Acetylene generator

“ 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. ”

“ It is not possible to give a direct answer to the question as to which is the best type of acetylene generator. There are no generators made by responsible firms at the present time which are not safe. Some may be easier to charge and clean than others; some require more frequent attention than others; some have moving parts less likely to fail, when handled carelessly, than others; some have no moving mechanism to fail. ”

“ EFFECT OF HEAT ON ACETYLENE.--The effect of excessive retention of heat in an acetylene generator upon the gas itself is very marked, as acetylene begins spontaneously to suffer change, and to be converted into other compounds at elevated temperatures. Being a purely chemical phenomenon, the behaviour of acetylene when exposed to heat will be fully discussed in Chapter VI. when the properties of the gas are being systematically dealt with. ”

“ Since of the substances in or about a generator water is the one which has by far the largest capacity for absorbing heat, and since it is the only substance to which any necessary quantity of heat can be safely or conveniently transmitted, it follows that the larger in size an acetylene generator is, or the more rapidly that generator is made to deliver gas, the more desirable is it to use water as the means for dissipating the surplus heat, and the more necessary is it to employ an apparatus in which water is in large chemical excess at the actual place of decomposition. ”

“ 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. ”

“ Economy apart, no difficulty arises from imperfect combustion or escape of unburnt gas from an outdoor lamp, but in a room the presence of unburnt acetylene must always be offensive even if it is not dangerous; while the combustion products of the impurities--and in a portable generator acetylene cannot be chemically purified--are highly objectionable. It is simply a matter of good design to render any form of portable apparatus safe against explosion (employment of proper carbide being assumed) , for one or more vent-pipes can always be inserted in the proper places ”

“ In the event of a subsequent fire, the gas in the generator would burn, but not explode; but in view of the greater illuminating power per unit volume of carbide than per equal volume of compressed oil- gas, a portable acetylene generator should be somewhat less objectionable than broken cylinders of oil-gas if a fire should follow a railway accident of the usual kind. ”

“ It will be seen that a sufficient removal of these generator impurities need throw no appreciable extra labour upon the consumer of acetylene, for he can readily select a type of generator in which their production is reduced to a minimum; while a cotton-wool or coke filter for the gas, a water washer, which is always useful in the plant if only employed as a non-return valve between the generator and the holder, and the indispensable chemical purifiers, will take out of the acetylene all the remaining generator impurities which need, and can, be extracted. ”

“ Probably in the generator acetylene is first given off so rapidly that the heat does not act on it, but as decomposition advances into the [Pg 345] center of the mass of carbide, the acetylene generated has to pass through the external layers, which, as shown, may be at high temperatures, above that at which acetylene decomposes; thus a considerable amount of gas is lost, and the tar formed may distill into the generator and tubes, clogging the tubes. A more serious evil is the deterioration in the illuminating quality of the gas. ”

“ Six more years' experience in the design and manufacture of acetylene plant has enabled the older firms of manufacturers to fix upon certain standard patterns for their apparatus, and it may confidently be anticipated that many of these will survive a longer period. Faulty devices and designs have been weeded out, and there are lessons of the past as well as theoretical considerations to guide the inventor of a new type of generator. ”

“ If, for example, the quantity of water present in a generator is more than chemically sufficient to attack all the carbide added, however largo or small that excess may be, no more, and, theoretically speaking, no less, acetylene can ever be evolved than 26 parts by weight of gas for every 64 parts by weight of calcium carbide consumed. ”

“ Imperfect decomposition of the carbide inside the generator not only means an obvious loss of economy, but its presence among the residues makes a careful handling of them essential to avoid accident owing to a subsequent liberation of acetylene in some unsuitable, and perhaps closed, situation. A residue which is not conspicuously saturated with water must be taken out of the generator- house into the open air and there flooded with water, being left in some uncovered receptacle for a sufficient time to ensure all the acetylene being given off. ”

“ If the temperature of an acetylene generator rises to such an extent that part of the gas is polymerised into tar, that tar naturally tends to coat the residual carbide lumps, and, being greasy in character, more or less completely protects the interior from further attack. ”

“ On the other hand, the generators must be cared for, must often be filled with fresh carbide, and from time to time must be cleaned. With the generator system acetylene is as safe as or safer than illuminating gas. Berthelot has shown that at pressures below two atmospheres a vessel filled with acetylene can not be exploded by the explosion of a cap of fulminating mercury within the vessel, nor by heating a wire which extends into the vessel to a white heat by an electric current. The reason is that the acetylene can not explode unless it is decomposed into its elements, carbon and hydrogen ”

“ In exposed situations, where the holder is out of doors, one branch of the flow-pipe should enter and travel round the seal as previously suggested. Most large country residences are already provided with suitable heating apparatus for warming the greenhouses, and part of the heat may be capable of diversion into the acetylene generator-shed if the latter is erected in a convenient spot. ”

“ A generator which is perfectly satisfactory in general behaviour, and which evolves a sufficient proportion of the possible total make of gas to be economical, does not of necessity decompose the carbide quantitatively; nor is it constructed in a fashion to render an exact measurement of the gas liberated at standard temperature and pressure easy to obtain. For obvious reasons the careful consumer of acetylene will keep a record of the carbide decomposed and of the acetylene generated--the latter perhaps only in terms of burner- hours, or the like ”

“ The remaining two-thirds of the space is naturally full of air when the container is first put into the generator, but the air is displaced by acetylene as soon as gas production begins. Whether that space, however, is occupied by air, by acetylene, or by a gradually growing loose mass of slaked lime, each separate lump of hot carbide is isolated from its neighbours by a material which is also a very bad heat conductor; and the heat has but little opportunity of distributing itself evenly. ”

“ This wastage is not due to any decomposition of the acetylene in contact with water, but depends on the various solubilities of the different gases which compose the product obtained from commercial calcium carbide. Inasmuch as an acetylene evolved in the best generator contains some foreign ingredients, and inasmuch as an inferior product contains more (cf. Chapter V.) , the contents of a holder are never pure; but as those contents are principally made up of acetylene itself, that gas stands at a higher partial pressure in the holder than the impurities. ”

“ When a large carbide-to-water generator is worked systematically so as to avoid more loss of acetylene by solution in the excess of liquid than is absolutely necessary, the liquid residues coming from it will be collected in some ventilated closed tank where they can settle quietly. The clear lime-water will then be pumped back into the generator for further use, and the almost solid sludge will be ready to be carried to the pit where it is to be buried. Special care must be taken in disposing of the residues from a generator in which oil is used to control evolution of gas. ”

“ 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. ”

“ 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. ”

“ Acetylene forms an explosive mixture with air; so does illuminating gas. The odor of acetylene is unpleasant; so is the odor of the water gas used generally in the United States, and the acetylene can be cheaply deodorized.
As the generator system, then, is the general one, the most important question to the consumer is what generator to buy, and it is a perplexing question. The carbide manufacture is so organized that it is everywhere under the control of powerful and responsible companies which sell a guaranteed product. ”

“ If the acetylene passes directly to the burner from a generator having carbide in excess without being washed or filtered in any way, the gas may carry with it particles of lime dust, which will collect in the pipes mainly at the points where they are constricted; and as the pipes will be of comparatively large bore until the actual burner is readied, it will be chiefly at the orifices where the deposition occurs. This cause, though trivial, is often overlooked. It will be obviated whenever the plant is intelligently designed. ”

“ The water-seal of a holder standing within the generator-house may eventually become so offensive to the nostrils that the liquid has to be renewed; but when this happens it is due to the accumulation in the water of the water-soluble impurities of the crude acetylene. If, as should be done, the gas is passed through a washer or condenser containing much water before it enters the holder the sulphuretted hydrogen and ammonia will be extracted, and the seal will not acquire an obnoxious odour for a very long time. ”

“ 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. ”

“ The generator must have a vent-pipe in case the gas-main leading to the holder should become blocked at any time, and the acetylene which continues to be evolved in all water-to-carbide apparatus, even after the supply of water has been cut off be unable to pass away. Theoretically a non-automatic apparatus does not require a vent-pipe in its generator because all the gas enters the holder immediately, and is, or should be, unable to return through the intermediate water seal; practically such a safeguard is absolutely necessary for the reason given. ”

“ FROTHING IN GENERATORS.--A very annoying trouble which crops up every now and then during the evolution of acetylene consists in the production of large masses of froth within the generator. In the ordinary way, decomposition of carbide is accompanied by a species of effervescence, but the bubbles should break smartly and leave the surface of the liquid reasonably free from foam. Sometimes, however, the bubbles do not break, but a persistent "head" of considerable height is formed. ”