Therefore, for water, the rise in temperature is--
27.41 X 1000 / 302 = 90.8° C.;
and for acetylene the rise is--
0.59 X 1000 / 26 / 0.25 = 90.8° C.]
If the generator were designed on lines to satisfy the United States Fire Underwriters, it would contain 8.33 lb. of water to every 1 lb. of carbide attacked; identical calculations then showing that the original temperature of the water and gas would be raised through 53.7° C. Provided the carbide is not charged into such an apparatus in lumps of too large a size, nor at too high a rate, there will be no appreciable amount of local overheating developed; and nowhere, therefore, will the rise in temperature exceed 91° in the first instance, or 54° C. in the second. Indeed it will be considerably smaller than this, because a large proportion of the heat evolved will be lost by radiation through the generator walls, while another portion will be converted from sensible into latent heat by causing part of the water to pass off as vapour with the acetylene.
EFFECT OF HIGH TEMPERATURES ON GENERATORS.--As the temperature amongst the carbide in any generator in which water is not present in large excess may easily reach 200° C. or upwards, no material ought to be employed in the construction of such generators which is not competent to withstand a considerable amount of heat in perfect safety. The ordinary varieties of soft solder applied with the bitt in all kinds of light metal-work usually melt, according to their composition, at about 180° C.; and therefore this method of making joints is only suitable for objects that are never raised appreciably in temperature above the boiling-point of water. No joint in an acetylene generator, the partial or complete failure of which would radically affect the behaviour of the apparatus, by permitting the charges of carbide and of water to come into contact at an abnormal rate of speed, by allowing the acetylene to escape directly through the crack into the atmosphere, or by enabling the water to run out of the seal of any vessel containing gas so as to set up a free communication between that vessel and the air, ought ever to be made of soft solder--every joint of this character should be constructed either by riveting, by bolting, or by doubly folding the metal sheets. Apparently, a joint constantly immersed in water on one side cannot rise in temperature above the boiling-point of the liquid, even when its other side is heated strongly; but since, even if a generator is not charged with naturally hard water, its fluid contents soon become "hard" by dissolution of lime, there is always a liability to the deposition of water scale over the joint. Such water scale is a very bad heat conductor, as is seen in steam boilers, so that a seam coated with an exceedingly thin layer of scale, and heated sharply on one side, will rise above the boiling-point of water even if the liquid on its opposite side is ice-cold. For a while the film of scale may be quite water-tight, but after it has been heated by contact with the hot metal several times it becomes brittle and cracks without warning. But there is a more important reason for avoiding the use of plumbers' solder. It might seem that as the natural hard, protective skin of the metal is liable to be injured or removed by the bending or by the drilling or punching which precedes the insertion of the rivets or studs, an application of soft solder to such a joint should be advantageous. This is not true because of the influence of galvanic action. As all soft solders consist largely of lead, if a joint is soldered, a "galvanic couple" of lead and iron, or of lead and zinc (when the apparatus is built of galvanised steel), is exposed to the liquid bathing it; and since in both cases the lead is highly electro-negative to the iron or zinc, it is the iron or zinc which suffers attack, assuming the liquid to possess any corrosive properties whatever. Galvanised iron which has been injured during the joint-making presents a zinc-iron couple to the water, but the zinc protects the iron; if a lead solder is present, the iron will begin to corrode immediately the zinc has disappeared. In the absence of lead it is the less important metal, but in the presence of lead it is the more important (the foundation) metal which is the soluble element of the couple. Where practicable, joints in an acetylene generator may safely be made by welding or by autogenous soldering ("burning"), because no other metal is introduced into the system; any other process, except that of riveting or folding, only hastens destruction of the plant. The ideal method of making joints about an acetylene generator is manifestly that of autogenous soldering, because, as will appear in Chapter IX. of this book, the most convenient and efficient apparatus for performing the operation is the oxy-acetylene blow-pipe, which can be employed so as to convert two separate pieces of similar metal into one homogeneous whole.
In less critical situations in an acetylene plant, such as the partitions of a carbide container, &c., where the collapse of the seam or joint would not be followed by any of the effects previously suggested, there is less cause for prohibiting the use of unfortified solder; but even here, two or three rivets, just sufficient to hold the metal in position if the solder should give way, are advisedly put into all apparatus. In other portions of an acetylene installation where a merely soldered joint is exposed to warm damp gas which is in process of cooling, instead of being bathed in hard water, an equal, though totally dissimilar, danger is courted. The main constituent of such solders that are capable of being applied with the bitt is lead; lead is distinctly soluble in soft or pure water; and the water which separates by condensation out of a warm damp gas is absolutely soft, for it has been distilled. If condensation takes place at or near a soldered joint in such a way that water trickles over the solder, by slow degrees the metallic lead will be dissolved and removed, and eventually a time will come when the joint is no longer tight to gas. In fact, if an acetylene installation is of more than very small dimensions, e.g., when it is intended to supply any building as large as, or larger than, the average country residence, if it is to give satisfaction to both constructor and purchaser by being quite trustworthy and, possessed of a due lease of life, say ten or fifteen years, it must be built of stouter materials than the light sheets which alone are suitable for manipulation with the soldering-iron or for bending in the ordinary type of metal press. Sound cast-iron, heavy sheet-metal, or light boiler-plate is the proper substance of which to construct all the important parts of a generator, and the joints in wrought metal must be riveted and caulked or soldered autogeneously as mentioned above. So built, the installation becomes much more costly to lay down than an apparatus composed of tinplate, zinc, or thin galvanised iron, but it will prove more economical in the long run. It is not too much to say that if ignorant and short-sighted makers in the earliest days of the acetylene industry had not recommended and supplied to their customers lightly built apparatus which has in many instances already begun to give trouble, to need repairs, and to fail by thorough corrosion--apparatus which frequently had nothing but cheapness in its favour--the use of the gas would have spread more rapidly than it has done, and the public would not now be hearing of partial or complete failures of acetylene installations. Each of these failures, whether accompanied by explosions and injury to persons or not, acts more powerfully to restrain a possible new customer from adopting the acetylene light, than several wholly successful plants urge him to take it up; for the average member of the public is not in a position to distinguish properly between the collapse of a certain generator owing to defective design or construction (which reflects no discredit upon the gas itself), and the failure of acetylene to show in practice those advantages that have been ascribed to it. One peculiar and noteworthy feature of acetylene, often overlooked, is that the apparatus is constructed by men who may have been accustomed to gas-making plant all their lives, and who may understand by mere habit how to superintend a chemical operation; but the same apparatus is used by persons who generally have no special acquaintance with such subjects, and who, very possibly, have not even burnt coal-gas at any period of their lives. Hence it happens that when some thoughtless action on the part of the country attendant of an acetylene apparatus is followed by an escape of gas from the generator, and by an accumulation of that gas in the house where the plant is situated, or when, in disregard of rules, he takes a naked light into the house and an explosion follows, the builder dismisses the episode as a piece of stupidity or wilful misbehaviour for which he can in nowise be held morally responsible; whereas the builder himself is to blame for designing an apparatus from which an escape of gas can be accompanied by sensible risks to property or life. However unpalatable this assertion may be, its truth cannot be controverted; because, short of criminal intention or insanity on the part of the attendant, it is in the first place a mere matter of knowledge and skill so to construct an acetylene plant that an escape of gas is extremely unlikely, even when the apparatus is opened for recharging, or when it is manipulated wrongly; and in the second place, it is easy so to arrange the plant that any disturbance of its functions which may occur shall be followed by an immediate removal of the surplus gas into a place of complete safety outside and above the generator-house.
GENERATION AT LOW TEMPERATURES.--In all that has been said hitherto about the reaction between calcium carbide and water being instantaneous, it has been assumed that the two substances are brought together at or about the usual temperature of an occupied room, i.e., 15 degrees C. If, however, the temperature is materially lower than this, the speed of the reaction falls off, until at -5 degrees C., supposing the water still to remain liquid, evolution of acetylene practically ceases. Even at the freezing-point of pure water gas is produced but slowly; and if a lump of carbide is thrown on to a block of ice, decomposition proceeds so gently that the liberated acetylene may be ignited to form a kind of torch, while heat is generated with insufficient rapidity to cause the carbide to sink into the block. This fact has very important bearings upon the manipulation of an acetylene generator in winter time. It is evident that unless precautions are taken those portions of an apparatus which contain water are liable to freeze on a cold night; because, even if the generator has been at work producing gas (and consequently evolving heat) till late in the evening, the surplus heat stored in the plant may escape into the atmosphere long before more acetylene has to be made, and obviously while frost is still reigning in the neighbourhood. If the water freezes in the water store, in the pipes leading therefrom, in the holder seal, or in the actual decomposing chamber, a fresh batch of gas is either totally incapable of production, because the water cannot be brought into contact with the calcium carbide in the apparatus, or it can only be generated with excessive slowness because the carbide introduced falls on to solid ice. Theoretically, too, there is a possibility that some portion of the apparatus--a pipe in particular--may be burst by the freezing, owing to the irresistible force with which water expands when it changes into the solid condition. Probably this last contingency, clearly accompanied as it would be by grave risk, is somewhat remote, all the plant being constructed of elastic material; but in practice even a simple interference with the functions of a generator by freezing, ideally of no special moment, is highly dangerous, because of the great likelihood that hurried and wholly improper attempts to thaw it will be made by the attendant. As it has been well known for many years that the solidifying point of water can be lowered to almost any degree below normal freezing by dissolving in it certain salts in definite proportions, one of the first methods suggested for preventing the formation of ice in an acetylene generator was to employ such a salt, using, in fact, for the decomposition of the carbide some saline solution which remains liquid below the minimum night temperature of the winter season. Such a process, however, has proved unsuitable for the purpose in view; and the explanation of that fact is found in what has just been stated: the "water" of the generator may admittedly be safely maintained in the fluid state, but from so cold a liquid acetylene will not be generated smoothly, if at all. Moreover, were it not so, a process of this character is unnecessarily expensive, although suitable salts are very cheap, for the water of the generator is constantly being consumed, [Footnote: It has already been said that most generators "consume" a much larger volume of water than the amount corresponding with the chemical reaction involved: the excess of water passing into the sludge or by- product. Thus a considerable quantity of any anti-freezing agent must be thrown aside each time the apparatus is cleaned out or its fluid contents are run off.] and as constantly needs renewal; which means that a fresh batch of salt would be required every time the apparatus was recharged, so long as frost existed or might be expected. A somewhat different condition obtains in the holder of an acetylene installation. Here, whenever the holder is a separate item in the plant, not constituting a portion of the generating apparatus, the water which forms the seal of a rising holder, or which fills half the space of a displacement holder, lasts indefinitely; and it behaves equally well, whatever its temperature may be, so long as it retains a fluid state. This matter will be discussed with greater detail at the end of Chapter III. At present the point to be insisted on is that the temperature in any constituent of an acetylene installation which contains water must not be permitted to fall to the freezing-point; while the water actually used for decomposition must be kept well above that temperature.
GENERATION AT HIGH TEMPERATURES.--At temperatures largely exceeding those of the atmosphere, the reaction between calcium carbide and water tends to become irregular; while at a red heat steam acts very slowly upon carbide, evolving a mixture of acetylene and hydrogen in place of pure acetylene. But since at pressures which do not materially exceed that of the atmosphere, water changes into vapour at 100° C., above that temperature there can be no question of a reaction between carbide and liquid water. Moreover, as has been pointed out, steam or water vapour will continue to exist as such at temperatures even as low as the freezing-point so long as the vapour is suspended among the particles of a permanent gas. Between calcium carbide and water vapour a double decomposition occurs chemically identical with that between carbide and liquid water; but the physical effect of the reaction and its practical bearings are considerably modified. The quantity of heat liberated when 30 parts by weight of steam react with 64 parts of calcium carbide should be essentially unaltered from that evolved when the reagent is in the liquid state; but the temperature likely to be attained when the speed of reaction remains the same as before will be considerably higher for two conspicuous reasons. In the first place, the specific heat of steam in is only 0.48, while that of liquid water is 1.0. Hence, the quantity of heat which is sufficient to raise the temperature of a given weight of liquid water through n thermometric degrees, will raise the temperature of the same weight of water vapour through rather more than 2 n degrees. In the second place, that relatively large quantity of heat which in the case of liquid water merely changes the liquid into a vapour, becoming "latent" or otherwise unrecognisable, and which, as already shown, forms roughly five-sixths of the total heat needed to convert cold water into steam, has no analogue if the water has previously been vaporised by other means; and therefore the whole of the heat supplied to water vapour raises its sensible temperature, as indicated by the thermometer. Thus it appears that, except for the sufficient amount of cooling that can be applied to a large vessel containing carbide by surrounding it with a water jacket, there is no way of governing its temperature satisfactorily if water vapour is allowed to act upon a mass of carbide--assuming, of course, that the reaction proceeds at any moderate speed, e.g., at a rate much above that required to supply one or two burners with gas.
The decomposition which with perfect chemical accuracy has been stated to occur quantitatively between 36 parts by weight, of water and 64 parts of calcium carbide scarcely ever takes place in so simple a fashion in an actual generator. Owing to the heat developed when carbide is in excess, about half the water is converted into vapour; and so the reaction proceeds in two stages: half the water added reacting with the carbide as a liquid, the other half, in a state of vapour, afterwards reacting similarly, [Footnote: This secondary reaction is manifestly only another variety of the phenomenon known as "after-generation" (cf. ante). After-generation is possible between calcium carbide and mechanically damp slaked lime, between carbide and damp gas, or between carbide and calcium hydroxide, as opportunity shall serve. In all cases the carbide must be in excess.] or hardly reacting at all, as the case may be. Suppose a vessel, A B, somewhat cylindrical in shape, is charged with carbide, and that water is admitted at the end called A. Suppose now (1) that the exit for gas is at the opposite end, B. As the lumps near A are attacked by half the liquid introduced, while the other half is changed into steam, a current, of acetylene and water vapour travels over the charge lying between the decomposing spot and the end B. During its passage the second half of the water, as vapour, reacts with the excess of carbide, the first make of acetylene being dried, and more gas being produced. Thus a second quantity of heat is developed, equal by theory to that previously evolved; but a second elevation in temperature, far more serious, and far less under control, than the former also occurs; and this is easily sufficient to determine some of those undesirable effects already described. Digressing for a moment, it may be admitted that the desiccation of the acetylene produced in this manner is beneficial, even necessary; but the advantages of drying the gas at this period of its treatment are outweighed by the concomitant disadvantages and by the later inevitable remoistening thereof. Suppose now (2) that both the water inlet and the gas exit of the carbide cylinder are at the same end, A. Again half the added water, as liquid, reacts with the carbide it first encounters, but the hot stream of damp gas is not permitted to travel over the rest of the lumps extending towards B: it is forced to return upon its steps, leaving B practically untouched. The gas accordingly escapes from the cylinder at A still loaded with water vapour, and for a given weight of water introduced much less acetylene is evolved than in the former case. The gas, too, needs drying somewhere else in the plant; but these defects are preferable to the apparent superiority of the first process because overheating is, or can be, more thoroughly guarded against.