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pressed air as an atomizing agent, especially where the waste of fresh water is of no consequence. But there are good reasons to believe that air alone, preferably heated, is fully as good to use as steam, and once for all it should be recognized that steam is not, and cannot be, a fuel, that already it is fully burned hydrogen, and as useless and impossible of use as fuel as a piece of quicklime or a heap of furnace slag. It is quite probable that twenty-five years ago, as to-day, the use of high pressure steam was found better than low pressure steam. This was simply because of its greater temperature on the one hand, and of its greater atomizing effect on the other hand. There was nothing chemical in its effect, and to-day hot air is so much better than cold air because it is hot.

It should be further noted in respect of the claimed duty of 46 pounds evapor ation, that the claimant himself stated truly that "Sextane," one of the rare hydrocarbons, the composition of which is C,H,, had an evaporative efficiency of only 28-72. The claim that steam was a fuel was as clear and unmistakable as it is! erroneous. Particular attention has been drawn to this phase of the liquid fuel problem because mischief inevitably arises as the result of unscientific statements: and Aydon's paper has so frequently been quoted as authoritative that it has be come looked upon as a classic. So much of the statistical work that it contains is impossible, that it, with the discussion it evoked, cannot be considered reliable in many of the figures that may be well within the range of probability, and must be dismissed as not furnishing information of serious value to-day, though in the dis cussion on the paper certain speakers pointed out the errors to which attention has been called above.

In all fresh departures it is of course inevitable that many workers in the field will be men who, while possessed of great energy, and patience, and determination. are yet but poorly equipped in scientific knowledge even of the most elementary order. Many things are done which do not commend themselves to men of better information, and a long process of quite unnecessary trial and error is gone through. Sometimes, after needless expense, success is attained, but frequently after a discouraging expenditure of capital which renders the capitalist unwilling to incur further expense even on the very threshold of success. Too often failure alone results. So it has been with liquid fuel. Many past attempts have failed for want of elementary knowledge, combined with claims of impossible achievement which are not fulfilled in practice and result in disappointment.

The first really practical and efficient employment of liquid fuel to locomotives for steam-raising purposes appears to be due to Mr. Thomas Urquhart, of the Grazi and Tsaritzin Railway of Russia, to whom the writer is indebted for information. Mr. Urquhart used the spraying system and obtained good results, and his paper of 1884 marks the beginning of the period of really useful work.

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The application of liquid fuel in the Caucasus owes its success to a combination of causes. Russian petroleum has less light oil in its composition, and therefore produces more astatki, i.e. mazut or residuum ; coal is dear in the district, and the man was present in Mr. Urquhart to render the application of liquid fuel successful, previous applications not having proved so.

Mr. Urquhart placed the use of liquid fuel on so sound a basis that his system has naturally received considerable attention in what follows, and many of his tables have been reproduced.

1 Institution of Mechanical Engineers, Minutes of Proceedings. 1884.

The Chicago Exhibition in the early nineties gave great impetus to the use of liquid fuel in America, for all the boilers there were arranged with oil fuel only.

In Great Britain the use of liquid fuel has not been extensive, but it has been marked by good practice and only bids fair to become extensive since the introduction of Texas oil. Previously the tendency had been to use the products of distillation of coal or oil in the shape of tars or creosotes.

To-day liquid fuel is well established and recognized as a fuel of extreme elasticity, and one that can be burned smokelessly. It is realized that the days of experiment are past, and that no serious difficulties now remain to be overcome.

At the same time the question must be considered from a conservative standpoint, because at present and for years to come the output of petroleum will not be sufficient to make liquid fuel a serious rival of coal in every use of fuel. There is no knowledge of the possibility of extensive petroleum production in even the distant future. Petroleum wells do not endure indefinitely. They are not like artesian water wells, fed from surface rainfall, and chemists cannot assure us that they are being fed in any case from still deeper sources, nor is it decided whether petroleum is of mineral or of organic origin. The future of petroleum is thus more or less uncertain.

GENERAL CONSIDERATIONS

A general idea of the liquid fuel problem should therefore be obtained before attempting to gauge its merits. The Author's summing up of the question in the Electrical Review is abstracted in what follows.

There is a very considerable lack of the sense of proportion in many who discuss the question of liquid fuel without a sufficiently full knowledge of facts.

In Great Britain alone over 200 million tons of coal are raised each year. In the United States the amount is still greater. The present production of mineral oil is a mere fraction of the 773 millions of tons of coal produced in the world. There exists among those who have oil to sell a habit of raising prices so soon as a demand has been created. Such an instance recently occurred where a prospective oil user, who had based his calculations upon a price of 32s. per ton, found that he was expected to pay 65s.

Similarly, when Holden's system was applied to some South American locomotives, the price of oil was raised as soon as all the engines were known to be fitted. In that instance the sellers reckoned without their host, and in ignorance of the mechanical details of the system, and felt much aggrieved when the engines all ran out on coal alone.

Liquid fuel has undoubted advantages in many cases, and probably nowhere could it be used to better advantage than in an electric light station.

One of the principal advantages of oil is its high calorific value per pound. This, with the best oils, is double the capacity of the inferior coals, and 30 per cent. better than the best coal. The ease with which it can be stored and moved from point to point is an advantage. It can be fired mechanically, makes no ash or clinker, can be burned at maximum rate or entirely turned off in a moment. Further, a very large power of boilers requires very little labour in the stokehold.

Petroleum consists of a very large variety of constituents; these are gaseous, liquid, or solid. The gas is marsh gas, CH,, and at once disappears; the lighter liquids are very volatile, and finally there are solid bodies at the end of a long series of liquids of varying degrees of volatility and specific gravity.

The chemical formula which cover most of the constituents of petroleum are C Han and C, H2n+2 These formulæ continue throughout the whole range from marsh gas, CH,, onwards.

2.

The Texas oil is used chiefly as it is found.

The Russian oil is used in the form of astatki or residuum, the proportion left after distilling off the lighting and lubricating oils. also used in the form of residuum.

Much of the American oil is

The proportion of carbon in all the liquids used as fuel varies very little from 84 per cent., the hydrogen amounting to 16 per cent. There is little else, so that petroleum is practically all combustible.

It may be laid down as a well established fact that there is at present only one way to burn liquid fuel for steam raising, and that is by atomizing the fuel in company with a sufficient amount of air around each atom. In order that the oil may atomize freely, it should be deprived of its viscidity. This is readily done by heat. and brings with it the further advantage that any water in the oil more easily separates out of hot oil than cold oil, first, because the heated oil, being more limpid. offers less resistance to the freeing of the water; and secondly, there is greater expansion of oil than of water due to the heat, and the water gains a relatively greater specific gravity.

Warming is done by means of a steam coil, and it may be merely local warming in the vicinity of the take-off valve in the tank. It is essential that water be fairly well separated, because if it comes through the burners in any quantity it may extinguish the fires, and the next following oil is apt to ignite very explosively.

In storing oil there is always apt to be some vapour given off, and an empty tank ought not to be entered with a light.

Though not nominally of double the calorific capacity of average fair coal, oil is found in practice to be worth double the price of coal, owing to the labour cost which it saves.

This is as regards marine service, for the oil can be carried in ballast tanks, and paying cargo is carried in the coal bunker space.

For land purposes, of course, these latter considerations do not weigh, and the relative value must be placed on the performance ratio of about 16 to 10, together with the economy of labour, cleaning, ash cartage, etc.

Above and beyond all these things, however, is the power which liquid fuel gives of immensely increasing the steam-production of a boiler at short notice.

In general practice a steam-boiler is designed with a given ratio of heating surface per unit of fuel burned. Any reduction of this ratio is accompanied by a poorer performance. Less steam is produced per pound of oil consumed. A reduction of the heating surface ratio does not, however, reduce the performance by anything

like the same ratio.

If a large demand for steam is made upon a boiler for a short fraction of its working hours, it may be cheaper to consume fuel at a high rate for a fraction of the time than to employ two or even three boilers at normal rates during a fraction of the day, the extra boilers remaining idle during the rest of the day; albeit when the heavy load is past these extra boilers are retired hot and full of energy. The saving by the first method is very considerable in respect of space occupied, buildings and capital cost generally, and if not carried too far it will outweigh the fuel cost of the short run at heavy output.

For this system of working, coal can, of course, be employed. Coal, however,

cannot be fired at abnormal rates with special ease. A mechanical stoker does not readily increase its rate of working. The better forms of stoker-on the coking principle-find it very difficult to put their whole grate surface into the new and forced condition. The sprinkler class, again, do not work so well at abnormal rates. Coal combustion is only to be regulated by draught intensity. With oil the supply is instantly variable to suit the steam required, and a boiler can be rapidly put under its fullest output rate of work. With boilers of the small tube type, especially, their Mr small water contents enables the engineer to leave them standing cold to within a short time of maximum output. Oil is then turned on, and in a few minutes the boiler is in full work. When a boiler is already at work the mere turn of a handle puts it into its maximum steam-producing condition.

As soon as the demand ceases the oil can be turned off, and the normal coal fire continued, or the boiler laid off entirely. By means of liquid fuel great elasticity is possible.

In a lighting station the load factor is very usually about 12 per cent. That is to say, about one-eighth of the plant is, on the average, at work all the working hours.

This excessive misproportion is remedied to any desired extent by means of accumulators, but it is not yet commercially economical to instal so high a proportion of battery power as to enable the power-plant to run at steady load all day. The peak of the load, however short in duration, cannot be surmounted without the aid of power, and it is to the height and small duration of the maximum load curve that the poor load factor of a lighting station is due. Accumulators for heavy output of short duration greatly improve the load factor, but, in any case, the number of boilers at work to tide over the peak is several times the mean number.

If, by means of liquid fuel, boilers can be heavily pushed for two, three, or four hours, the capital outlay on boilers will be much reduced. When the various points are taken into account, the boiler scheme that will probably suggest itself will be, first some boilers of the Lancashire type, economical and steady steamers; secondly, large tube boilers with a moderate water contents and large grate area, and with efficient steam driers or superheaters. These boilers can be heavily forced with some sacrifice of economy, but the priming due to heavy forcing must be eliminated by a good superheater. This is essential to economy. Thirdly, small tube boilers of very small water capacity, capable of being heavily forced, delivering their steam preferably above water level in the steam drum. If all these boilers are fitted with oil sprayers, the maximum demand for steam will be met with the minimum of capital outlay.

It is a common fallacy to suppose that boilers of small water capacity respond most readily to a sudden demand for steam. The claim is, it is true, often made for water tube boilers. Nothing could be more erroneous.

When a boiler is at work under full pressure, the whole of its water is at a temperature which corresponds with the pressure. Any addition to the furnace activity cannot add to the heat contents of the boiler, unless the pressure is allowed to rise; obviously, therefore, given the continuance of the same pressure, the boilers of large water contents will answer to an urged fire just as rapidly as a boiler of small water contents. When boilers are standing at rest, however, and cold, the boiler which contains the least water will, ceteris paribus, become most quickly hot. Such a boiler as, for example, the Solignac, which holds almost no water, can be made, by aid of oil fuel, to produce its maximum power in a few minutes after lighting up.

In this respect oil has a very decided advantage over solid fuel. To secure ! good fire with solid fuel there must be a thick bed of incandescent fuel on the grate. and this can only be built up with comparative slowness, and when its duty is over it remains a more or less wasted force. With oil, however, the maximum fire is instantaneous, and the only drawback is the cold brickwork of the setting, which must become hot before the maximum furnace duty is attained.

For ordinary economical work the number of heat units that a boiler can absorb per square foot of heating surface will not be changed when liquid fuel is employed, except so far as liquid fuel can possibly be burned without smoke more easily than solid hydrocarbons, such as coal, and that thereby the heating surface is maintamed clean and free from dust and soot, and therefore more efficient. But mere evaporative efficiency must not be allowed to outweigh the overall, or commercial efficiency. Exactly what governs the relation between evaporative and commerend efficiency cannot be stated positively. Indeed, commercial efficiency alone should be considered as the true basis of design. It may, however, be stated in general terms that plant which is on duty for long hours may probably be designed to work more economically as regards fuel than plant intended to work very

hours.

short

Let it be assumed that the boilers which are economical of fuel have an eff ciency of 72 per cent., and that the small highly pushed boilers are run at 60 per cent. efficiency for three hours.

Then, in course of a year, coal is wasted which represents 12 per cent. difference of efficiency lost for three hours daily.

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To enable this loss to be avoided there would be so many thousands of pounds extra capital cost in boilers, buildings, etc., and where oil is not employed, so much more labour cost as compared with oil. Properly equated at a suitable rate of interest and depreciation, the relative value of the alternative systems may found after the manner of the Kelvin law applied to cable work. In many stations the extra labour for the heavy duty period must be difficult to arrange satisfactorily. Men are probably employed more hours than they really work, and where it may be best to use coal for 10 hours, the labour cost may make it cheaper to use oil for 4 hours of a peak load, even if, in mere fuel cost per unit, the oil is more expensive. Recent trials with liquid fuel show that there is still much to be done in reducing the air supply. The air required to burn 1 unit weight of carbon is 113 unibe. An ordinary oil fuel requires fully 15 units with, of course, some additional excess as with solid fuel, But with oil fuel there ought to be better mixture of air and fuel, and therefore better combustion with less excess of air.

If we regard air as the fuel and coal or oil as the sustainer of combustion, as we have a chemical right to do, we shall arrive at the conclusion that, approximately, the calorific value of a fuel in actual duty done will not differ much from the chemical ratio of air required in the combustion process. The large amount of air per pound of oil arises from the large percentage of hydrogen in the oil, and it is the large capacity for oxygen possessed by hydrogen which renders the theoretical temperature of combustion so nearly like that of carbon, in spite of the high calorific capacity of hydrogen.

As regards the production of petroleum, that of the United States in the year 1901 was 69,389,194 barrels, valued at 663 million dollars. If each barrel is assumed to contain 360 lb., or say 6 barrels per ton, the total tonnage will be 11,565,000, and the value therefore, something under 23s. per ton, or practically $1 per

barrel.

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