Measure 1.2.1 Test boiler efficiency on a continuing basis.

Avoid Error Due to Extraneous Air

The combustion efficiency test involves measuring a small quantity of residual oxygen in the combustion gases. In a well tuned boiler, the gases contain less than one percent of free oxygen. Therefore, the test is seriously falsified by any outside air that enters the test probe.

Be careful to avoid getting outside air into the probe at the probe hole. Sloppy technique may allow outside air to travel along the shaft of the probe and enter the tip. To avoid this, aim the probe toward the boiler, preferably by bending the probe to that it faces into the flue gas stream well inside the flue. Wrap a piece of cloth around the probe to act as a gasket between the probe and the edge of the hole. It’s the same general idea as safe sex. See Figures 7 and 8.

If the flue contains a draft hood or atmospheric damper, tap the gas sample well below the point of entry of outside air. Turbulence in the flue may cause ambient air to backflow to the probe. Such backflow is especially likely to occur near bends in the ductwork.

Fig. 7 Small furnace, small tester Combustion efficiency testing is easy. It requires only a small hole in the flue. However, it is critical to keep air from leaking through the hole into the probe. Fig. 8 Large boiler, small tester Combustion efficiency testing is done the same way on all boilers and furnaces. Here, a test is conducted high on a large stack. The probe is located at the arrow, the tester is in the foreground. All these people are relaxing. It takes only one person to run the test.

Leakage of extraneous air into the flue is likely to occur if the boiler has an induced-draft fan, i.e., a fan that sucks combustion gases through the boiler rather than blowing them through. If you insert the probe at a point between the boiler outlet and the induced-draft fan, the induced-draft fan will draw outside air into the test hole. Air will enter at any leaks that exists in the flue ahead of the test probe. To avoid this problem, examine the boiler breeching ahead of the test hole, and plug any leaks. Drill the test hole as close to the boiler or economizer outlet as possible.

Errors from Sensing the Wrong Gases

A particular type of sensor may respond to more than one substance in the flue gas. This leads to a false reading if you are expecting to test for one substance but your tester registers another. The most common problem is that hydrogen and hydrogen sulfide register as carbon monoxide with some electrochemical carbon monoxide sensors. This is not a problem with clean fuels, which produce only carbon dioxide and water vapor as their principal end products. Beware of false readings when burning fuels, such as high-sulfur oil and coal, which produce gases other than carbon dioxide and water.

Errors Due to Internal Steam Leaks

If a boiler has an internal steam leak, the steam mixes with the flue gases. The steam is cooler than the combustion gases and lowers the temperature at the test point, giving an erroneously high efficiency calculation. If the steam leak is large, it may become apparent from an efficiency reading that seems abnormally high, especially at low loads. The rate of steam leakage remains constant at all firing rates, as long as the boiler pressure remains constant. Therefore, the error caused by the escaping steam is larger at low firing rates. If a boiler has a steam leak large enough to be detected by an efficiency test, it should be shut down immediately and repaired.

How to Test Boilers with Flue Gas Recirculation

To reduce the production of nitrogen oxides during the combustion process, many boilers now recirculate a portion of the flue gas. Exhaust gas is drawn from the flue with a fan and it is injected back into the boiler at the burner. The flue gas contains little oxygen and it is relatively cool. The purpose of this is to reduce the peak combustion temperature, since nitrogen oxides are formed mainly at high combustion temperatures.

Flue gas recirculation does not cause any problems with combustion efficiency testing. Efficiency testing works by measuring what goes into the boiler and what comes out. Recirculation is a process that occurs entirely within the boiler. Therefore, any effect that recirculation may have on efficiency is included in the flue gas analysis. Recirculation does not affect where you should tap your flue gas sample.

How to Test Boilers with Economizers and Air Preheaters

Economizers and air preheaters (covered in Subsection 1.7) are devices for capturing additional heat from the flue gases after they leave the boiler. The recovered heat typically is used to heat feedwater or combustion air, respectively. These devices lower the flue gas temperature, so they affect the efficiency test.

With both types of units, the proper place to measure the flue gas temperature is at the outlet of the heat recovery device, rather than at the outlet of the boiler. With a combustion air preheater, the place to measure inlet air temperature is at the inlet to the air preheater, not the inlet to the burner. In other words, do not measure the temperature of the heated air produced by the preheater.

With a combustion air preheater of the heat wheel type, the flue gas test may yield an erroneously high efficiency. This is because the rotation of the wheel carries a certain amount of cool ambient air into the exhaust stream. Some types of heat wheels have purge sections that deliberately recirculate a portion of the incoming air into the exhaust from the heat wheel. Furthermore, different portions of the heat wheel are at different temperatures, so the flue gas temperature leaving the heat wheel is non-uniform. There is no practical way to make an accurate correction for these factors.

Preparation and Testing Conditions

When you first start your efficiency testing program, do a test under optimum conditions to establish a baseline for future tests. Do the baseline test with the boiler in peak condition, and across the entire load range of the boiler. Compare later tests to this baseline. When the boiler efficiency falls too far below your measured baseline, it is time to take corrective action.

For this baseline test, expect to spend considerably more effort and time inspecting the boiler plant and correcting deficiencies than in doing the testing itself. Adjust, clean, and repair all parts of the boiler that affect efficiency. Clean firesides and watersides. Clean and adjust burners. Remove all looseness from air and fuel control linkages. Check the combustion air supply. Adjust flue draft. And so forth. Refer to the other Subsections for the Measures that optimize the efficiency of your particular types of boilers.

Do the tests while the boiler has a steady load. If the system is used primarily for comfort heating, you may not have enough load to test your boilers during warm weather. It is easier to conduct tests in a plant that has more than one boiler, so the boiler being tested can shift load to and from the other boilers.

Do not attempt to create a load on the boiler by warming it from a cold state. This produces an efficiency reading that is artificially high, because the flue gases are being cooled by the mass of the boiler and water.

You have to make certain adjustments to the boiler plant while the boiler is operating. For example, determine the optimum fuel oil heater temperature by adjusting the fuel oil temperature for peak boiler efficiency. Make these adjustments as the test progresses.

How Often to Repeat Efficiency Tests

The most common reason for a decline in efficiency is accumulation of fouling on firesides, watersides, and burners. Fouling accumulates at a fairly predictable rate, if the boiler has no defects. The appropriate interval for combustion efficiency testing depends on the type of boiler and the type of fuel. A simple gas-fired boiler with atmospheric burners may hold its efficiency for years without adjustment, and with minimal maintenance. On the other hand, a large pressure-fired boiler burning a variety of fuels may have a noticeable drift in efficiency over a period of days or weeks.

Even if the boiler has recently been cleaned, test for efficiency and make adjustments whenever anything changes that can reduce efficiency. For example, test efficiency when changing from one type of fuel to another, when changing batches of fuel, and when slack in the linkages controlling air-fuel ratio becomes apparent. If controls are subject to drift, determine the length of time that it takes a significant error to accumulate. (If drift is serious, improve the control system as recommended by Measures 1.3.2 and 1.3.3.)

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These excerpts shows the level of detail and accuracy that you'll find in the Energy Efficiency Manual by Donald Wulfinghoff. This 1500-page manual offers the most comprehensive coverage of energy-saving measures for buildings ever assembled under one cover. For more information and to view the complete table of contents, find the Energy Efficiency Manual in the Oikos Bookstore.