The efficiency of a boiler is an important part of a purchase evaluation since the annual cost of fuel can easily be 2 to 3 times the installed cost of the equipment. Hence, a difference in efficiency and the resultant difference in fuel cost can easily offset the difference of installing the product. In many cases, the fuel savings in the first year have exceeded the difference of the installed cost, paying for itself. And the savings continue to accrue year-after-year.
While it is important to consider efficiency in an equipment purchase, it is crucial to understand efficiency so the purchaser can be assured that values are being compared accurately among comparable products. Steam boiler efficiency is complex when all of the elements that effect efficiency are considered and a complete thermodynamic analysis is performed. Fortunately, it is not necessary to understand the process in detail, but a basic understanding of the terms can help ensure accuracy when comparing and evaluating efficiency. These factors are discussed in the context of the discussion on efficiency terms (below).
There are several terms used to qualify efficiency when used in the context of an industrial steam boiler. These include simple efficiency, boiler efficiency, thermal efficiency, combustion efficiency and fuel-to-steam efficiency. The terms simple efficiency and boiler efficiency are essentially meaningless by themselves since they must be qualified in order to understand their significance.
In general, thermal efficiency refers to the efficiency of a thermal process. This is opposed to mechanical efficiency, the efficiency of a mechanical process. When used in conjunction with boilers, thermal efficiency may refer to the efficiency of the heat exchanger. Hence, this term is not significant for purposes of comparing one boiler (steam generator) to another. While the thermal efficiency of the heat exchanger is an important factor, its importance lies in its contribution to the fuel-to-steam efficiency.
While the terms simple efficiency and thermal efficiency are not meaningful for comparing one boiler to another, the terms combustion efficiency and fuel-to-steam efficiency are meaningful. Fuel-to-steam efficiency is the more significant of the two terms, but is difficult to measure or calculate in real world application. Therefore, combustion efficiency, which is easily computed using a combustion gas analyzer, is frequently used for performance comparison purposes.
Combustion efficiency equals the total heat released in combustion, minus the heat lost in the stack gases, divided by the total heat released. For example, if 1000 BTU/hr is released in combustion and 180 BTU/hr are lost in the stack, then the combustion efficiency is 82%: (1000 – 180)/1000 = 0.82 or 82%.
Fuel-to-steam efficiency is the most important because it is a measure of the energy that is converted to steam and that is, after all, the reason a user installs a steam boiler—to produce steam. Fuel-to-steam efficiency is equal to combustion efficiency less the percent of heat losses through radiation and convection. In the example above, 20 BTU/hr are lost to convection and radiation so the convection and radiation losses are 2%: 20/1000 = .02 or 2%. If the combustion efficiency for this same case is 82% then the fuel-to-steam efficiency is 80%: 82% – 2% = 80%.
Operating efficiency: Each of the terms discussed above refer to the efficiency of a boiler when operating at a fixed condition; for instance, at 100% load, with specified air and feedwater temperatures, etc. These efficiencies are unquestionably important, but there are operational factors that affect the annual fuel bill. These factors can have an effect that may be greater than the difference of a point or two in the efficiency of the equipment when, for example, operating at 100%.
The boiler blowdown considerations article provides further information on the topic of blowdown and how it can affect operating efficiency.