Everything You Need to Know About Fire-Tube Boilers

Industrial boiler systems heat water or produce hot water and steam for various process and space heating applications. Different boiler models are best suited for different environments and needs. In general, fire-tube boilers tend to be more economical and require less technical skill to maintain.

Understanding the inner workings of a fire-tube boiler, its best-suited applications and how it differs from other types of boilers is essential to deciding whether to invest in one. Read on for your guide to how fire-tube boilers work and what sets them apart.

What Is a Fire-Tube Boiler?

A boiler operates by serving as an enclosed vessel that transfers heat from combustion products to water, producing  high-pressure steam or hot water as energy. Fire-tube boilers may heat organic and synthetic fluids in addition to water. Today’s larger fire-tube boilers produce more than 1,500 boiler horsepower, which is about 50,000 pounds per hour of steam output.

The fire-tube boiler first emerged during the Industrial Revolution and served as the original power source for providing steam to factories. They’ve been used historically in powering ships and trains, as well. Today, there are various types of fire-tube boilers, which primarily differ in their structure. The most common types include:

• Scotch or Scotch marine boilers: This type of fire-tube boiler is often used for high vapor capacities and high pressure. Scotch boilers have multiple small diameter tubes and use a chimney to release excess gas from their internal process. After each pass through the boiler, heat is transferred from the products of combustion to the water. This type of boiler is commonly used for commercial and industrial applications. 
• Vertical boilers: This boiler is a one-pass, simple type of fire-tube boiler, also available in water-tube options. It’s considered to have a small footprint and is built in such a way that it can be installed in areas with limited space. This factor makes many vertical boilers popular choices for restaurant, hospital and industrial applications.
• Locomotive boilers: As their name suggests, locomotive boilers are interactive models meant for train engines that feature a horizontal multi-tubular design. They can also be constructed as firebox boilers, which feature partially contained, water-cooled furnaces within the boiler.
• Horizontal return tubular boilers: This boiler is an older model that’s typically placed above a steel or masonry furnace. The combustion products of the furnace pass through the tubes and exit directly through the other end of the boiler.

Regardless of the specific application or industry, fire-tube boilers produce hot water and steam through their cyclical processes. They provide a unique heating method, different from that of water-tube boilers and heat recovery steam generators. Most of today’s fire-tube boilers are portable, making them excellent heating and power sources for sawmills, oil fields and construction sites. 

How Does a Fire-Tube Boiler Work?

The step-by-step functionality of a fire-tube steam boiler is slightly different from other types of boilers, such as water-tube. However, the purpose remains the same. Some fire-tube boilers are one-pass machines, whereas others are multipass. These boilers, especially the three-pass ones, are excellent for understanding how temperature is processed inside the boiler.

Fire-tube boilers produce heat and power through the circulation of hot gases within the system. When the fuel source combusts, it heats the surrounding air, which rises to the top of the chamber and enters the submerged fire tubes. As the flue gases move through the fire tubes, they heat the water through conduction and produce saturated steam. 

Normally, these gases pass through the tubes several times and transfer their heat through the tube walls. When it comes into contact with the hot tubes, the water on the other side begins to heat to produce steam or hot water depending on the boiler and application.

Throughout this process, the key to pressure-balancing is the water that submerges each set of tubes. A typical fire-tube boiler is filled most of the way with water but not entirely to the top. 

The durability of the front end and back door of the boiler is essential. The back door of a fire-tube boiler can have one of two configurations: 

• Dryback: The turnaround area contains a heat-resistant refractory lining that contains the heat within the system.
• Waterback: The turnaround area is water-cooled, removing the need for a refractory lining.

Pressure vessel design is a major contributing factor to a boiler’s efficiency. Each boiler’s pressure rating is all about the thickness of the metal. Although different boiler types are better for different applications, that’s the general rule. 

Fire-Tube vs. Water-Tube Boiler — What’s the Difference?

While fire-tube and water-tube boilers often exist with the same end goal, the primary difference between the two lies in their functionality, as the combustion gases and water are oppositely placed within the boiler. 

With fire-tube boilers, combustion gas passes inside the tubes, transferring heat to the water between the tubes and the outer shell. With water-tube boilers, water passes inside the tubes, but the flue gases stay in the shell and pass over the surfaces of the tubes. Fire-tube boilers are also internally heated and produce less steam, while water-tube boilers are externally fired and operate with a higher level of steam production.

Structurally, there are some characteristics that are unique to fire-tube boilers. Fire-tube boilers often contain a pressure gauge placed on the top of the outer shell that indicates the level of pressure inside the boiler. Since this type of boiler is filled with water surrounding the passing tubes, it also contains a water level indicator. 

Water-tube boilers can withstand higher internal pressure and can generate superheated steam. In some cases, this ability makes water-tube boilers preferable to fire-tubes due to their ability to perform well in operations that require higher boiler steam pressure capacity.

On this same note, fire-tube and water-tube boilers are often better suited for different applications. While water-tube boilers can withstand higher pressure, fire-tube boilers are the more economical choice if your project allows for it. Water-tube boilers are also great options for power plant energy since they’re a more simply constructed vessel that takes up less space.

Where Are Fire-Tube Boilers Used?

Fire-tube boilers are commonly used in industrial and commercial applications. Package boilers, boiler systems and various boiler room equipment often keep entire operations running behind the scenes. 

Common uses for fire-tube boilers include:

• Institutional heating: Many people may refer to a designated space within their business as the boiler room, where operations and electricity are housed. That’s because boilers, specifically fire-tube boilers, are useful for heating buildings of many different sizes. Hot water or steam produced by boilers can be circulated through building structures to maintain temperature.
• Health care facilities: Most hospitals need to maintain high air quality and specific humidity levels to combat harmful pathogens. Hot steam is also useful for equipment sterilization when pumped into an autoclave.
• Food production facilities: Anywhere food is being prepared requires the utmost quality of sterilization. Boilers assist in baking, brewing, pasteurizing, sanitizing and processing various foods. In agriculture, fire-tube boilers can also be used in soil sterilization, dairy product production and sugar refining.
• Cosmetics factories: In the cosmetics industry, boilers can assist in the creation of perfumes, creams and other cosmetics products.
• Paper and printing operations: Whether involved in the power of the printing or the drying process, boilers contribute to the creation of paper by supplying steam heat to drying tunnels and creating corrugated cardboards.

Package fire-tube boilers can power numerous industries, such as the textile industry, surface treatment industry, automobile industry and more. Institutional heating is one of the most common uses for boilers. No matter the application, you’ll likely be able to use fire-tube boilers to accomplish your needs.

Maintaining an Efficient Fire-Tube Boiler

If your current boiler model serves your business well, you may want to update it rather than go through the effort to replace it completely. Gas-fired and oil-fired boilers can be retrofitted to boost efficiency, so you don’t have to spend additional time or money on a replacement. Some existing fire-tube boilers can benefit from an updated, more efficient burner management control system. 

Typically, boiler maintenance should be monitored and performed by a professional. They may check for the condition of the venting system and exit chimney, controls of the boiler that affect pressure or temperature settings and the boiler’s combustion and fuel-to-steam efficiency. They’ll also look for leaks. Having an industry professional conduct routine maintenance checks and offer advice on purchasing new or retrofitting boilers will boost your company’s efficiency and potentially save you money.

Specific devices that should be regularly maintained include:

• Pressure gauges: Pressure gauges are one of the most critical safety features of any type of boiler. These gauges should be checked on a regular basis to ensure there’s proper calibration. Boiler experts can conduct this check with official test equipment.
• Pressure relief valves: Safety valves allow steam to escape when the pressure in the system is higher than the normal operating pressure. Without a pressure relief valve, the pressure in the vessel will continue to rise to unsafe levels.
• Safety sensors and controls: The safety features of a fire-tube boiler help prevent a malfunction or even explosion of the machine. Using the proper test equipment can help you check the temperature, gas pressure and several other factors.
• Combustion safety switches: Safety switches provide proper airflow to the combustion chamber and exhaust flue, which ensures your boiler is functioning properly.
• Feedwater level controls: Pump controls ensure that the proper amount of water enters the boiler.  Too much water in the boiler can put strain on the entire system, while too little water can cause the tubes to overheat.

Carbon monoxide detectors will also need to be checked regularly, such as every quarter. Carbon monoxide (CO) is a dangerous byproduct of incomplete combustion. Monitoring emission levels in your boiler room is a crucial safety measure because carbon monoxide is both colorless and odorless — if your boiler is burning fuel incorrectly, you may not know until it is too late. Most states already require carbon monoxide detectors in buildings, with states like Michigan and Florida having specific laws about using detectors in boiler rooms. 

Pumps, valves and insulation are also boiler elements that should be routinely monitored. Proper maintenance of fire-tube boilers ensures efficiency, lowers your business costs and helps with providing the utmost safety of those occupying any given room. 

Ensuring the quality of water in your boiler also prevents other issues from forming slowly over time. If the water in your boiler causes mineral buildup, these issues may create blockages within the boiler tubes. Installing proper water treatment equipment based on the quality of the incoming water source will help your boilers run as smoothly as possible. Different models of fire-tube boilers operate slightly differently, which may impact how often maintenance checks are performed and which type is best for your application.

Choosing a Fire-Tube Boiler

Depending on your industry and needs, you may be best suited for a water-tube boiler, waste heat recovery boiler, fire-tube heater or a different hot water or steam source. Fire-tube boilers are excellent choices for many applications involving heat and steam production.

Other factors impact the right model for your application. The size of different fire-tube boilers determines the amount of storage space required and how much steam or heat the boiler can provide. Ease of installation and management, price and the number of passes the boiler contains are other things to consider. Knowing which fuel source you’ll want to use may impact your choice, as well. Common combustion sources are natural gas, propane and fuel oil.

In some instances, you may only need a fire-tube boiler rental. If there’s a temporary issue with your steam or hot water source, many boilers are available for temporary use. Thermal fluid and hot water rental boiler packages meet different industrial needs.

When choosing a fire-tube boiler supplier, whether for purchase or rental, take into account the delivery method, how independent or assisted the startup process will be and whether your supplier offers emergency or general repair service for your product. Fire-tube boilers are powerful machines, and it’s crucial to invest in one that will best serve the needs of your location or project.

Fire-Tube Boilers From Smith Hughes

Smith Hughes specializes in industrial boiler supply for large production and utility plants. Our products and services are available nationwide.  Our fire-tube boilers come in space-saving, versatile and multi-pass models, so you can select a type that fits your needs best.

Replacement, repair and temporary rental boilers are all services and products we offer with our 24-hour emergency service. Keep in mind that even if you install multiple fire-tube boilers, no boiler works the same. That’s why we provide fire-tube boiler packages with assisted startup, installation and training so you can be confident about the safety and efficiency of your boilers.

To learn more about which fire-tube boiler best suits your industrial needs, contact us for a quote or consultation. Between our service technicians and management team, we can assist in the setup and maintenance of your boiler for various applications.