire tubes
These tubes are the primary heat transfer elements. They typically range in diameter from 51 mm to 102 mm and are made from seamless carbon steel tubes (ASTM A179, A192, or equivalent).
Fire-tube boilers are commonly designed with either two-pass or three-pass configurations to enhance heat exchange performance.
The spacing and layout of the tubes must comply with design codes to ensure adequate combustion gas flow and minimize pressure drops.
2.4. Steam chamber
Located in the upper section of the boiler shell, the steam chamber collects the saturated steam generated during operation. Pressure within the steam space is regulated by various safety and control devices.
It is equipped with:
- Water level gauges
- Safety valves
- Blowdown valves
- Steam outlet valves and pressure regulators
The size of the steam chamber directly influences steam quality – larger chambers produce drier steam.
2.5. Feed water system
The feed water supplied to the boiler must be softened and deaerated to prevent scaling and corrosion. The system typically consists of:
- A feed water pump (usually a multistage centrifugal type), selected based on required flow rate and head
- Check valves, gate valves, and flow meters
Depending on steam quality requirements, the system may also include a deaerator or chemical dosing unit.
2.6. Burner
The burner is the component responsible for injecting and igniting the fuel (such as DO, FO, CNG, LPG, or biomass). Most modern systems employ automatic burners equipped with:
- Electronic ignition systems
- Air-fuel ratio control for efficient combustion
- Well-known burner brands include Riello (Italy), Weishaupt (Germany), and Powerflame (USA), selected based on boiler capacity and installation conditions.
Design parameters such as fuel inlet pressure, heat output, and furnace volume must be harmonized to ensure compatibility.
2.7. Control system and safety devices
To ensure safe and automated operation, fire-tube boilers are equipped with:
- Pressure transmitters
- Multi-level water level sensors (Low, High, and Very Low cut-off)
- Safety valves (as required by TCVN 7704, capable of discharging the full rated steam capacity)
- A logic controller (either PLC-based or relay-based), managing burner ignition, feed water regulation, and alarm systems
Modern systems often include an HMI (Human-Machine Interface) panel to display operating parameters such as pressure, temperature, water level, and burner output in real time. For advanced applications, integration with SCADA systems allows for remote monitoring, operational logging, and real-time performance analysis.
3. Applications of Fire-Tube Boilers in industrial production
Thanks to their compact design, simple operation, and high thermal efficiency within medium capacity ranges (typically 0.5 to 20 tons of steam per hour), fire-tube boilers are widely used across various industries requiring saturated steam at pressures ranging from 6 to 16 bar. Below are key sectors where fire-tube boilers are commonly applied, along with specific roles in each production process.
3.1. Textile industry
In the textile sector, steam plays a critical role in multiple fabric treatment processes. Fire-tube boilers are typically installed in dyeing, finishing, and industrial garment workshops.
Typical applications:
- Fabric dyeing: Steam supplies dyeing machines at stable temperatures (around 130°C – 140°C), ensuring even dye penetration.
- Drying: Roller dryers or hot air dryers use steam to generate dry, hot airflow, preventing shrinkage or deformation of fabrics.
- Ironing and finishing: Heat presses and industrial ironing machines utilize pressurized steam to flatten and shape garments after sewing.
With continuous operation demands, fire-tube boilers help maintain stable steam pressure and flow, minimizing thermal fluctuations that may affect fabric quality.
3.2. Food and Beverage industry
In this industry, steam is used for both heating and sterilization, which requires a clean, impurity-free steam supply system.
Typical applications:
- Cooking: Jacketed kettles and stirred cooking tanks use saturated steam to heat ingredients like milk, fruit juice, soy sauce, etc.
- Sterilization: Steam is delivered to autoclaves for killing microorganisms in canned goods, glass bottles, and food packaging.
- Concentration and evaporation: Steam powers multi-effect evaporators to reduce water content without altering product flavor or aroma.
Thanks to their quick heat-up time and flexible operation, fire-tube boilers are well-suited for small and medium-scale food processing lines.
3.3. Chemical industry
In the chemical sector, many reactions and extraction processes require a continuous and stable heat source to maintain optimal temperature and pressure.
Typical applications:
- Reaction heating: Steam is supplied to jacketed reactors to maintain temperatures between 100°C – 160°C, depending on the specific reaction.
- Distillation: Steam provides heat to distillation columns, enabling the separation of components with different boiling points.
- Liquefaction and drying: Certain chemical processes require steam for liquefying or drying raw materials or final products.
While not commonly used in large-scale chemical plants (which often require higher-pressure systems), fire-tube boilers are popular in medium-sized chemical workshops.
3.4. Pulp and paper industry
Paper production from wood pulp requires a large, steady supply of steam, primarily for cooking, pressing, and drying processes.
Typical applications:
- Pulp cooking: Steam is used in alkaline or acid digesters to break down cellulose fibers from wood.
- Paper drying: Dryer rolls in paper production lines are steam-heated to evaporate moisture from wet pulp, improving paper strength and whiteness.
Fire-tube boilers, with their medium capacity range, are ideal for local or mid-sized paper manufacturing facilities.
3.5. Wood processing industry
Steam plays an essential role in controlling wood moisture content and shaping processes.
Typical applications:
- Wood drying: Industrial kilns use steam to create high-temperature environments, controlling moisture content to reduce cracking, warping, and pest infestation.
- Glue pressing and wood bending: Wood is softened with steam to facilitate molding into desired shapes or hot-press bonding with heat-resistant adhesives.
Thanks to their high combustion efficiency and low operating cost, fire-tube boilers are ideal for furniture production facilities, plywood factories, and timber processing plants.
3.6. Other industrial applications
Beyond the industries mentioned above, fire-tube boilers are also utilized in various other manufacturing sectors, such as:
- Industrial laundry: Supplying steam to roller irons, ironing tables, and starch cooking vessels.
- Plastics and rubber production: Using steam to heat molds or soften raw materials.
- Mechanical and metalworking industries: Supplying steam to paint booths, drying chambers, and heat treatment processes.
In summary, due to their optimized structure and reliable performance, fire-tube boilers are a suitable choice for industries requiring thermal energy at moderate pressure and temperature levels—especially those that prioritize stable steam output, fast startup, and ease of maintenance.
4. Key factors to consider when selecting a Fire-Tube Boiler
Choosing an appropriate boiler system not only determines production efficiency but also directly impacts economic performance, operational safety, and long-term system stability. For fire-tube boilers—commonly used in industries requiring medium-pressure steam—particular attention should be paid to the following factors:
4.1. Required steam capacity (Steam flow rate per hour)
Boiler capacity is defined by the steam flow rate (tons/hour or kg/hour) needed to meet the total demand of the production line under continuous operation.
Key considerations include:
- Total steam consumption at all usage points within the facility.
- Allowances for load fluctuations or simultaneous startup of multiple devices.
- Steam loss rate during distribution (typically 5% – 10%).
Undersized boilers may lead to steam shortages and reduced equipment performance, while oversized systems result in unnecessary investment and operational costs.
4.2. Steam output pressure and temperature
Fire-tube boilers are typically suitable for operating pressures ranging from 6 – 16 bar, with corresponding saturated steam temperatures between 158°C – 201°C. Requirements exceeding these limits may call for switching to water-tube boilers.
Before selection, determine:
- Required steam pressure for each application (e.g., dryers, cookers, presses, etc.).
- Stability of pressure under simultaneous steam demand from multiple points.
- Whether superheated steam is needed (fire-tube boilers are not ideal for superheating).
- Incorrect pressure specifications can reduce heat transfer efficiency, damage equipment, or pose safety risks.
4.3. Fuel type and Fuel cost
Fire-tube boilers offer flexibility in fuel use, including:
- Diesel Oil (DO)/Fuel Oil (FO)
- Natural Gas (CNG/LNG) or Liquefied Petroleum Gas (LPG)
- Biomass (sawdust, rice husks, cashew shells, etc.)
- Coal (less common due to environmental concerns)
Fuel cost and supply availability at the installation site can account for over 50% of monthly operational costs. Fuel selection should be based on:
- Average local fuel prices.
- Supply stability.
- Combustion chamber and flue gas treatment system requirements.
4.4. Boiler efficiency
Boiler efficiency reflects the ability to convert fuel energy into usable steam.
Fire-tube boilers using oil or gas typically achieve efficiencies > 85%.
Biomass-fueled systems range from 75% – 82%, depending on fuel quality and furnace design.
Select boilers with optimized heat transfer design, waste heat recovery systems (economizers), and automated control systems to maximize efficiency. A 1% increase in efficiency can translate into millions of VND saved monthly in fuel costs.
4.5. Installation space and plant layout
Fire-tube boilers are compact and suitable for limited-space installations. However, proper layout is essential to:
- Maintain safe distances as required by TCVN 7704:2007 and QCVN 01:2008/BLĐTBXH.
- Facilitate fuel supply, water feeding, blowdown, and maintenance operations.
- Ensure effective ventilation, fuel gas supply, and exhaust systems.
The layout should clearly indicate the location of the boiler, feedwater tanks, steam piping, electrical systems, and control panels.
4.6. Initial investment and operating/maintenance costs
Total costs to consider include:
- Boiler equipment and accessories (pumps, valves, control panels, etc.).
- Infrastructure costs: foundations, steam piping, chimneys, boiler house.
- Periodic maintenance: fire-tube cleaning, descaling, technical inspections.
- Lifetime fuel and feedwater costs.
Some businesses opt for boiler leasing models to reduce upfront investment, but the reliability of the service provider must be thoroughly assessed.
4.7. Safety standards and Environmental requirements
Boilers are subject to stringent safety regulations due to the risks of explosion and high-pressure accidents. Compliance with the following is essential:
- TCVN 7704:2007, TCVN 8366:2010, and QCVN 01:2008/BLĐTBXH standards.
- Initial and periodic inspections by certified third parties as per Circular 36/2019/TT-BLĐTBXH.
- Emission standards for gases, dust, NOx, SOx (especially when using solid fuels or FO).
Consider integrating flue gas treatment systems, waste heat recovery, and baghouse filters to meet environmental protection regulations and avoid penalties.
4.8. Supplier reputation and After-sales support
Beyond technical specifications, a sound decision must also be based on:
- Supplier’s track record and previous project experience.
- 24/7 technical support capabilities, genuine spare part availability, and operator training.
- Warranty terms, scheduled maintenance policies, and system improvement consultation.
Unreliable suppliers can lead to production disruptions and unforeseen costs. Prebecc provides turnkey design, fabrication, and installation services for fire-tube boilers, helping enterprises optimize investment while ensuring compliance with strict safety and environmental standards.
Selecting a fire-tube boiler is a multi-dimensional decision that requires a comprehensive assessment of production needs, operational conditions, and the company’s management capacity. A properly designed and selected boiler system not only meets steam demand but also becomes a key contributor to improved productivity, cost savings, and risk mitigation.