Steam Boilers 101

A steam boiler is a pressurized vessel that transfers heat to water to produce steam for a variety of applications.

This guide will provide a comprehensive and straightforward overview of what boilers do, how boilers function, and how to choose a safe, reliable and efficient steam boiler.

Table of contents

How Do Boilers Work?

Steam boilers produce pressurized steam power by heating water to its boiling point using a combustible fuel source.

How the water is heated depends on whether it is a firetube or watertube boiler. The primary differences between the two can be found in their names.

In a firetube boiler, the combustible fuel source is contained within a tube surrounded by a vessel filled with water. The tube gradually heats the water around it, eventually producing steam.

In a watertube boiler, the water is contained within multiple tubes, and heat from the combustible fuel source is applied to the outside of the tubes to produce steam.

The Anatomy of a Boiler System

• Pressure Vessel: A pressure vessel contains gases or liquids at high temperatures, usually under high pressure. In a boiler, the pressure vessel is constructed from a high-strength material, often steel.
• Burner: The burner provides heat to the boiler by combusting fuel and oxygen. Fuel sources include natural gas, low-pressure propane, No. 2 oil, coal and other fuels.
• Tubes: In watertube boilers, the metal tubes situated inside the boiler contain water and are externally heated. In firetube boilers, heated gas passes through one or more tubes, heating water surrounding the tubes.
• Economizer: An economizer is a heat exchange mechanism that transfers heat energy otherwise lost in exhaust gasses and uses that energy to heat the water entering the boiler. Consequently, less additional energy is required to heat the incoming water, making the boiler more efficient.
• Deaerator Tank: Deaerators are pressurized feedwater tanks that use pressure and heat to remove oxygen and other dissolved gases (notably, carbon dioxide) from the water fed into the boiler. Dissolved oxygen and carbon dioxide could otherwise cause serious corrosion to the boiler.
• Heat Exchanger: A heat exchanger transfers heat from one substance to another without those substances directly interacting. In a boiler, the hot gas’ heat is transferred to the water via a heat exchanger.
• Control Panel: The control panel enables operators to control boiler settings such as temperature and pressure. Commercial and industrial boiler control panels include detailed analytics.
• Feedwater Tank: The feedwater tank is a collection tank that supplies the water used by the boiler to create steam. The feedwater tank is where the treated water is collected and then pumped into the boiler. Boiler chemicals that remove oxygen and protect the metals within the boiler are injected and mixed within the feedwater tank. Facilities with condensate return lines can collect condensate water from steam that drops below the boiler point and fed back into the system to recapture the treated water.
• Combustion System: The combustion system works by combining air and fuel then igniting the mixture to produce heat. Ensuring the correct balance of air to fuel is an important component of the boiler combustion system.
• Reverse Osmosis System: Reverse Osmosis works by using a high-pressure pump to increase the pressure on the raw water side of the RO and force the water across the semi-permeable RO membrane, leaving almost all (around 95% to 99%) of dissolved impurities behind in the reject stream.
• Chemical Monitoring Systems: The carefully controlled addition of chemicals can enhance a boiler’s operations. Chemical monitoring systems precisely monitor chemical levels and provide ongoing analysis.
• Fuel Types: Combusting fuel is the ultimate source of the boiler’s heat. Gas, oil, and coal are common sources of fuel. When coal is used, it is often crushed and heated to improve efficiency. Less commonly, biomass such as wood chips or other natural materials may be used.
• Water Treatment: Water used by the boilers needs to be treated before entering the boiler to prolong the life of the boiler. Water softeners and Reverse Osmosis systems help to prepare the water by removing dissolved solids like calcium and magnesium to reduce the potential for scale build-up within the boiler. Feedwater tanks are also a part of the water treatment as they use heat to reduce the number of dissolved gases within the water that can contribute to oxidation and corrosion within the boiler.

Firetube vs. Watertube Boilers

As mentioned earlier, the two main types of boilers you will find on the market today are firetube boilers and watertube boilers. Let’s dive deeper into how each type works.

Firetube Boilers

As the name implies, in firetube boilers, a flame passes through an enclosed tube. The flame heats the ambient gas. That heat is transferred through the tube walls, heating the water contained in the vessel to the point that steam is produced.

Firetube boilers have a lot of history. After all, they powered some of the world’s first steam locomotives. Due to the immense stored pressure and inherent inefficiencies, more industrial processes are turning to watertube boilers for greater safety and efficiency.

Watertube Boilers

In watertube boilers, a furnace heats the gas which circulates tubes containing water. The heat is transferred through the tube walls, heating the water inside the tubes until steam is produced. Watertube boilers are typically capable of generating considerably more pressure than firetube boilers.

As watertube boilers do not enclose large volumes of water, their inherent risk is less than firetube boilers. Watertube boilers are also much more efficient, making them a preferred choice for intensive industrial processes.

Common Boiler Applications

Steam boilers are used for a wide variety of residential, commercial, and industrial applications.

Boilers intended for residential and commercial use typically have the lowest capacities. They are suitable for use in smaller-scale buildings and facilities that do not require a large amount of steam power.

Industrial boilers can handle a wide array of industrial processes that require greater horsepower. You can find industrial boilers in various settings, including hospitals, university campuses, chemical plants, breweries, food processing facilities, and manufacturing factories. In automotive production plants, steam is used for vulcanizing rubber for tires and other applications.

In food manufacturing and brewing, innumerable processes require steam, both in direct food production and sterilizing equipment and containers. High-temperature steam is also used to ensure food safety through pasteurization.

Boilers vs. Hot Water Heaters vs. Furnaces

Boilers, hot water heaters, and furnaces all produce heat, but they vary in their design and function. Let’s uncover the differences:

• Boilers heat water in a pressurized container to create steam. Many facilities will set a steam pressure, that has a direct correlation to steam temperature which can be adjusted for a wide range of commercial and industrial applications. When used domestically for central heating, the steam can be efficiently distributed to radiators to heat the house. Boilers vary in complexity and design, depending on their specific purpose.
• Compared to boilers, hot water heaters are more basic. Their single function is to heat water. Commonly, water is stored in a tank and heated with heating rods. Alternatively, instant water heaters rapidly warm water without the need for a storage tank.
• Rather than heating water, furnaces heat air that circulates through a home or building. Fuel burns to warm a heat exchanger, which heats the air before it distributes through a series of vents. The furnace temperature is controlled with a thermostat.

Boiler Efficiency

The efficiency of a boiler will directly affect the cost of running the boiler over its lifetime.

Boiler efficiency improves by optimizing the way boilers are used. Ultimately, however, the design of the boiler system determines much of the efficiency outcomes.

Functionally, boilers can be thought of as heat exchanging mechanisms. A boiler generates heat and ultimately transfers that heat to water — the less heat energy that is lost throughout the process, the more efficient the boiler.

Fuel-to-Steam Efficiency vs. In-Service Efficiency

The two industry standards for measuring efficiency are fuel-to-steam efficiency and in-service efficiency. Fuel-to-steam efficiency, also known as Annual Fuel Utilization Efficiency (AFUE), measures combustion efficiency. That is, what percentage of energy is converted to steam. A limitation of fuel-to-steam efficiency is that it only gives you a measurement of efficiency when the boiler runs at full capacity. Practically speaking, most boilers do not constantly run at full capacity. That’s why it’s important to consider in-service efficiency, which refers to the boiler’s total efficiency in its everyday function.

Boiler Turndown Ratios

Another important measure of operational efficiency is the boiler turndown ratio. Boilers with a high turndown ratio provide flexibility with the ability to produce lower amounts of steam than the maximum output. This allows facilities to deviate from the maximum steam output and conserve resources by turning down the boiler output to efficiently match the facility’s steam supply demands.

NOx Ratings

NOx is a blanket term for a group of nitrogen oxides responsible for smog and air pollution (nitric oxide and nitrogen dioxide). Any form of combustion is likely to produce NOx emissions. As boilers combust fuel to produce heat, NOx is produced. NOx emissions are regulated because they can harm human health and damage the environment.

Miura boilers produce less NOx by reducing the flame temperature. As the temperature rises so does NOx creation.

The Most Efficient Boilers

Traditional firetube boilers can take a long time to heat up and begin generating steam — all while consuming large amounts of fuel in the process. In contrast to firetube boilers, watertube boilers are more efficient because the reduced water content means there is less wasted energy used to heat the system when beginning operation.

For greater efficiency, consider a modular boiler system. Instead of a single massive boiler, modular boilers systems are a group of small boilers that work together to meet your steam demands precisely.

Essentially, each boiler in the modular boilers system operates independently. The independent boilers integrate into a single system, with a single master controller switching them ON or OFF as needed to meet demand. All modules may operate simultaneously, or individual modules can be shut down and quickly restarted as needed. This means you’re only producing steam when you need it.

At Miura America, we specialize in modular water tube boiler systems. There are several advantages of using a Miura boiler system:

• Each module starts up in less than five minutes, making the system more flexible and heat efficient.
• Smaller modular boiler units take up less floor space.
• You can rotate the use of individual units to reduce wear and tear.
• Operators can turn off individual boilers when not needed to save fuel.
• If one unit stops working, the rest continue to function. There’s less chance of downtime.
• The system is completely scalable. You can add more units as needed.

Boiler Safety

In the early days of steam power, boilers were volatile and not always securely constructed. Boiler explosions were commonplace, resulting in injury and even death.

Today, boiler design follows rigorous safety guidelines overseeing both the production and use of boilers. Let’s explore a brief history of boiler safety and identify the safest boiler option currently available.

Boiler Safety Through the Decades

Most early boiler explosions were caused by a breakdown in the parts of the pressure vessel, either through corrosion or poor build quality.

A turning point in the regulation of boilers came about in the wake of the Grover Shoe Factory disaster in 1905. A boiler located at the RB Grover shoe factory in Brockton, Massachusetts exploded, causing widespread injury and loss of life.

There were no reliable boiler operation guidelines in the decades preceding the disaster, and industrial safety inspections were uncommon. As a result, thousands of boiler explosions had occurred.

Following a successful campaign by the American Society of Mechanical Engineers (ASME) to persuade industrialists of the need for regulation, Massachusetts passed a law regulating boiler use in 1907. Those state laws would eventually inform a national safety code.

The ASME’s first Boiler and Pressure Vessel Code was published in 1915. Today, the ASME has issued 28 books, covering a comprehensive range of operational issues, running into tens of thousands of pages.

The safest modern boilers virtually eliminate the possibility of catastrophic failure, creating a much safer work environment.

So, What’s the Safest Boiler on the Market?

Modern boilers are robustly built, with additional features to prevent corrosion and damage that might cause safety issues. Effective monitoring and robust safety measures are essential to ensure ongoing safety, particularly for commercial and industrial applications.

In terms of boiler design, the safest type of boiler is a watertube boiler because it is designed to operate with less water volume than a firetube boiler. Additionally, should a watertube boiler fail, that failure would be contained within the tubes rather than exploding outward like a firetube boiler failure.

Boilers should also be designed to minimize the chance of thermal shock or even eliminate it. Thermal shock occurs when cold water enters a boiler and reacts with the extremely hot water already in the boiler. The ensuing rapid contraction and expansion of components could lead to catastrophic failure. Some measures can be taken to reduce the likelihood of thermal shock (such as reducing daily operating cycles). Still, ideally, a boiler should be designed to prevent thermal shock from occurring under any conditions.

Boiler Maintenance & Monitoring

Effectively monitoring and maintaining boilers before any problems arise will help ensure your facility stays safe and productive.

Chemical Water Treatment

Chemical water treatments are used to reduce dissolved oxygen within the water or treat metal surfaces to prevent degradation. Using these treatments allows facilities to control pH, prevent scale, reduce corrosive ions, and ensure the boiler’s reliable function.

Water Softeners

Water softeners remove hard metals (notably calcium and magnesium) from the water used in the boiler. Hard water can cause scale build-up and can damage boiler components over time.

Controllers and Monitoring Systems

Modern boiler controllers include sophisticated diagnostics to help users identify potential issues before they become serious problems. Comprehensive monitoring systems can provide analytics on everything from current steam pressure to water hardness.

Learn More About Steam Boilers

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