Measure 1.2.1 Test boiler efficiency on a continuing basis.

• What "Efficiency Testing" Means in Boilers
• Limitations of the Combustion Efficiency Test
• How to Select Test Equipment
• Oxygen and Carbon Dioxide Testers
• Testers for Carbon Monoxide and Specialized Gases
• Smoke Testers
• Thermometers
• Avoid Error Due to Extraneous Air
• Errors from Sensing the Wrong Gases
• Errors Due to Internal Steam Leaks
• How to Test Boilers with Flue Gas Recirculation
• How to Test Boilers with Economizers and Air Preheaters
• Preparation and Testing Conditions
• How Often to Repeat Efficiency Tests
• Economics
• Traps and Tricks

Periodic efficiency testing is an important part of the management of any boiler plant, for these reasons:

• a large fraction of a facility’s total energy consumption flows through its boilers, so even a small drop in efficiency represents a large amount of energy and cost in absolute terms. In facilities where boilers have operated for long periods of time without being tested, it is not uncommon to find that efficiency has fallen by five or ten percent, which may represent a very large cost.
• efficiency testing is the most accurate indicator for adjusting the boiler and its auxiliary equipment, such as adjusting the air-fuel ratio (covered in Subsection 1.3). Efficiency testing tells you the “bottom line” economic performance of the boiler, unlike indirect clues, such as flame color.
• you can localize most boiler problems by knowing how to exploit the full range of efficiency test methods. Efficiency tests are to boilers what blood tests are to human beings.
• efficiency testing is the first step in estimating the benefit of potential boiler improvements, such as adding an economizer or an air-fuel control system.

Boilers are vulnerable to conditions, such as tube fouling, that may reduce their efficiency over relatively short periods of time. Therefore, you should test boiler efficiency on a regular basis. Fortunately, boiler testing is easy. It is practical for most facilities to have at least one person who is proficient in testing. This individual need not be a boiler operator, or even a member of the physical plant staff.

You may need several types of efficiency tests to get a complete picture of boiler performance. There is one main type of test, called a “combustion efficiency” test. You may have to supplement this with one or two specialized tests, such as a test for carbon monoxide, to get a complete picture of your boilers’ performance. This Measure tells you how to select the appropriate tests for your boilers, how to do them, and the weaknesses of each method. In a short time, you can be testing your boiler efficiency with confidence.

What "Efficiency Testing" Means in Boilers

Efficiency is defined as the ratio of the useful output energy produced by a system to the raw energy input into the system. In principle, it is possible to test the overall efficiency of a boiler plant. You could do this by measuring the energy content of the steam or hot water that is exported by the boiler plant and the total quantity of energy that is input to the boiler plant, including fuel for the boilers and other energy for the auxiliary equipment.

In actual boiler plants, such a comprehensive test is impractical. Measuring steam or hot water energy with reasonable accuracy, say within one percent, is possible only under laboratory conditions. Field measurements of flow are vulnerable to error, even if they are done by expensive consultants. Measuring input energy is also subject to error. You would have to analyze the energy content of your fuels, make corrections for ambient conditions, and create very stable test conditions. This is not possible under realistic conditions.

Fortunately, you don’t need to measure the total efficiency of a boiler plant if your objective is to tune up the plant’s efficiency, rather than running a testing laboratory. You can use an easy procedure, called a “combustion efficiency” test, to measure the aspects of boiler efficiency that cause most losses in normal operation. The “combustion efficiency” test determines how completely the fuel is burned, and how effectively the heat of the combustion products is transferred to the steam or water.

The efficiency of the other boiler plant components, such as pumps, fans, and motors, tends to remain constant, so you don’t need to test their efficiency on a continuing basis.

Limitations of the Combustion Efficiency Test

The combustion efficiency test is your primary tool for monitoring boiler efficiency. You can achieve accuracy in the range of one percent of efficiency if you do the testing carefully and use equipment of good quality. However, be sure that you understand what the test is telling you. The combustion efficiency test does not account for:

• Types of auxiliary equip standby losses. You perform a combustion efficiency test when the boiler is operating under a steady load. Therefore, the combustion efficiency test does not reveal standby losses, which occur between firing intervals. You cannot measure standby losses directly. You have to estimate them from the type of boiler and the firing schedule. See Measure 1.5.3 for an explanation of standby losses.
• heat loss from the surface of the boiler to the surrounding space. As a practical matter, you cannot measure this loss. Typical estimates state that the loss from surface radiation is about two percent of the boiler’s full load energy consumption. (Subsection 1.11 shows you how to reduce surface heat loss and recover the heat.)
• blowdown loss. The amount of energy wasted by blowdown varies over a wide range. (Subsection 1.8 shows you how to minimize blowdown loss and how to recover heat from blowdown.)
• soot blower steam. The amount of steam used by soot blowers is a variable that depends on the type of fuel and the judgment of the staff. (Subsection 1.6 shows you how to minimize soot blower steam consumption.)
• auxiliary equipment energy consumption. The combustion efficiency test does not account for the energy use by auxiliary equipment, such as burners, fans, and fuel pumps. (To minimize this energy consumption, see the Subsections that cover the specific treatment.)

While the combustion efficiency test does not give the overall efficiency of the boiler plant, it is by far the easiest method of tracking moment-to-moment, day-to-day, and season-to-season variations of boiler efficiency. Combustion efficiency testing tells you how far boiler efficiency drifts away from the best efficiency that you can achieve when the boiler is fully tuned up.

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