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An efficient Clayton steam boiler

A steam boiler with Combustion Efficiency and Fuel-to-Steam Efficiency

Steam Basics – Boiler Efficiency

The Efficiency of a boiler should be 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. Therefore, a difference in efficiency and the resultant difference in fuel cost can easily offset a difference in capital cost. In many cases, the fuel savings in the first year alone can exceed a difference in capital cost and, of course, fuel savings are on-going – year after year, after year.

While it is important to consider efficiency in an equipment purchase, it is equally important to understand efficiency to the point that the purchaser can be assured that values are being compared on an apples-to-apples basis. The subject of efficiency for a boiler is rather complex when all of the elements that affect 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 a good apples-to-apples efficiency evaluation. These factors are examined in the context of a discussion on efficiency terms.

Efficiency Terms used to qualify efficiency in the context of a boiler include simply efficiency, boiler efficiency, thermal efficiency, combustion efficiency and fuel-to-steam efficiency.

The terms, Efficiency and Boiler Efficiency, by themselves are, essentially, meaningless since they must be qualified in order to understand their significance.

In general, the term, Thermal Efficiency refers to the efficiency of a thermal process. This is as opposed to Mechanical Efficiency – the efficiency of a mechanical process. When used in conjunction with boilers, Thermal Efficiency sometimes refers to the efficiency of the heat exchanger. In any event, this term is not significant for purposes of comparing one boiler, or 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 Efficiency and Thermal Efficiency are not meaningful for comparing one boiler to another, the terms Combustion Efficiency and Fuel-to-Steam Efficiency are. Of these, Fuel-to-Steam Efficiency is the most significant but is difficult to measure or calculate in real world situations. Therefore, Combustion Efficiency that can be easily computed using a combustion gas analyser 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 300 kW are released in combustion and 53 kW are lost in the stack, then the combustion efficiency is 82%: (300 – 53) / 300 = 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. For example, as in the example above, 6 kW are lost to convection and radiation then the convection and radiation losses are 2%: 6 / 300 = .02 or 2%. If the combustion efficiency for this same case is 82% then the Fuel-to-Steam efficiency is 80%: 82% - 2% = 80%.

(Note: When comparing efficiencies it is important to know if the efficiency is based on the Higher Calorific Value (HCV, also known as the Gross Calorific Value GCV) or Lower Calorific Value (LCV, also known as the Nett Calorific Value NCV) of the fuel. Both are essentially "correct" but comparing an efficiency based on HCV to one based on LCV would not be correct. In Europe they are, typically, based on the LCV and result in a higher value than when based on HCV. The general relationship is: Efficiency based on LHV = Efficiency based on HHV X 1.11 for natural gas and X 1.06 for diesel fuel oil.)

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 and can have an effect that may be greater than the difference of a point or two in the efficiency of the equipment when, for instance, operating at 100%. These factors are discussed on the Fuel Savings page.

The Blowdown Considerations article provides further information on the topic of blowdown and how it can affect operating efficiency.

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