Boiler Blog | Nationwide Boiler Inc.

Given today’s awareness to the advantages of minimizing energy usage and carbon footprint, boiler operators and plant managers are always on the lookout for ways to improve boiler efficiency and ensure emissions compliance. With improved efficiency, fuel usage is minimized which in turn reduces the carbon footprint (i.e. reduces CO2 emissions from the boiler) and reduces issues around emissions compliance. One way to increase boiler efficiency is to use oxygen trimming, or O2 Trim, at the stack.

A typical burner will operate from 3 to 4% O2 at 50% boiler load and higher. This stack O2 concentration corresponds to the amount of excess air at the burner, and excess air is required for burner operation to assure complete combustion of the fuel.  For example, for natural gas firing, 3% O2 corresponds to 15% excess air. During commissioning, the burner service engineer will set the fuel / air ratio so that there is always excess air over the firing range of the burner.  The service engineer must also keep in mind that ambient conditions (mainly air temperature changes) will affect air density which will affect the burner fan air flow output.  On cold days the fan will flow more air due to a higher air density, and on hotter days the flow will be less. Varying air flow conditions can adversely affect burner operation.

Boiler efficiency is affected by the excess air concentration in the flue gas. The rule of thumb is that for every 5% more excess air, boiler efficiency decreases by 0.5%. If not adjusted, the boiler stack can vary by at least 2% O2 (i.e., if normal operation is 3% O2, it can increase to 5% O2 on a cold day). That corresponds to about 1% boiler efficiency loss. Saving 1% efficiency over a year operation can save big on fuel costs. If the normal fuel bill is $10,000,000 per year, you would save $100,000. Adding an O2 Trim system would cost a fraction of that amount (assuming a 150,000 lb/hr steam boiler or smaller), providing a quick and worthwhile ROI. So, what exactly is an O2 Trim System?

Many burners use a control system where the fan air flow does not vary based on air temperature.  As explained above, the air flow can vary based on ambient conditions causing the stack O2 to vary; this can be solved by adding O2 Trim to the control system. O2 Trim is an air flow trimming system where stack O2 is measured (using an O2 probe) and the air flow is adjusted (trimmed) based on the reading. It’s a closed loop control system since changes in air flow will directly affect the stack O2 reading (assuming fuel flow is the same). By maintaining a consistent air flow rate, O2 trim reduces fuel usage in turn increasing boiler efficiency.

In addition to increased boiler efficiency, utilizing O2 Trim will ensure stable and safe O2 levels. On hot days with reduced fan air flow, the stack O2 level can drop to dangerously low levels and boiler emissions can go out of compliance. With O2 monitoring, alarms can be created to alert the boiler operator to either reduce fuel flow or increase air flow, to return to safe operating conditions.  

O2 Trim isn’t ideal for every boiler, though. Due to the residence time in the boiler and ducting, it takes time for the changes in burner fan flow to reach the stack. This causes a time delay with an O2 Trim system, which works well for boilers with slow load changes. However, for systems with rapid boiler load changes, the O2 Trim system typically can’t keep up easily and it is often “hunting” for the optimum air flow.

If you are interested in learning more about whether an O2 Trim System will benefit your operations, reach out to one of our qualified parts specialists or call 800-227-1966. Check out other articles on Nationwide’s Boiler Blog for more tips and tricks for improved boiler efficiency, routine maintenance, and more!

With over 30 years of experience, our General Manager at Pacific Combustion Engineering, Jack Valentine, is an expert when it comes to the design of combustion control systems. In the Winter 2021 edition of Today’s Boiler, Jack discusses the features and options typical of Combustion Control Systems today. Let’s take a peek at what he had to say.

Combustion equipment safety is essential for the daily operation of facilities and safety of plant personnel. Safety protocols and mechanisms in industrial plants have improved drastically in the last century, but incidents still occur far too frequently. Because boiler systems are inherently dangerous, safety must be factored into the design of not just the boiler, but also the burner, combustion control, and the overall operation of the system.  

The Combustion Control System (CCS) on a boiler, also known as the Boiler Control System (BCS), refers to the set of instrumentation and controls that modulates the firing rate of the burner in response to load demand while maintaining the proper air/fuel ratio (AFR). It works in conjunction with the Burner Management System (BMS) that provides safeguards before, during the initial light-off of the burner, and at shutdown. The BMS also provides the flame safeties and interlocks required to keep the boiler safe during continuous operation. Depending upon the complexity of the boiler, the CCS can also provide other functions such as drum level control and draft control.  With Low NOx burners, it also controls the proper amount of flue gas recirculation (FGR) to the burner.

For the sake of simplicity, the various types of CCS described below will be for boilers firing a single fuel gas only; fuel oil and solid fuel systems add an entirely new level of complexity.

Boilers are designed to produce steam to accomplish a multitude of tasks; from powering a plant to sterilizing hospital equipment. In simple terms, a boiler is a closed container in which water is heated to its boiling point to produce usable steam. In order to produce steam, there are two key items that must be involved: water and heat.

A boiler is comprised of two seperate systems: the steam-water system (waterside), and the fuel-air-flue system (fireside). As you might have guessed, water is first introduced into the waterside of the boiler. Alternatively, the fireside of the boiler provides heat, produced through the combustion of fuel (commonly natural gas or fuel oil, but can be another source) and air, which is controlled by the burner.  The heat that is created within the fireside is transferred to the waterside to produce steam.

To complete a boiler system, additional elements are required. This typically includes the following major components:

•  Burner: a mechanical device that supplies the required fuel and air for proper combustion.
•  Controls: the BMS (burner management system) protects the equipment and personnel from safety issues.
      The CCS (combustion control system) controls the air and fuel for proper combustion.
•  Fan: supplies air for the combustion to take place.
•  Water Softener: pre-treats the boiler feedwater for removal of hardness, which would otherwise cause detrimental scale
   inside the boiler system.
•  Deaerator / Feedwater System: removes oxygen and gases from boiler feedwater supply (which will also damage boiler
  internals), and feeds it to the boiler system via high pressure feedwater pumps.

All of these elements come together to create a robust steam supply system that is utilized in an abundance of processes throughout many different industries. Boilers truly are a work of art, with many pieces working as one system to make something extremely powerful and impactful, which is why we at Nationwide Boiler are so passionate about what we do.

Stay tuned for the next article in ur Boiler Basics 101 series to learn more about common types of boiler systems.  

Selecting the right boiler for your facility is more than just a design choice – it directly impacts efficiency, reliability, operating costs, and emissions compliance. Among the most common designs are firetube boilers and watertube boilers, each with unique advantages depending on the application. These boilers can also be classified by pressure (low or high), output (steam or hot water), and steam temperature (saturated or superheated), adding further considerations to the decision-making process.

The following overview outlines the key differences between firetube and watertube boilers to help facility managers, engineers, and plant operators choose the system that best fits their needs.

What is a Firetube Boiler?

A firetube boiler, also known as Scotch Marine boiler, is one of the most traditional and widely used boiler designs. In this system, a large pressure vessel holds water, and tubes carrying hot combustion gases run through the vessel. As the gases pass through these tubes, heat is transferred to the surrounding water, gradually increasing its temperature until it produces either hot water or steam depending on the application. Because the “fire” or combustion gases are inside the tubes, the design is known as a firetube boiler.

Key Advantages:

• Lower upfront cost and simpler design
• Easier to operate and maintain
• Suitable for small to medium-sized facilities

Limitations:

• Slower to respond to load swings due to large water volume
• Design pressure capability is more compared to watertube designs

Firetube boilers are commonly utilized in industries and facilities such as small food processing plants, hospitals, schools, universities, and other heating applications where dependable steam or hot water is needed for steam systems operating at 150 psig and less.

What is a Watertube Boiler?

A watertube boiler operates differently than a firetube boiler. Instead of hot gases flowing through tubes surrounded by water, a watertube design allows water to circulate inside the tubes while the combustion gases pass around them. This configuration enables the system to handle much higher pressures and produce greater steam capacities than firetube units.

Key Advantages:

• Handles higher range of pressures, commonly up to 750 psig but pressures over 1000 psig are possible.
• Faster startup and load response
• More compact water content, reducing risk of catastrophic failure

Watertube boilers are commonly used in high-demand industries such as power plants, petrochemical plants, refineries, pulp and paper mills, large food processing, and large-scale manufacturing facilities, where high-pressure steam and higher capacities are required.

Limitations:

• Higher initial cost due to design and manufacturing differences
• Space requirements
• Typically, higher operation and maintenance costs

High Pressure vs. Low Pressure Boilers

Boilers are also defined by their maximum allowable working pressure (MAWP):

• Low Pressure Boilers: Operate at 15 psig or below. Commonly used for heating systems and hot water supply, they require less maintenance and are easier to manage.
• High Pressure Boilers: Designed for pressures above 15 psig. These units are essential for industrial steam production, power plants, and manufacturing processes where higher output is required.

Firetube boilers can be built for both pressure ranges, but watertube boilers are almost always high pressure due to their design.

Hot Water Boilers vs. Steam Boilers

While often confused, hot water boilers and steam boilers are separate classifications that can be applied to either firetube or watertube designs.

• Hot Water Boilers: Operate like large fuel-fired water heaters, producing hot water in the range of 120 - 220°F. These are primarily used for building heat, hydronic heating systems, and domestic hot water.
• Steam Boilers: Heat water beyond the boiling point to create steam. These systems are more powerful and used in industrial processing, sterilization, district energy systems, and power generation. Steam boilers may generate either saturated steam or superheated steam, depending on the application.

Saturated vs. Superheated Steam

When discussing steam boilers, it's important to understand the distinction between saturated steam and superheated steam, as each serves different industrial needs.

• Saturated steam is steam that is in equilibrium with water at the same temperature and pressure. It contains no additional heat beyond what's needed to convert water into steam. This type of steam is commonly used in heating applications and processes where direct contact with the product is required, such as food production or sterilization.
• Superheated steam, on the other hand, is produced by adding more heat to saturated steam without increasing its pressure. This results in steam at a higher temperature, which is ideal for driving turbines and other mechanical equipment. Because it doesn’t condense as easily, superheated steam is more efficient for energy transfer over long distances.

Understanding which type of steam your application requires can help you choose the right boiler and optimize performance.

Electric Boilers

Electric boilers are gaining popularity as a clean and efficient alternative to traditional fuel-fired systems. Instead of burning gas, oil, or coal, electric boilers use electrical resistance or induction to generate heat.

Key advantages include:

• Zero emissions at the point of use, making them ideal for facilities with strict environmental regulations.
• Compact design and quiet operation.
• High efficiency, often approaching 100%, since nearly all the electrical energy is converted into heat.

While electric boilers may have higher operating costs depending on electricity rates, they offer a low-maintenance, sustainable solution for many commercial and industrial applications—especially where fossil fuel infrastructure is limited or undesirable.

Making the Right Choice for Your Operation

Every boiler, whether firetube, watertube, steam, or hot water, has its unique strengths. The right choice depends on:

• Your unique operational needs; pressure requirements and steam capacity
• Facility size and available footprint
• Operator experience and maintenance resources
• Energy efficiency and emissions requirements

With modern advances such as ultra-low NOx burners, economizers, and Nationwide’s proven CataStak™ SCR System for near-zero NOx performance, today’s boilers are more efficient and environmentally friendly than ever before.

Whether you’re evaluating a firetube boiler for a commercial application or a watertube boiler for high-pressure steam production, understanding the differences between boiler types is essential for making the right investment.

For more details and guidance, explore our resource: What Boiler Is Best for You.

This article has been updated to reflect its original publication date of April 2019, while incorporating current insights and clarifications to ensure continued relevance and accuracy.