Fig. 22. Orsat Apparatus
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 centimeters 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 are at the zero mark. This operation is necessary in order to obtain the zero reading at atmospheric pressure.
The apparatus should be carefully tested for leakage as well as all connections leading thereto. Simple tests can be made; 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, 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 in the pipette B 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 being the percentage of CO 2 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 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 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 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 weight of each gas, and divide the products by the sum of all the products; the quotients will be the percentages by weight. For most work sufficient accuracy is secured by using the even values of the molecular weights.
The even values of the molecular weights of the gases appearing in an analysis by an Orsat are:
| Carbon Dioxide | 44 | |
| Carbon Monoxide | 28 | |
| Oxygen | 32 | |
| Nitrogen | 28 |
[Table 33] indicates the method of converting a volumetric flue gas analysis into an analysis by weight.
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Application of Formulae and Rules —Pocahontas coal is burned in the furnace, a partial ultimate analysis being:
| Per Cent | |
|---|---|
| Carbon | 82.1 |
| Hydrogen | 4.25 |
| Oxygen | 2.6 |
| Sulphur | 1.6 |
| Ash | 6.0 |
| B. t. u., per pound dry | 14500 |
The flue gas analysis shows:
| Per Cent | |
|---|---|
| CO 2 | 10.7 |
| O | 9.0 |
| CO | 0.0 |
| N (by difference) | 80.3 |
Determine: The flue gas analysis by weight (see [Table 33] ), the amount of air required for perfect combustion, the actual weight of air per pound of fuel, the weight of flue gas per pound of coal, the heat lost in the chimney gases if the temperature of these is 500 degrees Fahrenheit, and the ratio of the air supplied to that theoretically required.
Solution: The theoretical weight of air required for perfect combustion, per pound of fuel, from formula ( [11] ) will be,
| W | = | 34.56 |
| = | 10.88 pounds. |
If the amount of carbon which is burned and passes away as flue gas is 80 per cent, which would allow for 2.1 per cent of unburned carbon in terms of the total weight of dry fuel burned, the weight of dry gas per pound of carbon burned will be from formula ( [16] ):
| W | = |
| = | 23.42 pounds |
and the weight of flue gas per pound of coal burned will be .80 × 23.42 = 18.74 pounds.
The heat lost in the flue gases per pound of coal burned will be from formula ( [15] ) and the value 18.74 just determined.
Loss = .24 × 18.74 × (500 - 60) = 1979 B. t. u.
The percentage of heat lost in the flue gases will be 1979 ÷ 14500 = 13.6 per cent.
The ratio of air supplied per pound of coal to that theoretically required will be 18.74 ÷ 10.88 = 1.72 per cent.
The ratio of air supplied per pound of combustible to that required will be from formula ( [14] ):
| = | 1.73 |
The ratio based on combustible will be greater than the ratio based on fuel if there is unconsumed carbon in the ash.
Unreliability of CO 2 Readings Taken Alone —It is generally assumed that high CO 2 readings are indicative of good combustion and hence of high efficiency. This is true only in the sense that such high readings do indicate the small amount of excess air that usually accompanies good combustion, and for this reason high CO 2 readings alone are not considered entirely reliable. Wherever an automatic CO 2 recorder is used, it should be checked from time to time and the analysis carried further with a view to ascertaining whether there is CO present. As the percentage of CO 2 in these gases increases, there is a tendency toward the presence of CO, which, of course, cannot be shown by a CO 2 recorder, and which is often difficult to detect with an Orsat apparatus. The greatest care should be taken in preparing the cuprous chloride solution in making analyses and it must be known to be fresh and capable of absorbing CO. [Pg 163] In one instance that came to our attention, in using an Orsat apparatus where the cuprous chloride solution was believed to be fresh, no CO was indicated in the flue gases but on passing the same sample into a Hempel apparatus, a considerable percentage was found. It is not safe, therefore, to assume without question from a high CO 2 reading that the combustion is correspondingly good, and the question of excess air alone should be distinguished from that of good combustion. The effect of a small quantity of CO, say one per cent, present in the flue gases will have a negligible influence on the quantity of excess air, but the presence of such an amount would mean a loss due to the incomplete combustion of the carbon in the fuel of possibly 4.5 per cent of the total heat in the fuel burned. When this is considered, the importance of a complete flue gas analysis is apparent.
[Table 34] gives the densities of various gases together with other data that will be of service in gas analysis work.
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1942 Horse-power Installation of Babcock & Wilcox Boilers and Superheaters in the Singer Building, New York City