Fire Tube Boiler

How to Select the right boiler ?

Capacity | Efficiency | Start-up Time | Steam Quality | Furnace Size | Total Contained Energy

When selecting the ideal boiler, several factors should be consider the overall assessment of the unit’s effectiveness. However, the relevance of each factor can vary depending on the specific application.

Understanding how the boiler will be used is crucial in determining which performance factors are key. Here are the key factors to select the ideal boiler:

Table of Contents


The capacity of the boiler, which refers to the amount of energy it can handle, is a fundamental performance factor. This includes factors such as the BTU/hr input, boiler horsepower (BHP), or pounds of steam per hour (PPH) production. The capacity requirement depends on the intended application and the expected demand for steam or hot water.


Efficiency measures how efficiently a boiler transforms fuel energy into usable steam energy. This influencing operational costs significantly. However, it’s not the sole determinant of a boiler’s operational expenses. Evaluating a boiler’s overall efficiency involves more than just numerical analysis. While most boilers provide a steady-state efficiency value, this figure applies only to continuous operation. For boilers with intermittent operation, such as daily running hours with cycles and weekends off, start up and shutdown costs play a crucial role in overall efficiency. The importance of specific efficiency factors varies depending on the application’s operational demands.

There are 2 main types of efficiencies for industrial boilers: steady state and dynamic efficiency.

Steady State Efficiency, also known as input-output efficiency, measures the effectiveness of a boiler when operating at a constant input and after the unit has been warmed up. Determining this efficiency requires making certain assumptions, which may not always align with the specific application. Efficiency varies based on factors like firing rate, steam pressure, excess air, fuel type, and boiler room conditions. Different assumptions can lead to varying efficiency results, such as those related to fuel composition, ambient temperature, steam temperature, and heat loss through the boiler shell. Consistency in these assumptions is crucial for meaningful comparisons between different units, as they can significantly impact efficiency.

Dynamic Efficiency refers to the efficiency of a boiler when cycled on and off. Cycling a boiler on and off requires additional energy to heat the water, refractory materials, and steel components. Energy is lost to the environment during off cycles. The physical size of the boiler and the amount of water it contains influence the energy consumption during these on and off cycles.

Start-up Time

The start-up time of a boiler, from cold to operating steam pressure, varies depending on the boiler type. Some boilers are specifically designed for rapid start-up and can achieve operating steam pressure within minutes. In contrast, traditional multi-pass Firetube and Industrial Watertube boilers typically require several hours to reach operating pressure. This extended start-up time is attributed to the larger water and material weight that needs heating, as well as the time required for uniform expansion of materials as they heat up.

Boilers with shorter warm-up times are characterized by tube arrangements that facilitate uniform heat absorption or flexible tube positions, combined with low water and material content. Such boilers are available in both Firetube and Watertube designs.

In scenarios where a backup boiler is needed, opting for a boiler with quick start-up capabilities can eliminate the need to maintain a hot standby boiler, which can be costly. A smaller-sized boiler with reduced heat loss can significantly decrease the energy required to provide backup boiler services, especially when a rapid response is necessary.

Steam Quality

Steam quality refers to the moisture content in steam, with lower moisture levels being preferable for most applications due to potential disruptions caused by excessive moisture. Achieving low moisture content becomes more challenging at lower operating pressures because steam occupies a larger space per pound. In Firetube boilers, steam quality is typically evaluated based on several factors:

Disengaging Area: This refers to the surface area where steam separates from water. A larger disengaging area reduces the chance of water droplets being carried along with the steam.

Steam Chest Size: The steam chest is the space where steam moves from the water surface to the steam outlet. A larger steam chest results in slower steam velocity, increasing the likelihood of water droplets falling out.

Internal Steam Velocity: This is the actual velocity of steam within the steam chest, influenced by factors such as steam nozzle size and location. Higher velocities can cause water droplets to remain in the steam, leading to issues like water level swelling and slug feeding.

Mechanical Separation: Various mechanical devices can be incorporated into boilers to improve steam quality by removing moisture or preventing high steam velocities from affecting water levels.

Operating pressure significantly affects steam quality. Lower-pressure Steam Boilers, such as those operating at 25 PSI, may struggle to produce dry steam due to higher internal steam velocities, whereas higher-pressure units, like those operating at 300 PSI, find it easier to achieve dry steam.

Furnace Size

In boilers, the size of the furnace is crucial for effective combustion. A larger furnace is typically necessary to achieve optimal combustion, especially for applications requiring low NOx emissions and firing with oil or solid fuels. The shape of the furnace also matters; for instance, Firetube boilers often feature round furnaces that accommodate flames better. Watertube boilers generally have larger furnaces, making them more suitable for solid fuels.

Manufacturers sometimes employ hybrid designs, combining Watertube and Firetube sections, to optimize furnace size and combustion efficiency. In boilers equipped with pre-mix burners, which produce smaller flame sizes, a separate large furnace may not be necessary. This is commonly seen in smaller and some larger boilers.

Total Contained Energy

Dynamic efficiency, a key consideration in boiler performance, focuses on the energy required to bring the boiler up to steam pressure. This aspect is crucial for applications with frequent off cycles, as they can lose significant energy during these periods. Generally, boilers with ample heating surface exhibit good steady-state efficiency, but their larger size results in more water, steel, and refractory to heat up, leading to higher heat loss during off cycles. In contrast, compact boiler designs require less energy to reach operating pressure and experience lower energy loss during off cycles.

Traditional Scotch Marine Firetube boilers, characterized by their weight and water content, require substantial energy to heat their water to steam temperature due to the high water content. Notably, water is the primary energy holder, with 1000 pounds of water requiring significantly more energy to heat compared to an equivalent weight of steel. This high water content distinguishes traditional Scotch Marine Firetube boilers from Watertube boilers, which typically have lower water content.

It is important to mention that there are many other Firetube and Watertube boilers with reduced water content and physical size, often employing different heat transfer methods to achieve good efficiency with smaller vessels.

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