Measure 1.2.1 Test boiler efficiency on a continuing basis.

How to Select Test Equipment

Efficiency test equipment must be accurate. The objective is to measure efficiency within a fraction of a percent, because a difference this small can correspond to a large amount of energy. The entire range of possible boiler efficiencies corresponds to a range of flue gas oxygen content from zero to twenty percent. At the high end of the efficiency range, an error of two percent in flue gas oxygen results in a half percent error in combustion efficiency.

You don’t need the skill of a surgeon to do combustion efficiency testing, but you do have to be careful. Some types of test equipment require more finesse than others. Let’s look at what is available.

Oxygen and Carbon Dioxide Testers

The pioneer of combustion efficiency testers is the Orsat analyzer, a chemical testing apparatus in which flue gases are mixed with a liquid reagent that changes volume as a result of the reaction. The apparatus displays the volume change accurately, providing an accurate indication of combustion efficiency. Oxygen, carbon dioxide, carbon monoxide, and other gases can be analyzed.

The Orsat analyzer is simple and accurate. A portable version is available at moderate price. However, the apparatus is built of delicate glass tubing and requires a fine touch. Modern testers are much more rugged and much easier to use, so the Orsat analyzer is now a museum piece.

A similar chemical approach is used by the vintage Bacharach “Fyrite” tester, which is compact and made of plastic. One version of this unit tests for oxygen, and a different version tests for carbon dioxide. They are shown in Figure 3. The “Fyrite” is less accurate than the Orsat, but it is rugged and fairly easy to operate. It is the least inexpensive of all combustion gas analyzers. You need to read a table to use this device. These testers require replacing their chemicals after a certain number of tests, and the chemicals have a limited storage life.

In recent years, electronic testers have become available for analyzing flue gases. Some typical units are shown in Figure 4. Most electronic testers use an electro-chemical cell as the gas sensing element. A different cell is required for each gas being measured.

The most common sensor used in oxygen testers is a zirconium oxide element that develops a voltage difference across two sides if there is a difference in oxygen concentration. The zirconium oxide element has the advantage that it can be exposed to flue gases for thousands of hours.

Fig. 3 Chemical combustion efficiency testers One tests for flue gas oxygen, the other for carbon dioxide. The rubber hose draws a flue gas sample through a liquid in the tester. A chemical reaction changes the liquid volume, revealing the amount of gas. Fig. 4 Electronic flue gas analyzers These test for a variety of gases. Oxygen and carbon dioxide testers can calculate combustion efficiency directly.

The overwhelming advantage of electronic testers is that they provide an immediate and continuous readout simply by inserting a probe into the flue gases. This allows you to see the effect your of boiler adjustments immediately.

Some combustion efficiency testers avoid the need to use a table to calculate efficiency. They perform the efficiency calculation with a microcomputer and read out combustion efficiency directly. There is a potential for error with such testers because efficiency depends on the temperature of the inlet air as well as the temperature of the flue gas. Some testers use the temperature of the air surrounding the tester as the inlet air temperature. This introduces error because the air near the flue may be much warmer than the air entering the boiler. Better testers provide a separate remote thermometer for measuring the air inlet temperature while you take the flue gas sample.

Electronic testers of good quality are substantially more expensive than chemical testers. Still, the price is small compared to the benefit of keeping boiler efficiency optimized (unless your boilers are very small). As with most electronic equipment, the quality of electronic flue gas analyzers varies widely. The best of the lot may be capable of better accuracy than the inexpensive chemical testers. The worst of the lot are incapable of providing useful accuracy.

A weakness of all electronic gas analyzers is that they must be re-calibrated frequently because the sensor output drifts. Fortunately, oxygen testers may be self-calibrating by using the oxygen content of the atmosphere as a calibration point. Testers for other gases require calibration using pre-mixed gases. Since the inner workings of these instruments are invisible, calibration is an act of faith in the calibration gases.

Most inexpensive electronic combustion analyzers do not measure carbon dioxide directly. Some units provide a carbon dioxide reading, but this is derived by internal calculation from the oxygen measurement. If you want to use a carbon dioxide test to check the oxygen test, a value that is calculated from the oxygen measurement is useless. Measure carbon dioxide with a separate instrument designed for that purpose.

Testers for Carbon Monoxide and Specialized Gases

You can find electronic testers for gases other than oxygen or carbon dioxide, but these tend to be expensive and specialized. The most economical testers for carbon monoxide and specific pollutant gases use disposable chemical capsules for each type of gas. These units are reliable and easy to use. See Figure 5.

For example, one type of tester uses disposable chemical capsules in the form of slender tubes. The tester consists of a holder for the capsule that is equipped with a pump that draws the gas sample through the capsule. The gas concentration is indicated by the length of a color change inside the capsule.

Smoke Testers

You can measure smoke density cheaply and reliably with a tester that is constructed like a bicycle pump. It pulls a measured amount of flue gas through a piece of filter paper. You compare the blackness of the smoke spot on the filter paper to a chart that comes with the tester. See Figure 6.

Another method of testing smoke density is to install an optical densitometer in the flue. This is a simple device that shines a light beam across the flue. The amount of light that is absorbed by the smoke is measured by a photocell. This device provides a continuous reading. In fact, some older oil-fired boilers use this device as the primary means of adjusting air-fuel ratio.

Fig. 5 Chemical carbon monoxide tester The simple device on the right draws a flue gas sample through a glass tube containing a chemical that reacts with carbon monoxide to change color. Each tube is used one time. By using different chemicals, the device can test for other flue gases. Fig. 6 Smoke density tester This device, similar to a bicycle tire pump, sucks a flue gas sample through a piece of filter paper. The resulting smoke spot is compared with the chart behind the pump.

Thermometers

The thermometer that you use for flue temperature measurements must be accurate. An error of 40°F in the flue temperature measurement can result in a one percent error in boiler efficiency. Combustion efficiency testing is no place for a shirt pocket thermometer. Use a laboratory grade liquid-in-glass thermometer. Mercury-in-glass thermometers are available with maximum temperatures up to 1,200°F. To read the thermometer most accurately, get one that has a maximum temperature close to the maximum temperature of the flue gases. A 500°F (or 300°C) thermometer is appropriate for efficient boilers. A 750°F (or 400°C) thermometer is adequate for most others. Good mercury thermometers are inexpensive, but they are fragile and difficult to read. Buy a couple of spares, along with a protective storage case.

Dial thermometers use a bimetallic sensing element that may drift with time. The only merit of dial thermometers is that they are easy to read. In a pinch, you can calibrate or check the low temperature end of a bimetallic thermometer by placing it in boiling water, which provides a reliable standard of 212°F (100°C) at sea level. If you use this method, correct for your altitude, because water boils at lower temperature at higher altitudes.

Electronic thermometers have high-tech appeal, but they may be inaccurate, just a toy dressed up with a digital output. Electronic temperature sensing elements are inherently non-linear. Most of the cost of the electronic thermometer goes to compensating for the weaknesses of the sensing element. Some electronic thermometers are accurate, but you typically have no way of distinguishing these units from the toys. Even with an electronic unit of high quality, be aware of the guaranteed accuracy range. For example, a one percent error in absolute temperature corresponds to an error of about 10°F at typical flue temperatures.

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