Boiler efficiency is extremely important. If your boiler isn’t burning fuel at a satisfactory rate, or if the heat is being lost through the surrounding system without being put to use, you end up with a lot of waste and can even run into problems meeting emissions standards.
The thing is, “boiler efficiency” is a loose term that can mean several different things. It’s often used to refer to thermal efficiency or fuel-to-steam efficiency, for instance, but it can also be used as shorthand for combustion efficiency. To help clarify, we thought we would summarize each of the types of boiler efficiency. They all calculate efficiency differently and can therefore that have their own particular economic implications.
Combustion efficiency measures how effectively a boiler transfers the chemical energy stored in fuel into usable heat. Reaching optimum combustion efficiency typically requires pumping additional air into the combustion chamber so that all fuel can be burned.
Without enough oxygen, some fuel will be left unburned and end up as soot, smoke, or carbon monoxide. Too much oxygen, on the other hand, will create excessively hot flue gas, which isn’t able to transfer all of its heat into steam production. According to combustion efficiency calculations, a boiler is considered efficient when it has low levels of unburned fuel and does not require much extra air.
Thermal efficiency measures how well a boiler’s heat exchanger transfers heat from the combustion process to the water or steam in the boiler. A simple ratio expressed as a percentage, thermal efficiency is basically the input energy transferred to the water or steam heating process divided by the output energy of the combustion process.
Thermal efficiency’s shortcoming as a measure of boiler efficiency is that it only measures the effectiveness of the heat exchanger. Because thermal heat can also be lost through radiation and convection via other boiler components, such as the shell or water column, thermal efficiency is limited as a calculation for measuring boiler efficiency.
Correcting the deficiencies of thermal efficiency calculations, fuel-to-steam efficiency attempts to measure the overall efficiency of a boiler’s transfer of heat from the combustion process to the water or steam in the boiler. In doing so, it measures the efficiency of the heat exchanger while also accounting for radiation and convection losses through other areas in the boiler system.
Fuel-to-steam efficiency (also referred to as “fuel efficiency”) can be calculated in two ways: the Input-Output Method or the Energy Balance Method.
The Input-Output Method divides the boiler output (measured in BTUs), by the boiler input (also measured in BTUs) and then multiplies the result by 100 so that it can be expressed as a percentage.
Energy Balance Method
The Energy Balance Method consists of subtracting the actual stack, radiation, and convection losses, expressed in percentages, from an ideal 100 percent efficiency, while also accounting for any system efficiency credits.
Stack losses can be ascertained by measuring the stack temperature, or the temperature of the combustion gases leaving the boiler. The higher the temperature of the flue gases, the less heat transferred from the combustion process to the water or steam.
When in operation, boilers unavoidably get hot themselves. The heat that they give off, which is lost from the process, is radiation loss. Radiation loss is generally viewed as a constant for a particular boiler, although there is a lot of variation across boilers. It is measured in BTU/hr.
In any situation, boilers come into contact with air flowing across their exterior surfaces. This airflow cools the boiler through convection, dissipating heat and removing it from the process. Like radiation loss, convection loss for any boiler installation is typically established as a constant and is quantified in BTU/hr.
The mathematical relationship between the two to calculating boiler efficiency can be expressed as follows:
Any reputable boiler manufacturer or boiler service provider should know the difference between these terms and should be willing to specify which type of efficiency they’re talking about when they refer to “boiler efficiency.” All of them have their uses, whether for regulation compliance or system optimization. It’s just a matter of knowing which one should be used when.
source - nationalboiler , Wikipedia