URNACE EFFICIENCY AND COMBUSTION.
Many engineers are satisfied when their oil fires show a bright clear flame without smoke. They over look the fact that much unnecessary air may be pass ing through the furnace, under these conditions; and if low efficiencies are the result, they lay the blame on the oil burner, without making any effort to investi gate or better the conditions. The time has come, however, when all engineers should pay more atten tion to scientific combustion. It is impossible to know AN hen combustion is complete (without excess of air) without the use of flue gas analysis instruments ; and as these are relatively cheap, they should be a part of every fire-room equipment.
The result of complete combustion of carbon is carbon dioxide, CO.. When completely burned fuel consisting of 100 per cent carbon will show a volu metric percentage of 20.7 CO. in the furnace gases. Fuel oils contain a high percentage of carbon, and the average per cent of CO. obtained in oil burning furnaces where furnace conditions are watched is from 13 to 14 per cent, the theoretical maximum being 15.6 per cent. The lower the percentage of CO., the greater the waste due to excess air. For example, if the amount drops from 13 per cent to 10 per cent, it is an indication of 54 per cent excess air, and a loss of 2.9 per cent of fuel. The table below shows the corresponding losses that occur when various per centages of CO. are indicated in the flue gases.
To be able to conduct the tests necessary to de termine the efficiency of combustion, the engineer should be provided with a practical gas analysis appa ratus. There are many such instruments on the mar ket. The manually operated type gives oxygen (o), CO, and CO., while the automatic CO2 recorder indi cates at certain intervals the percentage of CO. pres ent in a sample of the flue gases, and automatically registers it on a chart.
In using these instruments, the sampling tube should be inserted at the base of the chimney, or in the last pass in the boiler, as near as possible to the point where the gases leave the heating surface. In order to determine the air leakage, a reading should also be taken at the furnace and compared with the reading from the last pass ; the furnace should show a little higher percentage of CO.. Careful attention
should be given to the leakage of air through boiler settings, and analyses should be taken comparing the CO* present to compare the difference about six inches from the wall with the amount near the center of the furnace. Such a comparison will give a good idea of the leakage that is taking place through cracks in the walls, or through porous bricks. A saving of over 5 per cent has been effected by stopping such leaks in a boiler setting that was apparently in perfect con dition.
A draft gauge should be used while making the gas analyses, for a slight variation of the draft enter ing the furnace raises or lowers the percentage of CO•. If the percentage of CO. is found to be low, indicating excess air, the draft should be adjusted until the per centage of CO2 has increased.
Apparatus for Flue Gas Analysis. The Orsat ap paratus, illustrated in Fig. 107, is generally used for analyzing flue gases. 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 facili tating 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 in the capillary tube.