COLOUR OF SPENT CARBIDE.--In the early days of the industry, it was frequently taken for granted that any degradation in the colour of the spent lime left in an acetylene generator was proof that overheating had taken place during the decomposition of the carbide. Since both calcium oxide and hydroxide are white substances, it was thought that a brownish, greyish, or blackish residue must necessarily point to incipient polymerisation of the gas. This view would be correct if calcium carbide were prepared in a state of chemical purity, for it also is a white body.

Commercial carbide, however, is not pure; it usually contains some foreign matter which tints the residue remaining after gasification. When a manufacturer strives to give his carbide the highest gas-making power possible he frequently increases the proportion of carbon in the charge submitted to electric smelting, until a small excess is reached, which remains in the free state amongst the finished carbide. After decomposition the fine particles of carbon stain the moist lime a bluish grey tint, the depth of shade manifestly depending upon the amount present. If such a sludge is copiously diluted with water, particles of carbon having the appearance and gritty or flaky nature of c.o.ke often rise to the surface or fall to the bottom of the liquid; whence they can easily be picked out and identified as pure or impure carbon by simple tests. Similarly the lime or carbon put into the electric furnace may contain small quant.i.ties of compounds which are naturally coloured; and which, reappearing in the sludge either in their original or in a different state of combination, confer upon the sludge their characteristic tinge. Spent lime of a yellowish brown colour is frequently to be met with in circ.u.mstances that are clearly no reproach to the generator. Doubtless the tint is due to the presence of some coloured metallic oxide or other compound which has escaped reduction in the electric furnace. The colour which the residual lime afterwards a.s.sumes may not be noticeable in the dry carbide before decomposition, either because some change in the colour-giving impurity takes place during the chemical reactions in the generator or because the tint is simply masked by the greyish white of the carbide and its free carbon.

Hence it follows that a bad colour in the waste lime removed from a generator only points to overheating and polymerisation of the acetylene when corroborative evidence is obtained--such as a distinct tarry smell, the actual discovery of oily or tarry matters elsewhere, or a grave reduction in the illuminating power of the gas.

MAXIMUM ATTAINABLE TEMPERATURES.--In order to discover the maximum temperature which can be reached in or about an acetylene generator when an apparatus belonging to one of the best types is fed at a proper rate with calcium carbide in lumps of the most suitable size, the following calculation may be made. In the first place, it will be a.s.sumed that no loss of heat by radiation occurs from the walls of the generator; secondly, the small quant.i.ty of heat taken up by the calcium hydroxide produced will be ignored; and, thirdly, the specific heat of acetylene will be a.s.sumed to be 0.25, which is about its most probable value. Now, a hand-fed carbide-to-water generator will work with half a gallon of water for every 1 lb. of carbide decomposed, quant.i.ties which correspond with 320 grammes of water per 64 grammes (1 molecular weight) of carbide.

Of those 320 grammes of water, 18 are chemically destroyed, leaving 302.

The decomposition of 64 grammes of commercial carbide evolves 28 large calories of heat. a.s.suming all the heat to be absorbed by the water, 28 calories would raise 302 grammes through (28 X 1000 / 302) = 93 C., _i.e._, from 44.6 F. to the boiling-point. a.s.suming all the heat to be communicated to the acetylene, those 28 calories would raise the 26 grammes of gas liberated through (28 X 1000 / 26 / 0.25) = 4308 C., if that were possible. But if, as would actually be the case, the heat were distributed uniformly amongst the 302 grammes of water and the 20 grammes of acetylene, both gas and water would be raised through the same number of degrees, viz., 90.8 C. [Footnote: Let x = the number of large calories absorbed by the water; then 28 - x = those taken up by the gas.

Then--

1000x / 302 = 1000 (28 - x) / (26 X 0.25)

whence x = 27.41; and 28 - x = 0.59.

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 pa.s.s 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, a.s.suming 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 h.o.m.ogeneous whole.

In less critical situations in an acetylene plant, such as the part.i.tions 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 const.i.tuent 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 acc.u.mulation 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 a.s.sertion 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 a.s.sumed 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 pa.s.sing into the sludge or by- product. Thus a considerable quant.i.ty 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 const.i.tuting a portion of the generating apparatus, the water which forms the seal of a rising holder, or which fills half the s.p.a.ce 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 const.i.tuent 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 quant.i.ty 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 quant.i.ty 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 quant.i.ty 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 a.n.a.logue 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 ma.s.s of carbide--a.s.suming, 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 quant.i.tatively 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 pa.s.sage 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 quant.i.ty 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.

PRESSURE IN GENERATORS.--Inasmuch as acetylene is p.r.o.ne to dissociate or decompose into its elements spontaneously whenever its pressure reaches 2 atmospheres or 30 lb. per square inch, as well as when its temperature at atmospheric pressure attains 780 C., no pressure approaching that of 2 atmospheres is permissible in the generator. A due observance of this rule, however, unlike a proper maintenance of a low temperature in an acetylene apparatus, is perfectly easy to arrange for. The only reason for having an appreciable positive pressure in any form of generating plant is that the gas may be compelled to travel through the pipes and to escape from the burner orifices; and since the plant is only installed to serve the burners, that pressure which best suits the burners must be thrown by the generator or its holder. Therefore the highest pressure it is ever requisite to employ in a generator is a pressure sufficient (_a_) to lift the gasholder bell, or to raise the water in a displacement holder, (_b_) to drive the gas through the various subsidiary items in the plant, such as washers and purifiers, (_c_) to overcome the friction in the service-pipes, [Footnote: This friction manifestly causes a loss of pressure, _i.e._, a fall in pressure, as a gas travels along a pipe; and, as will be shown in Chapter VII., it is the fall in pressure in a pipe rather than the initial pressure at which a gas enters a pipe that governs the volume of gas pa.s.sing through that pipe. The proper behaviour and economic working of a burner (acetylene or other, luminous or incandescent) naturally depend upon the pressure in the pipe to which the burner is immediately attached being exactly suited to the design of that burner, and have nothing to do with the fall in pressure occurring in the delivery pipes. It is therefore necessary to keep entirely separate the ideas of proper burner pressure and of maximum desirable fall in pressure within the service due to friction.] and (d) to give at the points of combustion a pressure which is required by the particular burners adopted. In all except village or district installations, (_c_) may be virtually neglected. When the holder has a rising bell, (_a_) represents only an inch or so of water; but if a displacement holder is employed the pressure needed to work it is entirely indeterminate, being governed by the size and shape of the said holder. It will be argued in Chapter III. that a rising holder is always preferable to one constructed on the displacement principle. The pressure (d) at the burners may be taken at 4 inches of water as a maximum, the precise figure being dependent upon the kind of burners--luminous, incandescent, boiling, &c.--attached to the main. The pressure (_b_) also varies according to circ.u.mstances, but averages 2 or 3 inches. Thus a pressure in the generator exceeding that of the atmosphere by some 12 inches of water--_i.e._, by about 7 oz., or less than half a pound per square inch--is amply sufficient for every kind of installation, the less meritorious generators with displacement holders only excepted. This pressure, it should be noted, is the net or effective pressure, the pressure with which the gas raises the liquid in a water-gauge gla.s.s out of the level while the opposite end of the water column is exposed to the atmosphere. The absolute pressure in a vessel containing gas at an effective pressure of 12 inches of water is 7 oz. plus the normal, insensible pressure of the atmosphere itself--say 15-1/4 lb. per square inch. The liquid in a barometer which measures the pressure of the atmosphere stands at a height of 30 inches only, because that liquid is mercury, 13.6 times as heavy as water. Were it filled with water the barometer would stand at (30 X 13.6) = 408 inches, or 34 feet, approximately. Gas pressures are always measured in inches of water column, because expressed either as pounds per square inch or as inches of mercury, the figures would be so small as to give decimals of unwieldy length.

It would of course be perfectly safe so to arrange an acetylene plant that the pressure in the generating chamber should reach the 100 inches of water first laid down by the Home Office authorities as the maximum allowable. There is, however, no appreciable advantage to be gained by so doing, or by exceeding that pressure which feeds the burners best. Any higher original pressure involves the use of a governor at the exit of the plant, and a governor is a costly and somewhat troublesome piece of apparatus that can be dispensed with in most single installations by a proper employment of a well-balanced rising holder.

CHAPTER III

THE GENERAL PRINCIPLES OF ACETYLENE GENERATION--ACETYLENE GENERATING APPARATUS

Inasmuch as acetylene is produced by the mere interaction of calcium carbide and water, that is to say, by simply bringing those two substances in the cold into mutual contact within a suitable closed s.p.a.ce, and inasmuch as calcium carbide can always be purchased by the consumer in a condition perfectly fit for immediate decomposition, the preparation of the gas, at least from the theoretical aspect, is characterised by extreme simplicity. A cylinder of gla.s.s or metal, closed at one end and open at the other, filled with water, and inverted in a larger vessel containing the same liquid, may be charged almost instantaneously with acetylene by dropping into the basin a lump of carbide, which sinks to the bottom, begins to decompose, and evolves a rapid current of gas, displacing the water originally held in the inverted cylinder or "bell." If a very minute hole is drilled in the top of the floating bell, acetylene at once escapes in a steady stream, being driven out by the pressure of the cylinder, the surplus weight of which causes it to descend into the water of the basin as rapidly as gas issues from the orifice. As a laboratory experiment, and provided the bell has been most carefully freed from atmospheric air in the first instance, this escaping gas may be set light to with a match, and will burn with a more or loss satisfactory flame of high illuminating power. Such is an acetylene generator stripped of all desirable or undesirable adjuncts, and reduced to its most elementary form; but it is needless to say that so simple an apparatus would not in any way fulfil the requirements of everyday practice.

Owing to the inequality of the seasons, and to the irregular nature of the demand for artificial light and heat in all households, the capacity of the plant installed for the service of any inst.i.tution or district must be amply sufficient to meet the consumption of the longest winter evening--for, as will be shown in the proper place, attempts to make an acetylene generator evolve gas more quickly than it is designed to do are fraught with many objections--while the operation of the plant, must be under such thorough control that not only can a sudden and unexpected demand for gas be met without delay, but also that a sudden and unexpected interruption or cessation of the demand shall not be followed by any disturbance in the working of the apparatus. Since, on the one hand, acetylene is produced in large volumes immediately calcium carbide is wetted with water, so that the gas may be burnt within a minute or two of its first evolution; and, on the other, that acetylene once prepared can be stored without trouble or appreciable waste for reasonable periods of time in a water-sealed gasholder closely resembling, in everything but size, the holders employed on coal-gas works; it follows that there are two ways of bringing the output of the plant into accord with the consumption of the burners. It is possible to make the gas only as and when it is required, or it is possible in the s.p.a.ce of an hour or so, during the most convenient part of the day, to prepare sufficient to last an entire evening, storing it in a gasholder till the moment arrives for its combustion. It is clear that an apparatus needing human attention throughout the whole period of activity would be intolerable in the case of small installations, and would only be permissible in the case of larger ones if the district supplied with gas was populous enough to justify the regular employment of two men at least in or about the generating station. But with the conditions obtaining in such a country as Great Britain, and in other lands where coal is equally cheap and accessible, if a neighbourhood was as thickly populated as has been suggested, it would be preferable on various grounds to lay down a coal- gas or electricity works; for, as has been shown in the first chapter, unless a very material fall in the price of calcium carbide should take place--a fall which at present is not to be expected--acetylene can only be considered a suitable and economical illuminant and heating agent for such places as cannot be provided cheaply with coal-gas or electric current. To meet this objection, acetylene generators have been invented in which, broadly speaking, gas is only produced when it is required, control of the chemical reaction devolving upon some mechanical arrangement. There are, therefore, two radically different types of acetylene apparatus to be met with, known respectively as "automatic" and "non-automatic" generators. In a non-automatic generator the whole of the calcium carbide put into the apparatus is more or less rapidly decomposed, and the entire volume of gas evolved from it is collected in a holder, there to await the moment of consumption. In an automatic apparatus, by means of certain devices which will be discussed in their proper place, the act of turning on a burner-tap causes some acetylene to be produced, and the act of turning it off brings the reaction to an end, thus obviating the necessity for storage. That, at any rate, is the logical definition of the two fundamentally different kinds of generator: in automatic apparatus the decomposition of the carbide is periodically interrupted in such fashion as more or less accurately to synchronise with the consumption of gas; in the non-automatic variety decomposition proceeds without a break until the carbide vessels are empty.

Unfortunately a somewhat different interpretation of these two words has found frequent acceptance, a generator being denominated non-automatic or automatic according as the holder attached to it is or is not large enough to store the whole of the acetylene which the charge of carbide is capable of producing if it is decomposed all at once. Apart from the fact that a holder, though desirable, is not an absolutely indispensable part of an acetylene plant, the definition just quoted was sufficiently free from objection in the earliest days of the industry; but now efficient commercial generators are to be met with which become either automatic or non-automatic according to the manner of working them, while some would be termed non-automatic which comprise mechanism of a conspicuously self- acting kind.

AUTOMATIC AND NON-AUTOMATIC GENERATORS.--Before proceeding to a detailed description of the various devices which may be adopted to render an acetylene generator automatic in action, the relative advantages of automatic and non-automatic apparatus, irrespective of type, from the consumer's point of view may be discussed. The fundamental idea underlying the employment of a non-automatic generator is that the whole of the calcium carbide put into the apparatus shall be decomposed into acetylene as soon after the charge is inserted as is natural in the circ.u.mstances; so that after a very brief interval of time the generating chambers shall contain nothing but spent lime and water, and the holder be as full of gas as is ever desirable. In an automatic apparatus, the fundamental idea is that the generating chamber, or one at least of several generating chambers, shall always contain a considerable quant.i.ty of undecomposed carbide, and some receptacle always contain a store of water ready to attack that carbide, so that whenever a demand for gas shall arise everything may be ready to meet it. Inasmuch as acetylene is an inflammable gas, it possesses all the properties characteristic of inflammable gases in general; one of which is that it is always liable to take fire in presence of a spark or naked light, and another of which is that it is always liable to become highly explosive in presence of a naked light or spark if, accidentally or otherwise, it becomes mixed with more than a certain proportion of air. On the contrary, in the complete absence of liquid or vaporised water, calcium carbide is almost as inert a body as it is possible to imagine: for it will not take fire, and cannot in any circ.u.mstances be made to explode. Hence it may be urged that a non-automatic generator, with its holder always containing a large volume of the actually inflammable and potentially explosive acetylene, must invariably be more dangerous than an automatic apparatus which has less or practically no ready-made gas in it, and which simply contains water in one chamber and unaltered calcium carbide in another. But when the generating vessels and the holder of a non-automatic apparatus are properly designed and constructed, the gas in the latter is acetylene practically free from air, and therefore while being, as acetylene inevitably is, inflammable, is devoid of explosive properties, always a.s.suming, as must be the case in a water-sealed holder, that the temperature of the gas is below 780 C.; and also a.s.suming, as must always be the case in good plant, that the pressure under which the gas is stored remains less than two atmospheres absolute. It is perfectly true that calcium carbide is non-inflammable and non-explosive, that it is absolutely inert and incapable of change; but so comprehensive an a.s.sertion only applies to carbide in its original drum, or in some impervious vessel to which moisture and water have no access. Until it is exhausted, an automatic acetylene generator contains carbide in one place and water in another, dependence being put upon some mechanical arrangement to prevent the two substances coming into contact prematurely. Many of the devices adopted by builders of acetylene apparatus for keeping the carbide and water separate, and for mixing them in the requisite quant.i.ties when the proper time arrives, are as trustworthy, perhaps, as it is possible for any automatic gear to be; but some are objectionably complicated, and a few are positively inefficient.

There are two difficulties which the designer of automatic mechanism has to contend with, and it is doubtful whether he always makes a sufficient allowance for them. The first is that not only must calcium carbide and liquid water be kept out of premature contact, but that moisture, or vapour of water, must not be allowed to reach the carbide; or alternatively, that if water vapour reaches the carbide too soon, the undesired reaction shall not determine overheating, and the liberated gas be not wasted or permitted to become a source of danger. The second difficulty encountered by the designer of automata is so to construct his apparatus that it shall behave well when attended to by completely unskilled labour, that it shall withstand gross neglect and resist positive ill-treatment or mismanagement. If the automatic principle is adopted in any part of an acetylene apparatus it must be adopted throughout, so that as far as possible--and with due knowledge and skill it is completely possible--nothing shall be left dependent upon the memory and common sense of the gasmaker. For instance, it must not be necessary to shut a certain tap, or to manipulate several c.o.c.ks before opening the carbide vessel to recharge it; it must not be possible for gas to escape backwards out of the holder; and either the carbide-feed gear or the water-supply mechanism (as the case may be) must be automatically locked by the mere act of taking the cover off the carbide store, or of opening the sludge-c.o.c.k at the bottom. It would be an advantage, even, if the purifiers and other subsidiary items of the plant were treated similarly, arranging them in such fashion that gas should be automatically prevented from escaping out of the rest of the apparatus when any lid was removed. In fact, the general notion of interlocking, which has proved so successful in railway signal-cabins and in carburetted water gas-plant for the prevention of accidents duo to carelessness or overnight, might be copied in principle throughout an acetylene installation whenever the automatic system is employed.

It is no part of the present argument, to allege that automatic generators are, and must always be, inherently dangerous. Automatic devices of a suitable kind may be found in plenty which are remarkably simple and highly trustworthy; but it would be too bold a statement to say that any such arrangement is incapable of failure, especially when put into the hands of a person untrained in the superintendence of machinery. The more reliable a piece of automatic mechanism proves itself to be, the more likely is it to give trouble and inconvenience and utterly to destroy confidence when it does break down; because the better it has behaved in the past, and the longer it has lasted without requiring adjustment, the less likely is it that the attendant will be at hand when failure occurs. By suitable design and by an intelligent employment of safety-valves and blow-off pipes (which will be discussed in their proper place) it is quite easy to avoid the faintest possibility of danger arising from an increase of pressure or an improper acc.u.mulation of gas inside the plant or inside the building containing the plant; but every time such a safety-valve or blow-off pipe comes into action a waste of gas occurs, which means a sacrifice of economy, and shows that the generator is not working as it should.

As gla.s.s is a fragile and brittle substance, and as it is not capable of bearing large, rapid, and oft-repeated alterations of temperature in perfect safety, it is not a suitable material for the construction of acetylene apparatus or of portions thereof. Hence it follows that a generator must be built of some non-transparent material which prevents the interior being visible when the apparatus is at work. Although it is comparatively easy, by the aid of a lamp placed outside the generator- shed in such a position as to throw its beams of light through a window upon the plant inside, to charge a generator after dark; and although it is possible, without such a.s.sistance, by methodical habits and a systematic arrangement of utensils inside the building to charge a generator even in perfect darkness, such an operation is to be deprecated, for it is apt to lead to mistakes, it prevents any slight derangement in the installation from being instantly noticed, and it offers a temptation to the attendant to break rules and to take a naked light with him. On all those grounds, therefore, it is highly desirable that every manipulation connected with a generator shall be effected during the daytime, and that the apparatus-house shall be locked up before nightfall. But owing to the irregular habits engendered by modern life it is often difficult to know, during any given day, how much gas will be required in the ensuing evening; and it therefore becomes necessary always to have, as ready-made acetylene, or as carbide in a proper position for instant decomposition, a patent or latent store of gas more than sufficient in quant.i.ty to meet all possible requirements.

Now, as already stated, a non-automatic apparatus has its store of material in the form of gas in a holder; and since this is preferably constructed on the rising or telescopic principle, a mere inspection of the height of the bell--on which, if preferred, a scale indicating its contents in cubic feet or in burner-hours may be marked--suffices to show how near the plant is to the point of exhaustion. In many types of automatic apparatus the amount of carbide remaining undecomposed at any moment is quite unknown, or at best can only be deduced by a tedious and inexact calculation; although in some generators, where the store of carbide is subdivided into small quant.i.ties, or placed in several different receptacles, an inspection of certain levers or indicators gives an approximate idea as to the capacity of the apparatus for further gas production. In any case the position of a rising holder is the most obvious sign of the degree of exhaustion of a generator; and therefore, to render absolutely impossible a failure of the light during an evening, a non-automatic generator fitted with a rising holder is best.

Since calcium carbide is a solid body having a specific gravity of 2.2, water being unity, and since 1 cubic foot of water weighs 62.4 lb., in round numbers 137 lb. of _compact_ carbide only occupy 1 cubic foot of s.p.a.ce. Again, since acetylene is a gas having a specific gravity of 0.91, air being unity, and since the specific gravity of air, water being unity, is 0.0013, the specific gravity of acetylene, water being unity, is roughly O.00116. Hence 1 cubic foot of acetylene weighs roughly 0.07 lb. Furthermore, since 1 lb. of good carbide evolves 5 cubic feet of gas on decomposition with water, acetylene stored at atmospheric pressure occupies roundly 680 times as much s.p.a.ce as the carbide from which it has been evolved. This figure by no means represents the actual state of affairs in a generator, because, as was explained in the previous chapter, a carbide vessel cannot be filled completely with solid; and, indeed, were it so "filled," in ordinary language, much of its s.p.a.ce would be still occupied with air. Nevertheless it is incontrovertible that an acetylene plant calculated to supply so many burners for so long a period of time must be very much larger if it is constructed on the non-automatic principle, when the carbide is decomposed all at once, than if the automatic system is adopted, when the solid remains unattacked until a corresponding quant.i.ty of gas is required for combustion. Clearly it is the storage part of a non-automatic plant alone which must be so much larger; the actual decomposing chambers may be of the same size or even smaller, according to the system of generation to which the apparatus belongs. In practice this extra size of the non-automatic plant causes it to exhibit two disadvantages in comparison with automatic apparatus, disadvantages which are less serious than they appear, or than they may easily be represented to be. In the first place, the non- automatic generator requires more s.p.a.ce for its erection. If acetylene were an illuminating agent suitable for adoption by dwellers in city or suburb, where the back premises and open-air part of the messuage are reduced to minute proportions or are even non-existent, this objection might well be fatal. But acetylene is for the inhabitant of a country village or the occupier of an isolated country house; and he has usually plenty of s.p.a.ce behind his residence which he can readily spare. In the second place, the extra size of the non-automatic apparatus makes it more expensive to construct and more costly to instal. It is more cosily to construct and purchase because of its holder, which must be well built on a firm foundation and accurately balanced; it is more costly to instal because a situation must be found for the erection of the holder, and the apparatus-house may have to be made large enough to contain the holder as well as the generator itself. As regards the last point, it may be said at once that there is no necessity to place the holder under cover: it may stand out of doors, as coal-gas holders do in England, for the seal of the tank can easily be rendered frost-proof, and the gas itself is not affected by changes of atmospheric temperature beyond altering somewhat in volume. In respect of the other objections, it must be remembered that the extra expense is one of capital outlay alone, and therefore only increases the cost of the light by an inappreciable amount, representing interest and depreciation charges on the additional capital expenditure.

The increased cost of a year's lighting due to these charges will amount to only 10 or 15 per cent, on the additional capital sunk. The extra capital sunk does not in any way increase the maintenance charges; and if, by having a large holder, additional security and trustworthiness are obtained, or if the holder leads to a definite, albeit illusive, sense of extra security and trustworthiness, the additional expenditure may well be permissible or even advantageous.

The argument is sometimes advanced that inasmuch as for the same, or a smaller, capital outlay as is required to instal a non-automatic apparatus large enough to supply at one charging the maximum amount of light and heat that can ever be needed on the longest winter's night, an automatic plant adequate to make gas for two or three evenings can be laid down, the latter must be preferable, because the attendant, in the latter case, will only need to enter the generator-house two or three times a week. Such an argument is defective because it ignores the influence of habit upon the human being. A watch which must be wound every day, or a clock which must be wound every week, on a certain day of the week, is seldom permitted to run down; but a watch requiring to be re-wound every other day, or a fourteen-day clock (used as such), would rarely be kept going. Similarly, an acetylene generator might be charged once a week or once a day without likelihood of being forgotten; but the operation of charging at irregular intervals would certainly prove a nuisance. With a non-automatic apparatus containing all its gas in the holder, the attendant would note the position of the bell each morning, and would introduce sufficient carbide to fill the holder full, or partly full, as the case might be; with an automatic apparatus he would be tempted to trust that the carbide holders still contained sufficient material to last another night.

The automatic system of generating acetylene has undoubtedly one advantage in those climates where frost tends to occur frequently, but only to prevail for a short period. As the apparatus is in operation during the evening hours, the heat evolved will, or can be made to, suffice to protect the apparatus from freezing until the danger has pa.s.sed; whereas if the gas is generated of a morning in a non-automatic apparatus the temperature of the plant may fall to that of the atmosphere before evening, and some portion may freeze unless special precautions are taken to protect it.

It was shown in Chapter II that overheating is one of the chief troubles to be guarded against in acetylene generators, and that the temperature attained is a function of the speed at which generation proceeds. Seeing that in an automatic apparatus the rate of decomposition depends on the rate at which gas is being burnt, while in a non-automatic generator it is, or may be, under no control, the critic may urge that the reaction must take place more slowly and regularly, and the maximum temperature therefore be lower, when the plant works automatically. This may be true if the non-automatic generator is unskilfully designed or improperly manipulated; but it is quite feasible to arrange an apparatus, especially one of the carbide-to-water or of the flooded-compartment type, in such fashion that overheating to an objectionable extent is rendered wholly impossible. In a non-automatic apparatus the holder is nothing but a holder and may be placed wherever convenient, even at a distance from the generating plant; in an automatic apparatus the holder, or a small similarly constructed holder placed before the main storage vessel, has to act as a water-supply governor, as the releasing gear for certain carbide-food mechanism, or indeed as the motive power of such mechanism; and accordingly it must be close to the water or carbide store, and more or less intimately connected by means of levers, or the like, with the receptacle in which decomposition occurs. Sometimes the holder surrounds, or is otherwise an integral part of, the decomposing chamber, the whole apparatus being made self-contained or a single structure with the object of gaining compactness. But it is evident that such methods of construction render additionally awkward, or even hazardous, any repair or petty operation to the generating portion of the plant; while the more completely the holder is isolated from the decomposing vessels the more easily can they be cleaned, recharged, or mended, without blowing off the stored gas and without interfering with the action of any burners that may be alight at the time. Owing to the ingenuity of inventors, and the experience they have acquired in the construction of automatic acetylene apparatus during the years that the gas has been in actual employment, it is going too far boldly to a.s.sert that non-automatic generators are invariably to be preferred before their rivals. Still in view of the nature of the labour which is likely to be bestowed on any domestic plant, of the difficulty in having repairs or adjustments done quickly in outlying country districts, and of the inconvenience, if not risk, attending upon any failure of the apparatus, the greater capital outlay, and the larger s.p.a.ce required by non-automatic generators are in most instances less important than the economy in s.p.a.ce and prime cost characteristic of automatic machines when the defects of each are weighed fairly in the balance. Indeed, prolonged experience tends to show that a selection between non-automatic and automatic apparatus may frequently be made on the basis of capacity. A small plant is undoubtedly much more convenient if automatic; a very large plant, such as that intended for a public supply, is certainly better if non-automatic, but between these two extremes choice may be exercised according to local conditions.

CONTROL OF THE CHEMICAL REACTION.--Coming now to study the principles underlying the construction of an acetylene generator more closely it will be seen that as acetylene is produced by bringing calcium carbide into contact with water, the chemical reaction may be started either by adding the carbide to the water, or by adding the water to the carbide.

Similarly, at least from the theoretical aspect, the reaction, may be caused to stop by ceasing to add carbide to water, or by ceasing to add water to carbide. Apparently if water is added by degrees to carbide, until the carbide is exhausted, the carbide must always be in excess; and manifestly, if carbide is added in small portions to water, the water must always be in excess, which, as was argued in Chapter II., is emphatically the more desirable position of affairs. But it in quite simple to have carbide present in large excess of the water introduced when the whole generator is contemplated, and yet to have the water always in chemical excess in the desired manner; because to realise the advantages of having water in excess, it is only necessary to subdivide the total charge of carbide into a number of separate charges which are each so small that more than sufficient water to decompose and flood one of them is permitted to enter every time the feed mechanism comes into play, or (in a non-automatic apparatus) every time the water-c.o.c.k is opened; so arranging the charges that each one is protected from the water till its predecessor, or its predecessor, have been wholly decomposed. Thus it is possible to regard either the carbide or the water as the substance which has to be brought into contact with the other in specified quant.i.ty. It is perhaps permissible to repeat that in the construction of an automatic generator there is no advantage to be gained from regulating the supply of both carbide and water, because just as the mutual decomposition will begin immediately any quant.i.ty of the one meets any quant.i.ty of the other, so the reaction will cease (except in one case owing to "after-generation") directly the whole of that material which is not in chemical excess has been consumed-quite independently of the amount of the other material left unattacked. Being a liquid, and possessing as such no definite shape or form of its own irrespective of the vessel in which it is held, water is by far the more convenient of the two substances to move about or to deliver in predetermined volume to the decomposing chamber. A supply of water can be started instantaneously or cut oil as promptly by the movement of a c.o.c.k or valve of the usual description; or it may be allowed to run down a depending pipe in obedience to the law of gravitation, and stopped from running down such a pipe by opposing to its pa.s.sage a gas pressure superior to that gravitational force. In any one of several obvious ways the supply of water to a ma.s.s of carbide may be controlled with absolute certainty, and therefore it should apparently follow that the make of acetylene should be under perfect control by controlling the water current. On the other hand, unless made up into b.a.l.l.s or cartridges of some symmetrical form, calcium carbide exists in angular ma.s.ses of highly irregular shape and size. Its lumps alter in shape and size directly liquid water or moisture reaches them; a loose more or loss gritty powder, or a damp cohesive mud, being produced which is well calculated to choke any narrow aperture or to jam any moving valve. It is more difficult, therefore, by mechanical agency to add a supply of carbide to a ma.s.s of water than to introduce a supply of water to a stationary ma.s.s of carbide; and far more difficult still to bring the supply of carbide under perfect control with the certainty that the movement shall begin and stop immediately the proper time arrives.

But a.s.suming the mechanical difficulties to be satisfactorily overcome, the plan of adding carbide to a stationary ma.s.s of water has several chemical advantages, first, because, however the generator be constructed, water will be in excess throughout the whole time of gas production; and secondly, because the evolution of acetylene will actually cease completely at the moment when the supply of carbide is interrupted. There is, however, one particular type of generator in which as a matter of fact the carbide is the moving const.i.tuent, viz., the "dipping" apparatus (cf. _infra_), to which these remarks do not apply; but this machine, as will be seen directly, is, illogically perhaps, but for certain good reasons, cla.s.sed among the water-to-carbide apparatus. All the mechanical advantages are in favour, as just indicated, of making water the moving substance; and accordingly, when cla.s.sified in the present manner, a great majority of the generators now on the markets are termed water-to-carbide apparatus. Their disadvantages are twofold, though these may be avoided or circ.u.mvented: in all types save one the carbide is in excess at the immediate place and time of decomposition; and in all types without exception the carbide in the whole of the generator is in excess, so that the phenomenon of "after- generation" occurs with more or less severity. As explained in the last chapter, after-generation is the secondary production of acetylene which takes place more or less slowly after the primary reaction is finished, proceeding either between calcium hydroxide, merely damp lime, or damp gas and calcium carbide, with an evolution of more acetylene. As it is possible, and indeed usual, to fit a holder of some capacity even to an automatic generator, the simple fact that more acetylene is liberated after the main reaction is over does not matter, for the gas can be safely stored without waste and entirely without trouble or danger. The real objection to after-generation is the difficulty of controlling the temperature and of dissipating the heat with which the reaction is accompanied. It will be evident that the balance of advantage, weighing mechanical simplicity against chemical superiority, is somewhat even between carbide-to-water and water-to-carbide generators of the proper type; but the balance inclines towards the former distinctly in the ease of non-automatic apparatus, and points rather to the latter when automatism is desired. In the early days of the industry it would have been impossible to speak so favourably of automatic carbide-to-water generators, for they were at first constructed with absurdly complicated and unreliable mechanism; but now various carbide-feed gears have been devised which seem to be trustworthy even when carbide not in cartridge form is employed.

NON-AUTOMATIC CARBIDE-TO-WATER GENERATORS.--There is little to be said in the present place about the principles underlying the construction of non-automatic generators. Such apparatus may either be of the carbide-to- water or the water-to-carbide type. In the former, lumps of carbide are dropped by hand down a vertical or sloping pipe or shoot, which opens at its lower end below the water-level of the generating chamber, and which is fitted below its mouth with a deflector to prevent the carbide from lodging immediately underneath that mouth. The carbide falls through the water which stands in the shoot itself almost instantaneously, but during its momentary descent a small quant.i.ty of gas is evolved, which produces an unpleasant odour unless a ventilating hood is fixed above the upper end of the tube. As the ratio of cubical contents to superficial area of a lump is greater as the lump itself is larger, and as only the outer surface of the lump can be attacked by the water in the shoot during its descent, carbide for a hand-fed carbide-to-water generator should be in fairly large ma.s.ses--granulated material being wholly unsuitable--and this quite apart from the fact that large carbide is superior to small in gas-making capacity, inasmuch as it has not suffered the inevitable slight deterioration while being crushed and graded to size. If carbide is dropped too rapidly into such a generator which is not provided with a false bottom or grid for the lumps to rest upon, the solid is apt to descend among a ma.s.s of thick lime sludge produced at a former operation, which lies at the bottom of the decomposing chamber; and here it may be protected from the cooling action of fresh water to such an extent that its surface is baked or coated with a hard layer of lime, while overheating to a degree far exceeding the boiling-point of water may occur locally. When, however, it falls upon a grid placed some distance above the bottom of the water vessel, the various convection currents set up as parts of the liquid become warm, and the mechanical agitations produced by the upward current of gas rinse the spent lime from the carbide, and entirely prevent overheating, unless the lumps are excessively large in size. If the carbide charged into a hand-fed generator is in very large lumps there is always a possibility that overheating may occur in the centre of the ma.s.ses, due to the baking of the exterior, even if the generator is fitted with a reaction grid.

Manifestly, when carbide in lumps of reasonable size is dropped into excess of water which is not merely a thick viscid cream of lime, the temperature cannot possibly exceed the boiling-point--_i.e._, 100 C.--provided always the natural convection currents of the water are properly made use of.

The defect which is, or rather which may be, characteristic of a hand-fed carbide-to-water generator is a deficiency of gas yield due to solubility. At atmospheric temperatures and pressure 10 volumes of water dissolve 11 volumes of acetylene, and were the whole of the water in a large generator run to waste often, a sensible loss of gas would ensue.

If the carbide falls nearly to the bottom of the water column, the rising gas is forced to bubble through practically the whole of the liquid, so that every opportunity is given it to dissolve in the manner indicated till the liquid is completely saturated. The loss, however, is not nearly so serious as is sometimes alleged, because (1) the water becomes heated and so loses much of its solvent power; and (2) the generator is worked intermittently, with sufficiently long intervals to allow the spent lime to settle into a thick cream, and only that thick cream is run off, which represents but a small proportion of the total water present. Moreover, a hand-fed carbide-to-water generator will work