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is most necessary that the mortar be weatherproof. Common building lime should be eliminated from every part of the structure. I n some cases, where the temperature of the acid smoke stream is continualIy high and the acids not very active, the same brick and mortar may be used, but a sectional lining must be constructed instead of an independent lining. (Fig. 3) This form of construction is less expensive. The corbels built out at intervals from the main walls and supporting the lining should have the inner joints pointed with acid-proof mortar. On the top of each corbel an apron of an acid-proof material should be set ’in such a manner that the lower lip projects down over the top of the section of lining below. The air space is then protected. I n addition to this the upper 12 in. or so of the air space under each corbel should be packed with flexible material not affected by the particular acid encountered. (Fig. 4) Where lightning rods are installed on acid chimneys, the upper 50 ft. or more of the complete rod should be covered with an armor to protect the copper from effects of the acid. Lead covering is in most cases effective. All chimneys handling acid gases should be equipped with an outside ladder, the upper portion of which should be covered with lead or an acid-resisting material. CAREOF CHIMNEYS DURING SHUTDOWNS Chimneys that have been in almost continual service for years without showing any effect from the smoke stream
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have been observed to develop defects, particularly in the upper portions, after they have been shut down,for a protracted period. Although the conditions of temperature, dilution, acid mixture, etc., may be such as not to cause damage while the chimney is in operation, yet an accumulation of dust on the inner walls, which is deliquescent by virtue of its acid content, may tend to do damage when the chimney is not in operation. The weather-rain, fog, snow, or a heavy humid atmosphere-furnishes the necessary water within the chimney to convert the previously inert dust with an acid content into a liquid acid which immediately becomes active. It is wise, therefore, when the chimney is shut down for a period, to cover the entire opening at the top with a temporary weatherproof lid. This can be made in sections of light wood and is easily placed and removed. Lugs protected by an acidproof armor should be built into the head to which the sections of the temporary lid may be fastened. A satisfactory arrangement in designing a plant in which acid fumes are to be carried off is to plan the boiler house so that the gases from the boilers and the acid fumes from the apparatus can be put in the same chimney. The boiler gases not only keep the temperatures up, but they dilute the smoke stream containing acid gases. No hard and fast rules can be laid down which will apply to every case where chimneys handle acid gases. The problem of design and materials used can be solved only by an intimate knowledge of the nature and effect of the particular gases, fumes, or dust to be disposed of.
Accurate Bubble Meters for Very Small Rates of Gas Flow’ By Tyler Fuwa and G. A. Shattuck MASSACHUSETTS INSTITUTB ofl TECHNOLOGY, CAMBRIDGE, MASS.
OR THE measurement of gas flow in the laboratory, the ordinary capillary tube flowmeter is the moPt satisfactory instrument where from 200 cc. to 50 liters or more of gas are flowing per minute.2 For rates of flow below 200 cc. per min., however, the capillary tube flowmeter is not, in general, satisfactory. It therefore seemed worth while to investigate the possibilities of standardizing some type of bubble meter that could be depended upon to give reproducible results. Such a type of meter has several great advantages-it is very easy to set up, has practically no lower limit at which it cease3 to measure with good accuracy, and a single calibration curve may be used for any gas which does not react with the liquid in which the bubbles are formed.
gas through bottles C and D, thus stabilizing the flow through B, which would otherwise vary considerably, since an airline
DESCRIPTION OF APPARATUS Plate 1 shows the apparatus as set up for the flow measurements. Bottles A and B contain the meter liquid, the flow through B being regulated by the pinchcock and screwclamp HI, the object being t o divert the greater part of the 1 Received August 11, 1922. Published as Contribution No. 00 from the Research Laboratory of Applied Chemistry, Massachusetts Institute of Technology. 2 For a n excellent discussion of capillary tube flowmeters, see “GaT Flow Meters for Small Rates of Flow,” by A . F. Benton, THISJ O U R N A L , 11 (1919), 623.
PI,ATED DIAGRAM OF BUBBLEM E T E S
March, 1923
INDUSTRIAL A N D ENGINEERING CHEMISTRY
connection is used. Bottle A is placed in the train in order to saturate the gas with the meter liquid, previous to its passage into the meter bottle B. The volume of the gas measured by the graduate, being under less than atmospheric pressure, must be corrected when the water column is near the top of the graduate. The maximum correction, however, is less than 2 per cent.
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slightly inclined. Variations of 10 or 15 degrees in this angle were found, however, to have an inappreciable effect on the results. Satisfactory tips were made by grinding the end of the cut tip on a fine abrasive wheel or oil stone. Another method is to grind the cut tip and finish off with a fire polish.in the Bunsen flame. Attention is also called to the method of H a r k i n ~ ,in~ which the tip is filled and surrounded with Wood’s metal, ground to a smooth, square end, and finally dipped in hot water to remove the metal. To determine the effect of a change of liquid head on the operation of the meter, two runs were made, in which the tip was under heads of 1 in. and 6 in., respectively. A tabulation of the results showed no appreciable variation in results, however. Using soap as one of the most effective materials for lowering surface tension in small amounts, runs were made with a number of different concentrations, which show that the effect of this factor is not a t all serious, especially a t the lower concentrations, although for a 0.03 per cent solution of ivory soap and water, which had a “static” surface tension about half that of water, the bubbles were about 12 per cent smaller. As indicated previously, it would be desirable to use various strengths of sulfuric acid, rather than water, as the metering liquid, in order to get any desired humidity. Results were obtained with a number of different concentrations, indicating that the volume of the bubbles decreases considerably with increasing acid concentrations. Results with 1.295 gravity acid are shown in Plate 3, and the bubble volumes are, on the average, 10 per cent smaller than for pure water. It is, therefore, necessary to calibrate the meter against the particular strength of sulfuric acid with which it is t o be used, and the same is probably true of other metering liquids, such as oil, which might be used under other conditions.
The first requirement of the metering liquid is that, it must not react with the gas. Water is inert to most common gases, except for the slight amount dissolved until it becomes saturated. Sulfuric acid solutions of regulated concentration are in many cases preferable to water as metering liquids, because it is thereby possible to give the gas stream any desired humidity between 0 and 100 per cent. The physical properties of the liquid which are likely to affect the results are: (a) its density, ( b ) its surface tension, and (c) its partial pressure. The last effect is calculable and generally small, while the first two are studied herein. The size and type of the exit tip are very important, since they determine the size of the bubbles formed, and consequently the number per unit volume of gas. The effect of changing the liquid head on the exit tip will be also to change the size of the bubbles, but, except for a small difference in head, the variation was found to be negligible. The absolute pressure in the bubbler bottle is generally nearly enough atmospheric not to necessitate any correction, and in any case does not affect the volume of the bubbles at the pressure in question. The effect of varying the size and type of tip wTas first investigated. Plate 2 shows the volume number relation for It was found that tips having an internal diameter of about four sizes of tips; it will be noted that the average size of 8 mm. gave the most reliable results’for rates of flow between bubbles is slightly less than directly proportional to the inside 30 and 180 cc. per min. For rates of flow below 30 cc. per min. area of the tip. smaller bubble tips should be used. With reasonable care, the Straight tips, horizontal tips with a 90-degree bend near average deviation from the calibration curves can readily the end, and vertical tips with a bend of 180 degrees were be kept below 2 per cent. used, but only the straight tips gave satisfactory results. The chief difficulty with the other types of tips was due to ACKNOWLEDGMEXT the tendency of the meter liquid to enter the lower part of In conclusion, the writers desire to acknowledge the the tip, and thus change the effective diameter. It might be expected that the straight tip would give different re- helpful suggestions of Prof. Robert E. Wilson. 3 Harkins and Brown, J . A m Chem. S O L 4 , 1 (1919), 499. sults, depending on whether it was precisely vertical, or
