Fire-tube steam boilers are classic steam generating equipment in the industrial field. Hot flue gas generated by fuel combustion flows inside the tubes, which are surrounded by water. Heat transfer through the tube walls causes the water to boil and produce steam. Their history dates back to the Industrial Revolution in the 19th century, when they were widely used in factories and railway locomotives. Although newer equipment such as water-tube boilers have advantages in high-pressure, large-capacity scenarios, fire-tube steam boilers are still widely used in industrial scenarios with moderate steam demand due to their simple structure, convenient operation, and low cost.
In industries such as food processing, textiles, and chemicals, which rely on a stable steam supply, fire-tube steam boilers are indispensable. They can meet the needs of cooking, fiber setting, and heating reactions, ensuring continuous industrial production.
From a professional definition, a fire-tube steam boiler is a boiler device that uses hot flue gas flowing inside tubes and water circulating around them to achieve heat transfer, thereby heating the water to produce steam. During operation, fuel burns in the furnace to produce high-temperature flue gas. This flue gas enters the fire tubes and flows along the pipes. During this process, the heat of the flue gas is transferred to the water outside the tubes through the fire tube walls, gradually increasing the water temperature and eventually converting it into steam.
The working principle of a fire-tube steam boiler is a complex process involving combustion, heat transfer, steam generation, and circulation. Each stage is interconnected and collectively determines the boiler's operating efficiency and steam output.
In the combustion stage, fire-tube steam boilers can use various fuels, such as coal, heavy oil, natural gas, or biomass. The combustion methods of different fuels differ slightly, but the overall process is similar: fuel is delivered into the furnace, thoroughly mixed with air preheated by an air preheater, and ignited by an ignition device, resulting in vigorous combustion. The combustion process releases a large amount of heat, causing the furnace temperature to rise rapidly, forming high-temperature flue gas, typically reaching 800-1200℃.
The generated high-temperature flue gas flows along a specific path, first entering the fire tubes from the furnace. Since the fire tubes are evenly distributed within the water inside the boiler shell, the flue gas exchanges heat thoroughly with the water outside the tubes as it flows. Subsequently, the flue gas flows out from the other end of the fire tubes and enters the smoke box, which collects the flue gas. Finally, the flue gas is discharged outside the boiler through the chimney. Throughout the flue gas flow, heat transfer continues, with the high-temperature flue gas transferring heat to the water layer outside the tubes through the fire tube walls.
The heat transfer process mainly relies on two modes: conduction and convection. Conduction occurs inside the fire tube walls, where the high-temperature flue gas transfers heat to the inner wall of the fire tubes, and then to the outer wall through conduction. Convection occurs between the outer wall of the fire tubes and the water, where heat from the outer wall is transferred to the surrounding water through convection, causing the water temperature to rise continuously. When the water temperature reaches the boiling point at the corresponding pressure, localized boiling begins. The surface heat transfer coefficient of the tubes and the flow velocities of flue gas and water significantly influence heat transfer efficiency. A higher surface heat transfer coefficient indicates stronger heat transfer between the tube walls and the water. Appropriately increasing the flow velocities of flue gas and water enhances convective heat transfer, accelerates heat transfer, and thus improves boiler thermal efficiency.
As heat is continuously transferred, the water inside the boiler shell gradually absorbs heat, and its temperature rises. When the water temperature reaches the saturation temperature at that pressure, the water begins to vaporize, gradually transforming into saturated steam. If the boiler is designed with a superheater, the saturated steam will further enter the superheater, absorb more heat, and rise above the saturation temperature, transforming into superheated steam. If there is no superheater, saturated steam is generated directly.
The generated steam has a relatively low density and accumulates at the top of the boiler shell, forming a steam space. As steam is continuously generated, the steam pressure within the steam space gradually increases. When the pressure reaches a set value, the steam is transported through the steam outlet pipe to the steam-using equipment, providing steam for industrial production or other applications.
During boiler operation, a natural circulation phenomenon occurs within the boiler, driven by the density difference between water and steam. Water near the fire tubes absorbs more heat, reaches a higher temperature, and partially vaporizes to form steam, which, being less dense, rises. Water further away from the fire tubes, being relatively cooler and denser, flows downwards, replenishing the area near the fire tubes, creating a continuous natural circulation. This natural circulation helps to achieve a more uniform water temperature distribution within the boiler shell, ensuring stable steam generation.
Simultaneously, the pressure within the boiler is constantly built up and maintained, with safety valves and steam inlets playing crucial roles. Safety valves are important safety protection devices for the boiler. When the steam pressure inside the boiler exceeds a set safety value, the safety valve automatically opens, releasing some steam to reduce the pressure inside the boiler and prevent an explosion due to overpressure. When the pressure drops below the safety value, the safety valve automatically closes, ensuring normal boiler operation. Steam inlets are responsible for delivering the generated steam to the steam-consuming equipment. They also control the steam output by adjusting the opening of the steam outlet valve to meet the needs of different steam-consuming equipment and maintain stable pressure inside the boiler.
The thermal efficiency of fire-tube steam boilers is affected by factors such as combustion quality, heat loss, feedwater quality, and maintenance. In rewriting, the core points are retained, descriptive statements are simplified, and similar influencing factors are integrated, such as combining the description of boiler shell heat dissipation and flue gas heat loss, making the content more concise and compact.
The thermal efficiency of fire-tube steam boilers is constrained by multiple factors, directly affecting energy consumption and operating costs. Combustion quality is crucial; the calorific value of the fuel determines the heat release capacity for the same consumption, while the precision of combustion control determines whether the fuel can burn completely. Incomplete combustion not only wastes fuel and reduces thermal efficiency but also increases pollutant emissions. Poor boiler shell insulation and flue gas heat loss cannot be ignored. High-temperature flue gas carries heat out, and the boiler shell dissipates heat to the environment. Good insulation and waste heat recovery can effectively reduce heat loss.
Poor feedwater quality leads to scale formation on the fire tubes and the inner wall of the boiler shell. Scale has poor thermal conductivity, hindering heat transfer, resulting in increased flue gas temperature, increased heat loss, and potentially damage to the fire tubes. In addition, the carbon deposits and soot accumulated on the inner walls of the fire tubes also affect heat transfer. Therefore, strengthening the insulation of the boiler shell and pipes, regularly cleaning the fire tubes and smoke box, optimizing the combustion control system, and rationally adjusting the air-fuel ratio to ensure complete combustion of fuel are key measures to improve thermal efficiency and reduce energy consumption.
A fire-tube steam boiler consists of several key components, each with a specific function, working together to ensure the boiler's safe, stable, and efficient operation.
The boiler shell is the core pressure-bearing component. As the main container for water and steam, it needs to withstand the pressure of the internal water and steam. Therefore, it is usually made of high-strength low-carbon steel or alloy steel and undergoes rigorous welding and flaw detection to ensure its structural strength and sealing. The size and shape of the boiler shell are designed according to the boiler's capacity and pressure requirements. The internal space is divided into a water space and a steam space. The water space is used to store water, and the steam space is used to collect the generated steam.
The furnace is the important space for fuel combustion and heat generation. Its structural design directly affects the fuel combustion efficiency. The furnace is usually cylindrical or square in shape, and contains a grate (for solid fuels) or a burner (for liquid or gaseous fuels). The fuel mixes and burns with air on the grate or burner. The inner wall of the furnace is usually lined with refractory materials to withstand the high-temperature combustion environment, reduce heat loss, and protect the boiler shell from direct high-temperature erosion.
Fire tubes are crucial conduits that carry high-temperature flue gas and transfer heat outwards. They are typically made of seamless steel pipes, possessing excellent heat resistance and thermal conductivity. The number and diameter of the fire tubes are determined based on the boiler's heat load and heat transfer requirements. Multiple fire tubes are evenly distributed within the water inside the boiler shell, connecting one end to the furnace and the other to the smoke box. As the high-temperature flue gas flows through the fire tubes, it transfers heat to the water outside the tubes through the tube walls, making it the core component for heat transfer.
The smoke box, together with the chimney, guides and exhausts the combustion gases. Located at the outlet end of the fire tubes, the smoke box collects the flue gas flowing from the fire tubes, ensuring it enters the chimney evenly. The smoke box typically incorporates baffles and other structures to alter the flow direction of the flue gas, prolonging its residence time and further improving heat recovery efficiency. The chimney is installed at the top of the smoke box, using its height to generate draft, smoothly expelling the flue gas outside the boiler while preventing backflow.
Auxiliary devices are indispensable components for ensuring the safe and stable operation of a boiler, including safety valves, pressure gauges, water level gauges, feedwater pumps, and blowdown valves. Safety valves automatically open to release pressure when the pressure inside the boiler exceeds a safe value, preventing overpressure explosion and ensuring safe boiler operation. Pressure gauges monitor the steam pressure inside the boiler in real time, allowing operators to adjust boiler operating parameters based on the readings. Water level gauges monitor the water level inside the boiler shell, preventing dry burning due to excessively low water levels or affecting steam quality due to excessively high water levels. Feedwater pumps deliver treated softened water to the boiler shell to replenish water consumed during operation and maintain a normal water level. Blowdown valves periodically remove scale, sludge, and other impurities from the bottom of the boiler shell, preventing their accumulation from affecting heat transfer efficiency and boiler safety.
These components work together to form the complete system of a fire-tube steam boiler. The normal operation of each component is crucial to the overall performance and safety of the boiler; a malfunction in any component can affect the normal operation of the boiler and even lead to a safety accident.
Based on different standards such as structure and application, fire-tube boilers can be divided into various types. Different types of boilers differ in structural characteristics and applicable scenarios, meeting diverse industrial needs.
Horizontal fire-tube boilers are one of the most widely used types. Their boiler shell is placed horizontally, with the fire tubes arranged horizontally inside the shell. This structural design makes the boiler's footprint relatively reasonable and facilitates cleaning and maintenance by operators. Horizontal fire-tube boilers have a large heat transfer area and high thermal efficiency, capable of stably generating medium-pressure steam. They are suitable for process steam in small and medium-sized factories, building heating, and small power plants, and are widely used in industries such as food processing, textiles, and chemicals.
Vertical fire-tube boilers have a vertically placed boiler shell, with the fire tubes also distributed vertically. The overall structure is compact, with a small footprint, but relatively high. Due to their small size, they are suitable for installation in space-constrained environments, such as small workshops, ships, and mobile equipment. However, vertical fire-tube boilers have relatively small water capacity and steam output, and are generally suitable for scenarios with low steam demand, such as heating in small workshops and steam supply for small equipment.
Cornish and Lancashire boilers are early classic fire-tube boiler designs originating in England. A Cornish boiler typically has only one large-diameter fire tube, with the furnace located at one end. High-temperature flue gas flows inside the large tube, transferring heat to the water outside the tube through the tube wall. A Lancashire boiler, on the other hand, has two large fire tubes, a more complex structure, a larger heat transfer area, and a higher steam output. These two boiler designs are relatively traditional and played an important role in early industrial production. They are still used in small quantities in some applications with high requirements for equipment retrofitting or in specific small-scale industrial settings.
Locomotive boilers are fire-tube boilers specifically designed for mobile equipment and rapid steam supply needs, mainly used in mobile steam equipment such as railway locomotives and steam locomotives. These boilers have a compact structure, with a specially optimized arrangement of the boiler shell and fire tubes, enabling stable combustion and rapid steam generation during movement to meet the power requirements of the locomotive. Locomotive boilers are typically manufactured using high-strength materials, capable of withstanding vibrations and impacts, and adaptable to the working environment of mobile equipment.
When selecting a fire-tube boiler type, multiple factors need to be considered, such as floor space, steam output, maintenance difficulty, and space constraints of the application scenario. For small and medium-sized factories with high steam demand and relatively ample space, horizontal fire-tube boilers are the ideal choice; if space is limited and steam demand is low, vertical fire-tube boilers are more suitable; and for mobile equipment or scenarios with specific retro requirements, locomotive boilers or Conesch and Lancashire boilers are better suited to meet the requirements.
Fire-tube steam boilers, with their stable steam supply capacity and moderate pressure and output characteristics, are widely used in various industries and occasions, providing important support for industrial production and people's lives.
In the manufacturing sector, fire-tube steam boilers are indispensable equipment in many production processes. In the food processing industry, steam is used for processes such as cooking, sterilization, and drying, including cooking in grain processing, sterilization in meat processing, and drying in biscuit production. A stable steam supply ensures food processing quality and production efficiency. In the textile industry, steam is used for fiber setting and drying after dyeing. By controlling the temperature and pressure of the steam, textile products can have a better feel and appearance. In the chemical industry, steam is used as a heating medium for heating chemical reaction vessels and preheating raw materials, providing the necessary temperature conditions for the smooth progress of chemical reactions.
In power supply, fire-tube steam boilers can be used in small power plants or as a steam source for heat tracing. For some remote areas or small industrial parks, building large power plants is too costly. Small power plants using fire-tube steam boilers generate steam to drive turbines and generate electricity, meeting part of the local electricity demand. Simultaneously, in large power plants or other industrial facilities, fire-tube steam boilers can also be used as heat tracing equipment to provide steam heat tracing for pipelines and equipment, preventing the medium from solidifying or freezing at low temperatures.
Building heating and district heating supply are also important application scenarios for fire-tube steam boilers. In some cold regions, heating is required for buildings during winter. Low- to medium-pressure steam generated by fire-tube steam boilers is transported through thermal pipelines to radiators in various buildings, transferring heat to the indoor air and achieving building heating. District heating involves centrally located fire-tube steam boilers at heating stations, supplying steam to multiple surrounding buildings or areas through a pipeline network, providing thermal support for residential life and industrial production. This centralized heating method has advantages such as high efficiency and low pollution.
The medical and pharmaceutical industries have extremely high requirements for the quality and stability of steam. Fire-tube steam boilers can meet these requirements and are widely used in steam sterilization and cleaning processes. In hospitals, steam is used for the sterilization and disinfection of medical devices, such as surgical instruments and syringes, ensuring the hygiene and safety of medical devices. In the pharmaceutical industry, steam is used for sterilizing and cleaning equipment and pipelines in the drug production process, preventing drug contamination and ensuring that drug quality meets standards.
Overall, fire-tube boilers have significant advantages and good adaptability in applications requiring stable, medium-pressure steam. They can provide a reliable steam supply according to the needs of different industries, promoting the stable development of various industries.
The long-term widespread use of fire-tube steam boilers in the industrial field is inseparable from their numerous advantages, making them highly competitive in specific scenarios.
Firstly, fire-tube steam boilers have a relatively simple structure, mainly composed of a boiler shell, furnace, fire tubes, and some auxiliary devices. The number of parts is relatively small, and the manufacturing process is relatively simple, resulting in lower manufacturing difficulty and cost. At the same time, due to its simple structure, the purchase cost is more advantageous than that of water-tube boilers of the same size, making it an economical choice for small and medium-sized enterprises with limited budgets.
Secondly, fire-tube steam boilers are small in size and require less floor space. Their compact design allows for installation in space-constrained factory environments, making them particularly suitable for small and medium-sized enterprises with limited production space. No need to reserve excessive space for the boiler, saving factory floor space and indirectly reducing the company's site costs.
Furthermore, fire-tube steam boilers are relatively easy to operate and maintain. Its simple structure makes it easier for operators to understand the boiler's working principles and operating procedures. Basic operation methods can be mastered with simple training, requiring lower technical skills from operators and thus less investment in personnel training for enterprises. In terms of maintenance, the fewer parts and simpler structure make daily inspections, component replacement, and troubleshooting more convenient, resulting in shorter maintenance cycles and lower maintenance costs. This reduces boiler downtime due to untimely or difficult maintenance, ensuring continuous production.
Furthermore, fire-tube steam boilers have a fast start-up speed. Compared to water-tube boilers of the same size, fire-tube boilers store less water in the boiler shell and have a shorter heat transfer path. During startup, they can quickly absorb heat, rapidly raising the water temperature and generating steam. Rated operating pressure can be reached quickly to meet the needs of steam-using equipment. This rapid start-up characteristic is particularly advantageous in scenarios where equipment starts and stops frequently or where additional steam supply is needed temporarily, allowing for better adaptation to changes in production load.
Finally, fire-tube steam boilers operate safely and reliably within a low to medium pressure range. Its boiler shell and fire tube structure are well-suited to withstand stress under low to medium pressures. Furthermore, the relatively stable internal water and air flow during long-term operation results in slower wear and aging. Simultaneously, comprehensive auxiliary safety devices such as safety valves and water level gauges further ensure safe operation of the boiler within this pressure range, minimizing safety accidents caused by pressure issues. This makes it suitable for small to medium-sized applications where high steam pressure requirements are not critical.
These advantages have led to the long-term and widespread adoption of fire-tube steam boilers in small to medium-sized industrial facilities, making them a crucial choice for steam supply in many enterprises.
To improve the energy efficiency of fire-tube steam boilers, reduce energy consumption and costs, and extend their lifespan, optimization can be implemented in multiple aspects. Carbon and dust buildup on the inner walls of the fire tubes, smoke box, and chimney can impede heat transfer and flue gas flow, necessitating regular mechanical or chemical cleaning and ash removal. Installing economizers in the flue can utilize the waste heat of high-temperature flue gas to heat the feedwater, reducing exhaust heat loss and lowering the boiler shell heating load. Insulating the boiler shell and steam pipelines with high-quality materials such as rock wool can reduce heat loss. Installing sensors and analyzers at the smoke box or chimney to monitor flue gas temperature and oxygen content in real time and adjusting operating parameters based on the data can prevent combustion problems. An automatic combustion control system can precisely adjust the fuel and air volume according to steam load requirements, maintaining the optimal mixing ratio and achieving automatic start-up, shutdown, and load regulation. Adhering to these measures can save energy costs, reduce pollution emissions, and decrease equipment wear, creating both economic and environmental benefits.
Safety and maintenance are fundamental to the reliable operation of fire-tube steam boilers. Regularly checking pressure gauges and safety valves, monitoring and maintaining water level gauges and the feedwater system, removing scale and sludge, conducting pressure tests and safety valve calibrations, adhering to regulations and standards, and establishing comprehensive maintenance records and plans can prevent accidents and extend equipment lifespan.
Safety and maintenance are the foundation of reliable operation of fire-tube steam boilers. Properly performing related work can effectively prevent safety accidents and extend equipment lifespan. Daily checks of pressure gauges and safety valves are crucial. Pressure gauges should be calibrated regularly to ensure accurate pressure monitoring and prevent overpressure. Safety valves require regular manual testing and comprehensive calibration to ensure accurate operation at the set pressure. The maintenance of water level gauges and the feedwater system is equally critical. Regularly cleaning water level gauges, inspecting pipelines, and maintaining feedwater pumps and valves can prevent accidents caused by dry burning due to water level monitoring or water supply malfunctions. Furthermore, regularly removing scale and sludge from the boiler shell and the inner walls of the fire tubes through chemical or mechanical descaling and sludge removal can maintain heat transfer efficiency and prevent tube wall corrosion. Pressure testing and safety valve calibration are also crucial safeguards. A hydrostatic test every two years verifies the strength and sealing of pressure-bearing components, and safety valves undergo regular offline calibration to ensure parameter accuracy. Enterprises must also strictly adhere to boiler regulations and standards, ensuring compliant inspections, certified operators, and regular training. Simultaneously, establishing comprehensive maintenance records and routine inspection plans, clearly defining the maintenance content, cycle, and responsible personnel for each stage, allows for proactive problem prevention and handling by analyzing equipment conditions, reducing malfunctions and accidents.
Fire-tube steam boilers, due to their simple structure, convenient operation, low cost, and rapid start-up, have become a reliable and economical steam supply solution for small to medium-sized industrial applications, widely used in industries such as food and textiles. A thorough understanding of their design and operating principles helps improve operation and maintenance levels, operational efficiency, and safety. Combined with energy efficiency optimization measures such as pipe cleaning and ash removal, it can further reduce energy consumption and costs, achieving a win-win situation for both economic and environmental benefits. In today's environment where energy efficiency and sustainable development are highly valued, even with the continuous emergence of new boiler types, optimized and maintained fire-tube steam boilers, with their advantages and adaptability, will remain an important choice for steam supply for small and medium-sized enterprises, continuously providing strong support for industrial production.
