CHAPTER XXXII

ANALYSIS BY WEIGHT, AND AIR THEORETICALLY REQUIRED IN FUEL OIL FURNACE

In the last discussion it was found that Orsat analyses of chimney gases are always made volumetrically. In computing combustion data from these analyses, however, it is often necessary to have the proportions or percentages by weight instead

FIG. 175.-Carbon dioxide recording machine located in the Long Beach Plant of the Southern California Edison Company.

of by volume. The volumes of carbon dioxide, oxygen, carbon monoxide, and nitrogen which constitute the chimney gas analysis of a sample volume by means of the Orsat apparatus will be represented by V1, V2, V3, V4, respectively in this discussion.

Let us now see how we may transfer this relationship so that proportions by weight of M1, M2, M3 and M4 pounds may respectively set forth the constituents of a flue gas sample of weight M pounds. Since we are only in search of proportions by weight-that is a ratio of M1 to M, M2 to M etc., it is evidently not necessary to actually know the quantitative values of the weights involved.

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FIG. 176.-Recording thermometer and draft gage on Stirling boilers. Pacific Gas and Electric Company, station C, Oakland, Cal.

Fundamental Laws Involved. In a previous discussion we found (see page 48) that all perfect gases follow the composite law-namely, that at any particular state the product of its pressure p and volume V is equal to the product of its weight M and absolute temperature T multiplied by a constant R, or mathematically expressed

tionships for the carbon dioxide, oxygen, carbon monoxide, and nitrogen of the flue gas.

It is to be remembered that in the case under consideration the pressure p and the temperature T have the same value for each component in the flue gas; consequently, we shall not put any individual subscript for the pressure p and temperature T, so that we may write these individual expressions as follows:

In our previous discussion on the elementary laws of gases, it was also found mathematically that the constant R for any perfect gas is obtained by dividing 1544 by the molecular weight of the gas in question (see page 47).

From any book on elementary chemistry we find the molecular weight m of carbon dioxide (CO2) is 44, that of oxygen (O2) is 32, that of carbon monoxide (CO) is 28, and that of nitrogen (N2) is 28.

Relationship of a Component Weight to the Whole.-Bearing this in mind, it is seen from the above mathematical relationships

FIG. 177.- View of the float arrangements, showing the valves which control

the inlet and outlet of oil from storage tanks of Long Beach Plant, Southern California Edison Company.

A Concrete Rule for Conversions. This last equation now gives us a simple and ready rule for determining proportions by weight if the proportions by volume are given. In other words, this rule may be stated as follows:

In any analysis by volume, the analysis by weight is found by first summing the products formed by multiplying each component volume by its particular molecular weight. If now this summation C, is divided into the product of a component volume

and its particular molecular weight, the proportion by weight of that component is at once ascertained.

An Illustrative Example. Thus, a flue gas analysis shows the following proportions by volume: carbon dioxide (CO2) 0.086; oxygen (O2) 0.110; carbon monoxide 0.011; and nitrogen (N2) 0.793 per cent. Let us determine the proportions by weight present in this particular flue gas.

Since the molecular weights of carbon dioxide, oxygen, carbon monoxide and nitrogen are respectively 44, 32, 28, and 28, we find that miVi is 3.782, m2V2 is 3.520, m3V3 is 0.308, and m4V4 is 22.200. The sum of these products C, is found to be 29.810. Hence since miV1 is 3.782, we now find that the carbon dioxide component obtained by dividing 3.782 by 29.810 is 0.1270. Similarly for the oxygen component the proportion by weight is 0.1182; for the carbon monoxide component it is 0.0103; and for the nitrogen component we have 0.7453. As a check on our work we find that the sum of these separate components is unity as it should be. Or expressed in percentages, we would have for a volumetric analysis consisting of 8.6 per cent. carbon dioxide, 11.0 per cent. oxygen, 1.1 per cent. carbon monoxide, and 79.3 per cent. nitrogen, that the percentages by weight become 12.70 per cent. carbon dioxide, 11.82 per cent. oxygen, 1.03 per cent. carbon monoxide, and 74.53 per cent. nitrogen, which foot up 100 per cent. in either case and thus check our work.

A Suggested Form of Tabulation. To expedite computation the work set forth in the above discussion may be tabulated. Below we have a form of tabulation which will prove useful for such transformations:

Weight of Air Theoretically Required for Perfect Fuel Oil Combustion. For economic combustion in the furnace a certain percentage of air over and above that theoretically required for