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to 200 percent of excess air is used with a resulting great loss of heat. The maximum excess air required should be 25 percent. Insufficient air gives incomplete combustion with a consequent loss in unburned heat units and an excess of air cools the flame and carries away large quantities of heat in the flue gases. The air excesses for various boiler efficiencies are given in Table 2.
Table 1.-POUNDS OF AIR PER POUND OF OIL AND RATIO OF AIR SUPPLIED TO THAT CHEMICALLY REQUIRED.
Figure 1 shows the heat losses due to excess air in burning
It is well known that with charcoal or coke a very intense combustion can be maintained with very little smoke and within a comparatively small space. The reason for this is that even at the highest temperature the fuel is solid. Therefore, no carbon can leave the fuel bed except as a constituent of CO or CO,. When carbon is not supplied with sufficient air for complete combustion it burns to CO instead of to CO,. When carbon is burned only to CO it provides only two-thirds of the heat which it is capable of yielding up when burned to CO2. When completely burned, fuel which consists of 100 per cent carbon will show a percentage by volume of 20.7 CO, in the flue gases. Under good furnace conditions, when burning fuel oil which contains a high percentage of carbon the average theoretical CO2 percentage of flue gases is from 13 to 14 per cent. Table 3 shows the corre
sponding losses that occur when various percentages of CO., are indicated in the flue gases.
In order to determine whether the fuel oil is obtaining the correct amount of air, it is necessary to analyze the flue gases. A flue gas analysis gives the proportion by volume of the principal constituent gases produced by the combustion of any fuel. The gases usually determined in such an analysis are CO2, O, and CO. The volume remaining after these gases are removed is considered to be nitrogen (N).
The apparatus most commonly used for flue gas analysis is known as the Orsat. The Orsat apparatus (See fig. 2) is de
Table 2.-BOILER EFFICIENCY FOR EXCESS AIR SUPPLY (OIL
scribed as follows: "The burette "a" is graduated in cubic centimeters up to 100, and is surrounded by a water jacket to prevent any change in temperature from affecting the density of the gas being analyzed. For accurate work it is advisable to use four pipettes, "b," "c," "d," "e," the first containing a solution of caustic potash for the absorption of carbon dioxide, the second an alkaline solution of pyrogallol for the absorption of oxygen, and the remaining two an acid solution of cuprous chloride for absorbing the carbon monoxide. Each pipette contains a number of glass tubes, to which some of the solution clings, thus facilitating the absorption of the gas. In the pipettes "d" and "e," copper wire is placed in these tubes to re-energize the solution as it becomes weakened. The rear half of each pipette is fitted with a rubber bag, one of which is shown at "k," to protect the solution. from the action of the air. The solution in each pipette should be drawn up to the mark on the capillary tube. The gas is drawn
into the burette through the U-tube "h," which is filled with spun glass, or similar material, to clean the gas. To discharge any air or gas in the apparatus, the cock "g" is opened to the air and the bottle "f" is raised, until the water in the burette reaches the 100 cubic-centimeter mark. The cock "g" is then turned so as to close the air opening and allow gas to be drawn through "h," the bottle "f" being lowered for this purpose. The gas is drawn into the burette to a point below the zero mark, the cock "g" then being opened to the air and the excess gas expelled until the level of the water in "f" and in "a" is at the zero mark. This operation is necessary in order to obtain the zero reading at atmospheric
FIG. 1.-Curves showing heat losses due to excess air. Calculated on following conditions: Oil as fired-18 633 B. t. u., 84.73 per cent carbon. 11.74 per cent hydrogen, 1.06 per cent sulphur, 5 per cent nitrogen, 0.87 per cent oxygen, 0.7 per cent moisture, and 0.4 per cent sediment; atmospheric temperature, 55° F.; humidity, 58; stack temperature, 500° F.; Kern oil, 15° B.
pressure. The apparatus should be carefully tested for leakage, as well as all connections leading thereto. Simple tests can be made as, for example: If after the cock "g" is closed, the bottle "f" is placed on top of the frame for a short time and again brought to the zero mark,, and the level of the water in "a" is above the zero mark, a leak is indicated. Before taking a final sample for analysis, the burette "a" should be filled with gas and emptied once or twice, to make sure that all the apparatus is filled
with the new gas. The cock "g" is then closed and the cock “i” is opened and the gas driven over into "b" by raising the bottle "f." The gas is drawn back into "a" by lowering "f" and when the solution in "b" has reached the mark in the capillary tube, the cock "i" is closed and a reading is taken on the burette, the level of the water in the bottle "f" being brought to the same level as the water in "a." The operation is repeated until a constant reading is obtained, the number of cubic centimeters, absorbed as shown by the reading, being the percentage of CO, in the flue gases. The gas is then driven over into the pipette "c" and a similar operation is carried out. The difference between the resulting
Table 3.-CO2 AND FUEL LOSSES."
reading and the first reading gives the percentage of oxygen in the flue gases. The next operation is to drive the gas into the pipette "d," the gas being given a final wash in “e," and then passed into the pipette "c" to neutralize any hydrochloric acid fumes which may have been given off by the cuprous chloride solution, which, especially if it be old, may give off such fumes, thus increasing the volume of the gases and making the reading on the burette. less than the true amount. The process must be carried out in the order named, as the pyrogallol solution will also absorb carbon dioxide, while the cuprous chloride solution will also absorb oxygen. As the pressure of the gases in the flue is less than the atmospheric pressure, they will not of themselves flow through
a. Weymouth, Trans. A. S. M. E., Vol. 30, p. 803.
the pipe connecting the flue to the apparatus. The gas may be drawn into the pipe in the way already described for filling the apparatus, but this is a tedious method. For rapid work a rubber bulb aspirator connected to the air outlet of the cock "g" will enable a new supply of gas to be drawn into the pipe, the apparatus K
then being filled as already described. Another form of aspirator draws the gas from the flue in a constant stream, thus insuring a fresh supply for each sample. The analysis made by the Orsat apparatus is volumetric. If the analysis by weight is required it can be found from the volumetric analysis as follows: Multiply the percentages by volume by either the densities or the molecular