setting, and this style of furnace therefore possesses the further advantage of prolonging the life of the setting by maintaining it at a uniform temperature at all times. (Trustees.) 3. In the manufacture of carburetted water gas it is customary to have the water gas pick up the oil, in a state of vapor, at the entrance to the fixing vessels and carry this vapor and the resulting gas through these vessels, the oil gas being thus made in the presence of the water gas. What advantages has this method over that of making the oil gas entirely separate from the water gas? Ans. The principal advantage that is thought to be obtained by carrying on the manufacture of carburetted water gas so that the oil gas portion is made in the presence of the water gas instead of being made entirely separate from the water gas, is that the presence of the water gas shields the oil gas from the heat and renders it less subject to over-decomposition in case the temperature of the heated surfaces is too high or the exposure to the heat too prolonged. The effect upon the oil vapors and gas of exposure to heat of too great intensity or for too long a time is to produce hydrocarbon gases of low illuminating values, or hydrocarbon vapors, which, although they are of high illuminating value, can be carried by the gas only to a limited extent. By separating and surrounding the molecules of the oil vapors and gas, the water gas reduces the extent to which they are exposed to the heat and so reduces the liability to over-decomposition. On the other hand, it is possible that an excess of water gas will exert so great an effect as to interfere with the complete gasification of the oil, and in cases where the amount of exposure is small, or the intensity of the heat low, the presence of water gas during the decomposition of the oil may be detrimental. This, however, will not occur in an apparatus that is properly designed and handled. Another advantage of making the oil gas in the presence of the water gas is that the latter will pick up and carry oil vapors that escape gasification, and, if the water gas was not present, would be condensed and left behind when the oil gas was cooled. These vapors are of value as illuminants, and so the candle power of the carburetted water gas made in this way will be higher for an equal amount of oil used than will be that of carburetted water gas made by making the water gas and oil gas separately and mixing them cold. This advantage can, however, be obtained to a great extent even when the two gases are made separately by mixing them hot. This method of making carburetted water gas also makes it possible to conveniently arrange the apparatus so that the heat required for the gasification of the oil can be furnished. very economically by the combustion, and partly by the sensible heat, of the producer gas formed in the generator while the fuel is being heated up to the temperature required for the manufacture of water gas. (Trustees.) 4. Give a description, illustrated with sketches if necessary, of one or more forms of apparatus for removing heavy tar from gas, and state the position in the sequence of apparatus that should be occupied by the tar extractors, with your reason for such location. Ans. One method of removing tar from hot gas is founded upon the principal that deposition of the vesicles of tar held in suspension in the gas may be effected by bringing these vesicles into contact with solid surfaces, or with each other. The Pelouze and Audouin, founded upon this principle and designed for use with hot gas, is a very efficient and compact form of apparatus for the removal of heavy tar from gas. The action of the apparatus is as follows: The gas to be purified is made to flow through a series of holes of small diameter, so forming jets, which strike against a surface situated directly opposite. In the passage of the gas through the holes, the liquid molecules are brought into close contact. with each other, and the operation is completed by the contact with the solid surface upon which the tarry matter is deposited. The apparatus is represented in the cut, part shown as broken away in order to explain more clearly its action. AAA is the outer case, to which are attached the inlet and outlet pipes, I and O. Concentric with the case is fixed an annular tank BB, in the middle of which is left a free passage for the This annular tank is charged by preference with dead oil obtained from tar, and in it is suspended the bell or holder C, counterbalanced as represented. The sides of the bell C for a portion of its height is formed of three or more concentric cylinders which are separated by a space of about three-eighths of an inch between them. cylinder is pierced, as represented at D, with a number of rows PANO A Condenser Ө of holes each about one-twentieth of an inch in diameter, which are so placed that the gas in passing through the orifices of one of the cylinders impinges against the plate of the next; by this means all the tarry vesicles are broken up in a manner so complete, that when the apparatus is dispensed with, a piece of writing paper being exposed to the influence of the gas issuing from a small jet is, in the course of two or three seconds, rendered almost black by the deposit of the tar; whereas, when the condenser is in operation under like conditions, the paper is not soiled in the slightest degree. The tar is deposited on the side of the bell, whence it passes off by a suitable opening to the pipe P. The bell, being balanced and free to move as shown, rises and falls automatically as the difference between the pressure inside and that outside of it increases or decreases, and so by the exposing, as the conditions demand, of a greater or smaller number of holes through which the gas can pass the pressure thrown by the apparatus is kept constant in spite of variations. in the amount of gas made. A pressure gauge should be provided to show when this governing action is being interfered with by the clogging of the holes in the bell. When this occurs the holes should be cleaned, and the apparatus is so designed as to permit such cleaning to be done. A form of apparatus known either as a hot scrubber or a dry scrubber is also used for this purpose. It consists usually of a rectangular shell, enclosing a space which is divided into several compartments, the gas passing through these compartments in succession. In each compartment is a series of plates so arranged that the gas is frequently baffled and forced to change the direction of its travel, and is at the same time exposed to friction in passing over the surface of the plates. The shock of the impingement on the baffle plates, added to the friction between the gas and the plates, causes the deposition of the tar just as in the P. and A. condenser. As, however, the gas is not divided into small streams, as in the latter form of apparatus, the dry scrubber must be made much larger if it is to handle the same quantity of gas per day. The heavy tar extractor should be so located in the sequence of apparatus as to receive and treat the gas before it has been cooled down below 100° F. It may be placed after the exhauster when the gas leaves the latter at a temperature above 100° F., but should be placed before the exhauster when the gas leaves the latter at a temperature below that point. The reason for so locating the tar extractor is that the heavy tar has, when cold, the property of absorbing from the gas the vapors of the lighter liquids, which vapors if not absorbed in this way would remain suspended in the gas. They are very efficient in adding to the light-giving value of the gas, and if the tar is allowed to absorb them the illuminating value of the finished gas is made lower than it will be when they are kept in it. At a temperature above 100° F. the tar does not seem to have any appreciable effect upon them, so if it is removed while the temperature is above that point it is prevented from exerting any harmful influence upon the illuminating value of the gas. (Trustees.) 5. Define the term specific gravity of a gas. Give the limits between which you would expect the specific gravity of coal gas to vary. Same for carburetted water gas. How do you account for the difference in specific gravity between the two gases? Ans. The specific gravity of a substance is the ratio between the weight of any given volume of the substance and that of an equal volume of some other substance selected as a standard. In the case of gases the standard commonly employed is atmospheric air, though in chemical work hydrogen is used as a standard. For our purposes the specific gravity of a gas may be defined as the ratio of the weight of any given volume of the gas to the weight of an equal volume of air, the air and the gas being measured and weighed at the same temperature and pressure. The specific gravity of ordinary coal gas ranges from .350 to.500, air being 1.000, increasing with the candle power of the gas. The specific gravity of carburetted water gas as ordinarily made ranges from .550 to .700. The greater specific gravity of carburetted water gas is due to the fact that it contains on the one hand smaller percentage of hydrogen, which is the lightest substance known, and of marsh gas, also a comparatively light gas (sp. gr. .554), and on the other hand larger percentages of the heavier substance carbon monoxide (sp. gr. .969) and of the heavy hydrocarbons (sp. gr. from .969 upwards) than does coal gas. Trustees.) 6. Give the weight of a cubic foot of each of the following |