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Boiler Basics 101: Types of Boilers

When we think about boilers, there are a two types that typically come to mind; firetube, or scotch marine, and watertube boilers. These types of boilers can be classified as hot water, steam, high pressure, and low pressure. In today’s blog post we will be answering the question: what are the basic differences between the different types of boilers?

Although their final function is the same, the main difference between a firetube and watertube boiler is the construction and design of each system. In a firetube boiler, water inside a vessel is surrounded by tubes that contain combustion gases. In other words, the ‘fire’ is inside the tubes, making it a ‘firetube’. Watertube boilers are essentially the opposite in design. Combustion gases surround a series of tubes that contain water, coining the name, watertube.

By definition, high pressure boilers are built to a maximum allowable working pressure (MAWP) above 15 psig, while low pressure boilers are designed for operation at 15 psig or below. Low pressure boilers are most commonly utilized in heating applications and require less maintenance than that of a high pressure unit. Furthermore, firetube boilers can be built for both low and high pressure applications, while watertube boilers are typically built for high pressure needs.

Some may think that firetube and watertube boilers are in the same category as hot water and steam boilers. However, steam and hot water boilers are actually a classification, and can be considered a subcategory to firetube & watertube boilers.

Hot water and steam boilers operate in a very similar manner, but hot water boilers don’t actually produce steam. In reality, a hot water boiler is just a fuel fired hot water heater, in which heat is added to increase the temperature to a level below the boiling point. Hot water boilers are not as powerful as steam boilers, which is why they are more commonly used in heating applications providing hot water at 120 – 220F.

Steam boilers heat water to levels that are above boiling point, in order to produce steam. They are much more powerful and are utilized in more industrial and heavy-commercial applications. Steam boilers can be designed to produce either saturated or superheated steam, which we will discuss further on in a future post.  

Be it a firetube, watertube, hot water, or steam boiler, they are all effective and efficient in their own unique ways. To learn in more detail about the differences between boiler types, visit the section on our website, “What Boiler Is Best For You”.
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Boiler Basics 101: Superheat vs. Saturated Steam

Playing off our last topic, some might argue that there is another boiler type, or classification, to consider. In today’s article, we will discuss the difference between saturated and superheated steam boilers, and the applications that they are most commonly used for.

Let’s begin by talking about the science behind these two types of steam. Simply put, when water is heated to its boiling point, it will begin to vaporize and saturated steam is produced. Superheated steam occurs when the water is continually heated to temperatures beyond the boiling point, without any increase in pressure. Also known as dry steam, superheated steam has a much lower density and produces zero condensate.

As with the other types of boilers previously discussed, saturated and superheated steam boilers each have their own unique advantages and disadvantages and are better geared for certain applications over others. Saturated steam has a high density and is an excellent heating source. Commonly utilized in food processing, sterilization, district heating, and pulp & paper processing, saturated steam has the following advantages: 

  • Produces fast and even heating due to latent heat transfer
  • The temperature can quickly be established through the control of pressure
  • Has a high heat transfer coefficient, which requires a smaller heat transfer surface and in turn, allows for reduced initial
  • equipment costs

Superheated steam is not typically utilized in heat transfer applications. However, due to its dry composition and ability to cool while remaining in the same physical state, it can be extremely versatile and is most commonly utilized in refineries, for generating electricity, and for powering turbines.

Superheated steam is ideal for powering turbines for the following reasons:

  • The dry steam allows for steam-driven equipment to function effectively and efficiently (while condensate from wet steamwould negatively affect performance of the equipment)
  • Improves thermal efficiency and work capabilities of turbines
  • Contains zero condensate, minimizing the risk of corrosion and erosion damage

With a low heat transfer coefficient that is equivalent to that of air, superheated steam has more energy and can work harder than saturated steam, but the heat content is less useful. In addition, boilers that are built to produce superheated steam require more expensive components on the boiler system, in comparison to a saturated steam boiler. Therefore, it is extremely important to do your homework ahead of time to determine which type of steam is best suited for your particular application.

Did you know that Nationwide Boiler maintains a fleet of both saturated and superheated steam boilers for rent and for sale? In fact, we own the World’s Largest 125,000 lb/hr saturated steam mobile boiler, and the World’s Largest 110,000 lb/hr superheated steam mobile boiler!  Visit our website at nationwideboiler.com to learn more.

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Boiler Basics 101: Understanding Air Permitting for Boilers in California - SCAQMD and RECLAIM

California is known for having one of the most stringent air emissions standards in the nation. Not only has the state been making environmentally conscious efforts since the 1960’s, it is also the only state that can write its own air pollution related laws and standards. When the Clean Air Act passed, Congress required the Environmental Protection Agency to grant California exemption, since the state was already developing innovative laws and standards to address the state’s major air pollution issues.


As a boiler owner in California, familiarizing yourself with local air laws and regulations can be overwhelming, which is why Nationwide Boiler is here to help. Let’s start with one of the two toughest air districts in the state, the SCAQMD.

The South Coast Air Quality Management District (SCAQMD) encompasses the Los Angeles County, Orange County, Riverside County, and San Bernardino County  . The SCAQMD implemented the REgional CLean Air Incentives Market (RECLAIM) Program in 1993 to reduce NOx and SOx emissions produced by the region’s facilities. Although the program calls for potentially expensive equipment upgrades or replacements to meet the new guidelines, companies may qualify for trading credits and other incentives through partnerships with local utilities (like SoCalGas).

aaaaaWithin the RECLAIM standards, Rule 1146 outlines specific guidelines for boilers, steam generators, and process heaters that have a heat input of 5 mmBtu/hr or greater, that are utilized in all industrial, institutional, and commercial operations. This rule has changed several times, and at the end of 2018, another revision to the rule was adopted. All Group 1 units (>/= 75MMBTu/hr) as well as Group II units (20 - 75 MMBTu/hr) with an existing permit limit greater than 2 ppm must comply with a 5 ppm NOx limit. In addition, facilities that qualify must be in compliance by 2022 – 2023. The table below outlines all equipment and current limits based on category and heat input.

If your facility falls into the RECLAIM bubble, Nationwide Boiler can assist in bringing your stationary equipment up to current standards to comply with the latest rules and regulations. Our CataStak™ SCR system is proven to reduce NOx emissions to as low as 2.5 ppm on boilers, fired heaters, and other demanding applications. Imagine what we can do for your facility to get you in compliance with RECLAIM!

Stay tuned for part 2, where we will outline specific rules and regulations within the SJVAPCD.

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Enhancing BMS Safety with the use of PLC-based Controls

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 burner management system (BMS) is just one of many safety devices built into a boiler system, designed to control the combustion process from beginning to end in a safe and reliable manner. It monitors high and low gas pressure, the combustion air fan, combustion air, and water levels, in addition to monitoring safety devices and controlling the sequence of lighting the burner. If any issues arise related to pressure or water level, it will initiate closure of the shut-off valves. To automate these processes, PLC-based controls are often used as a BMS. PLC based BMS gives you much more flexibility, the ability to use analog input signals as limits, limit voting and the versatility to have almost unlimited safety limits.

A Programmable Logic Controller (PLC) is a robust computer utilized for industrial automation. Although a PLC doesn’t physically look like a typical computer, it incorporates the very same technology seen in computers and smart devices that we use every day. The PLC receives information from connected sensors or input devices, processes the data, and triggers outputs based on pre-programmed parameters. It consists of a power supply, a CPU (central processing unit), input and output cards, and communication cards. A programming device (often a laptop computer) is utilized for writing programs into the PLC and HMI (human machine interface) which provides a visual model of the system as a whole.

Compared to traditional technologies, a PLC-based system is easier to troubleshoot, more reliable, more cost-effective, and much more versatile. In addition, PLC-based controls provide added levels of safety for the burner management system and overall operation of your boiler. PLCs are built in compliance with NFPA 85 and SIL2 requirements at minimum and can be configured to meet SIL3 standards as well. Therefore, we are seeing boiler control panels being built or updated to PLC-based systems more than ever.

Pacific Combustion Engineering has extensive knowledge and experience in the design, build, and programming of PLC-based combustion control systems. If you are in the market for an upgrade or brand-new panel that incorporates PLC, give us a call and we will design a system that fits your unique process needs.

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