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.
Several terms are used to qualify efficiency when used in the context of a boiler. These 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 it is difficult to measure or calculate in real-world situations. Therefore, combustion efficiency, which can be 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 1,000 BTU/hr are released in combustion and 180 BTU/hr are lost in the stack, then the combustion efficiency is 82 percent: (1,000 - 180)/1,000 = 0.82 or 82 percent.
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. To continue the example above, suppose 20 BTU/hr are lost to convection and radiation. Then the convection and radiation losses are 2 percent: 20/1,000 = 0.02, or 2 percent. Because we know that in this example, combustion efficiency is 82 percent, we can calculate the fuel-to-steam efficiency by subtracting the heat losses due to convection and radiation from the combustion efficiency. Numerically, it is 82 percent - 2 percent, which equals 80 percent fuel-to-steam efficiency.
A word of caution: when comparing efficiencies, it is important to know if the efficiency is based on the high heat value (HHV) or low heat value (LHV) of the fuel. Both are essentially “correct,” but comparing an efficiency based on HHV to one based on LHV would not be correct. In the United States, boiler efficiencies are typically based on the HHV. In Europe, they are typically based on the LHV and result in a higher value than when based on HHV. The general relationship is: Efficiency based on LHV = Efficiency based on HHV multiplied by 1.11 for natural gas and multiplied by 1.06 for diesel fuel oil.
Each of the terms discussed above refers to the efficiency of a boiler when operating at a fixed condition such as at 100 percent 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 affect that may be greater than the difference of a point or two in the efficiency of the equipment when, for instance, operating at 100 percent. Operational factors include boiler design, time required to startup, steam quality and level of blowdown required.
The steaming rate, or the rate at which a boiler produces steam, normally is expressed in terms of pounds per hour or kilograms per hour. It is frequently misunderstood, and such a misunderstanding can lead to the purchase of the wrong size boiler. It is, therefore, essential that the steaming rate be qualified when selecting a boiler size. The three common steaming rate terms are:
From and at 212°F (100°C) Steaming Rate.
Gross Steaming Rate.
Net Steaming Rate.
From and at 212°F is the steaming rate for a boiler producing steam, at the outlet flange, at 212°F, and 0 psig, with feedwater at the inlet flange at 212°F and 0 psig. This is the most common steaming rate term and is used most often when steaming rate information is provided. And, by definition, one boiler horsepower (BHP) is equivalent to 34.5 lb of steam per hour, from and at 212°F.
Gross Steaming Rate is the rate at which a boiler produces steam, at the outlet flange, based on application specific feedwater conditions at the inlet flange and application specific steam conditions. The gross steaming rate typically differs from the From and at 212°F steaming rate because both the feedwater inlet and the steam conditions are different than 212°F and 0 psig. A typical application may, for instance, have feedwater at 190°F (88°C) and produce saturated steam at 100 psig (338°F).
Because the inlet temperature is less than 212°F and the outlet temperature is greater than 212°F, the amount of heat needed to produce a pound of steam, at these conditions, is greater than the amount needed to produce a pound of steam with inlet and outlet temperatures of 212°F. The gross steaming rate is, therefore, frequently less than the From and at 212°F steaming rate. It may, however, actually be greater if the feedwater receiver is a pressurized deaerator that heats the feedwater to a temperature above 212°F.
Net Steaming Rate is the steaming rate at which a boiler produces steam to your plant or process and, thus, is the most important steaming rate to consider. Net steaming rate differs from gross steaming rate in that it takes into account the amount of steam needed to heat the feedwater in the feedwater receiver (deaerator or hotwell): Specifically, the net steaming rate equals the gross steaming rate minus the steaming rate to the feedwater receiver. Except for some very unusual applications, the net steaming rate is less than the gross steaming rate or the From and at 212°F steaming rate.
Take, for example, a 100 BHP boiler operating with 100 percent makeup water at 60°F (16°C) and producing steam at 125 psig. In this case, the From and at 212°F (100°C) Steaming rate is 3,450 lb/hr, but the net steaming rate is only 2,874 lb/hr -- 17 percent less than the From and at 212°F steaming rate.
The effect of feedwater heating is applicable in all applications and thus should always be considered. There is another factor that has an effect and can be significant in some applications: blowdown, which is required for the boiler to operate effectively. In this case, “blowdown” refers to the amount of water that must be removed from the boiler system, on a regular basis, in order to control the level of total dissolved solids (TDS) in the boiler. Water that is removed to control TDS has been heated, and the amount of energy needed to heat this water reduces the amount of energy that is available to produce steam.
In summary, users should be certain to qualify steaming rates when using them to define the size of a boiler. Boiler horsepower is a specific term and no further information is needed to select the size of a boiler. However, if a steaming rate is used to specify boiler size, then which steaming rate is being used must be qualified.
This article originally appeared with the title "Understanding Boiler Efficiencies" in the January 2006 issue of Process Heating.