This article describes several variables to consider when selecting a steam boiler — from choosing a boiler configuration to understanding boiler water chemistry.
Let’s assume that you are tasked with finding a suitable replacement for an aging boiler in your plant. This article will equip you with some key information about steam boilers, including fuel types, boiler configurations, and condensate system considerations.
Before you start the project, you need to arm yourself with some basic knowledge, such as your steam requirements, what type of boiler you prefer, and which manufacturer can meet your needs. You would not ask a car dealer “Which car do you think I should buy?” Therefore, you need to understand your own requirements to help you narrow down the options. So, before you can choose the best boiler for your application, determine your needs and research available technologies.
Ideally, you will have plenty of time to gather information and choose a piece of equipment that meets your needs. If, however, a boiler failure occurs and you are up against a deadline, you may have to get a rental boiler. There are many companies that can fulfill a rental request in about one to two weeks. Installing a replacement boiler in your facility within a week or so is generally not an option, as this can only be fulfilled by a boiler manufacturer that stocks replacement boilers that can match your steam demand; typically, only modular boiler manufacturers have this capability. For the purpose of this conversation, let’s assume you have some time to make an informed decision.
Coordinate the project team
When choosing the project team, picking one person to be the main point of contact for the project, both for internal and external personnel, may be the most important factor for team communication. This point of contact should fully understand what the goal of the boiler replacement project is, be easily accessible, and be flexible enough to handle frequent changes.
At the start of the project, schedule a team meeting and ask these questions at the meeting:
- What do we need? Do we need steam or hot water? Do we need humidity? Do we need electricity? Do we just need a certain temperature for our process? How often do we need it?
- How critical is our process? Are we replacing equipment? Are we starting from scratch?
- Are we looking at a 5-yr or 10-yr payback period? Or, is there another driving cost factor?
- Does anyone on this team understand boiler water chemistry?
- Do we want to perform the maintenance on this equipment, or will we outsource maintenance? Are there any guarantees or warranties from any of the manufacturers associated with bundling services?
- What does our state say about boiler operations? What does our insurance company say about boilers?
You should know the answers to these questions to fully understand the scope of the project. There are many organizations that you can hire to answer these questions for you and deliver the optimal boiler system to you on a platter. But if you are a do-it-yourself kind of organization, then you may want to familiarize yourself with the topics addressed in this article before you begin to answer these questions (or start making phone calls to external companies).
The most basic definition of a boiler is a closed vessel in which water is heated until the water has reached the desired pressure and temperature. This definition is extremely broad and the project team will need to narrow down the specific type of boiler that will be required. Some manufacturers use the term boiler to refer to different subcategories of boilers, such as power boilers, high-temperature water boilers, steam heating boilers, or hot-water heating boilers.
The American Society of Mechanical Engineers’ (ASME) Boiler and Pressure Vessel Code (BPVC) regulates the design and construction of boilers (1, 2). Your boiler requirements will be influenced by how you plan to use the boiler in your process. Do you need power, heating, or something else? ASME divides boilers into two categories: high pressure/temperature and not high pressure/temperature.
ASME BPVC Section I: Power boilers. This section of the code specifies requirements for all methods of construction of power and electric boilers, as well as high-temperature water boilers, heat-recovery steam generators, solar receiver steam generators, and certain fired pressure vessels (1). These are steam boilers in which the steam pressure exceeds 15 psi, and water boilers in which the pressure exceeds 160 psi and/or the temperature exceeds 250°F.
ASME BPVC Section IV: Heating boilers. This section of the code provides requirements for design, fabrication, installation, and inspection of steam heating boilers, hot-water heating boilers, hot-water supply boilers, and potable water heaters intended for low-pressure service that are directly fired by oil, gas, electricity, coal, or other solid or liquid fuels (2). These are steam boilers in which the steam pressure does not exceed 15 psi, and water boilers in which the pressure does not exceed 160 psi and temperature does not exceed 250°F.
You can use steam or hot water boilers to achieve high temperatures, but the infrastructure of the distribution systems will be different. Hot-water distribution systems are typically more efficient than steam distribution systems — because most hot-water systems are closed-loop, they need very little makeup water and there are no flash steam losses that are associated with steam system steam traps. Also, dissolved solids do not accumulate in the boiler of a hot-water system, so there is no need for blowdown.
Although some manufacturers use a similar design for their steam boilers and hot-water boilers, hot-water boilers are typically more efficient than steam boilers. Because steam boilers require much more heat to create steam, there is much more wasted heat energy than in hot-water systems. Some of this heat can be recaptured through the use of economizers, but buying an economizer will usually require an extra cost analysis to determine whether it is needed.
Hot-water systems have some drawbacks over steam systems. Hot water cannot provide temperatures as high as steam can. Also, hot water needs to be pumped to higher elevations, whereas steam can travel through piping to higher elevations without pumping because it travels from areas of higher pressure to areas of lower pressure.
A boiler is very simple in concept, but selecting the right boiler requires a lot of consideration in order to make sure that you are getting the desired output. When evaluating your boiler options, consider the long-term economics (i.e., money saved or money spent over time). Make sure you understand that the decisions you make today will affect costs in the future.
For the rest of this project, let’s focus on steam boilers.
Steam boiler system basics
A steam boiler is used to heat water into steam, and the steam is circulated through a closed-loop piping system to transfer heat and/or humidity to a process. If you have processes that require different steam pressures and temperatures within your system, you can use pressure reducers to modify the pressure of the steam for specific pieces of equipment. Alternatively, you can use separate steam boilers with different pressure ratings for each...
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