Boiler efficiency is undoubtedly the most important and widely used term when the subject of purchasing a new industrial steam system begins. Unfortunately, the goal of some boiler manufacturers at times seems to be to make the process of understanding efficiency as difficult as possible. (It is not uncommon to see a cluster of terms and equations provided in the language of a seasoned engineer. Often, the differences among these terms are not clear to a casual user.) While there are numerous factors that can contribute to boiler efficiency, having a perfect understanding of a boiler’s efficiency is not critical. A general understanding of the terminology and principles, however, can go a long way in helping prospective users make a confident decision.
The following article will break down the subject of boiler efficiency and how it relates to making a new purchase. It also will include tips for the prospective buyer as well as a few of the hard questions to ask any potential supplier.
Why Boiler Efficiency Matters
Boiler efficiency is important because of the fuel bill. In the average industrial scenario, the final purchase cost of a new steam system, including delivery and installation, pales in comparison to the amount that will be spent on fuel over the lifetime of the boiler. It is not unheard of to spend two to three times the purchase amount in fuel in the first year alone. This represents an annual expense that will be incurred as long as the boiler is in operation.
This is why efficiency is so important. It directly impacts the annual operating costs.
As little as 1 percent difference in terms of efficiency can equal tens of thousands of dollars. While the natural inclination may be to pursue a lower cost upfront, this decision could result in hundreds of thousands of dollars in fuel bills in the years to come.
With fuel supply dominating nearly 80% of your operational costs, maintaining a high level of efficiency is important to maximizing your annual savings.
What Is Efficiency?
Boiler efficiency represents the difference between the energy input and output, expressed as a percentage. Another way to think of it is as how much steam a boiler is able to produce relative to the fuel usage.
As a term, however, “boiler efficiency” often is used as a general blanket description to summarize any and all of the factors that may be contributing. As a result, when the term “boiler efficiency” is used, it is important to understand what is or is not being represented. This is something your potential supplier should be able to provide to you.
For an existing boiler, to get a quick breakdown of the efficiency rating, simply take the boiler’s output and divide by the input. This will provide a starting point for any inquiries. Keep in mind, however, that this does not take into consideration all of the factors that may affect efficiency.
Oftentimes, a supplier may provide this number as the final efficiency rating, so be sure to ask the supplier. Further, energy inputs and outputs should be listed clearly on any boiler company’s website in their technical documentation, so anyone can run the numbers themselves. If these numbers are difficult or impossible to find, ask the supplier why.
To get a quick breakdown of your efficiency rating, simply take your boiler’s output and divide by your input.
Terminology and Calculations
Several considerations contribute to determining a boiler’s true efficiency. Here are some of the terms that may be encountered when discussing this subject.
Thermal Efficiency. When used in reference to steam boilers, thermal efficiency refers to the effectiveness of the heat exchanger or, more directly, how well heat is being transferred from the fuel supply into usable steam. The focus of this measurement is on the heat exchanger and not on the boiler, however, even though this term is often mistakenly applied to boilers. Thermal efficiency is a factor in the overall efficiency, but it is not the be all and end all. As a result, it is not a useful term when comparing one boiler to another.
Combustion Efficiency. A measure of how well your boiler is burning fuel, combustion efficiency specifically refers to the total heat released during combustion minus heat that is lost during the combustion process.
Fuel-to-steam efficiency is the best possible measurement of efficiency. It considers the final amount of energy that is being converted into steam, taking into consideration all of the factors that contribute to the boiler’s overall efficiency.
Under ideal circumstances, a boiler’s combustion will result in low levels of unburned fuel and low levels of excess air. In real-world applications, however, combustion conditions are never perfect. They will always require some measure of excess air to burn completely, and finding this balance becomes crucial. Not enough air will result in unused fuel and the production of soot and carbon monoxide. Too much air will result in heat losses. Both scenarios have a negative impact on cost savings.
Taking a combustion efficiency measurement is done easily through an analysis of the combustion air by means of a combustion analyzer. This is a good way to compare performance between boilers.
Heat Losses. While not technically a measure of efficiency, heat losses affect every boiler and contribute to the overall efficiency in the form of wasted energy. Three types of heat loss can occur:
- Convection losses, which refer to heat that is lost from airflow surrounding the boiler.
- Radiation losses, which refer to losses from heat that radiates from the boiler shell.
- Stack losses, which refer to heat that is lost by exhaust gases that exit the boiler stack, having never transferred their heat into the feedwater.
When entering a boiler room, do you immediately notice how hot the room is? Do you hesitate to get too close to the boiler out of fear of burning yourself? These are signs that a boiler is experiencing high heat loss.
Fuel-to-Steam Efficiency. This is the best possible measurement of efficiency. It considers the final amount of energy that is being converted into steam, taking into consideration all of the factors that contribute to a boiler’s overall efficiency. When a potential supplier begins to discuss products, this is what should be presented.
Efficiency is so important because in the average industrial scenario, the final cost of your new steam system, delivery and installation pales in comparison to the amount you will spend in fuel over the lifetime of the boiler.
A Few Words of Caution
As can see seen, there is a great deal to cover when it comes to boiler efficiency. Moreover, there are other factors to take into consideration — examples include feedwater temperature, steam pressure and blowdown rates — when determining the true boiler efficiency. Many industry tricks can be used to artificially boost efficiency numbers, and you do not want to find out too late that you are losing money.
The best way to avoid this situation is to ask any potential supplier to provide a real-world efficiency rating for their boiler. Different manufacturers may call this by different names, but what you should get is a realistic efficiency rating based on the conditions of your plant. Do not settle for a manufacturer providing you with a hypothetical rating based on “ideal conditions.” (The easiest way to boost efficiency ratings is to run the boiler under perfect conditions with ideal water temperature, perfect fuel combustion — and without any heat losses whatsoever.) These conditions seldom exist in the real world, so make sure that the number being provided is realistic to you and your needs. What temperature will your feedwater be? At what pressure will you be operating?
There is an old saying about comparing apples to apples and how it relates to drawing an even comparison. Comparing a real-world rating with one under “ideal conditions” is not an equal assessment. Getting this information from multiple suppliers allows you to make an accurate decision among them.
This article has only just scratched the surface when it comes to breaking down boiler efficiency. You do not need an encyclopedic knowledge, however, to make an informed decision. By following a few basic steps and asking a few questions of any sales personnel, you will be well on your way to making an informed decision about your new steam system. PH
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