PRESSURE IN GENERATORS.--Inasmuch as acetylene is prone 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 passing 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 circumstances, 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 glass 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 space, 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 glass 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 institution 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 space 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 circumstances; 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 quantity 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 circumstances 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 assuming, as must be the case in a water-sealed holder, that the temperature of the gas is below 780° C.; and also assuming, 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 assertion 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 quantities 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 cocks 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-cock 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 accumulation 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 glass 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 assistance, 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 quantity 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 quantities, 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 space. 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 space 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 space 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 quantity 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 space 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 space 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.