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SUCCESSFUL management of a boiler consists in getting the desired amount of work out of it, and at the same time keeping the expenses as low as possible. To be able to do this, a boiler must above all things be well designed for its work; it should be handled with the proper amount of
any defects which may show themselves should be made good at once and their causes removed.
Lighting Fires.-The fires in the main boiler are usually lighted by throwing some burning fuel from the donkey boiler into the main furnaces; but if this cannot be done, a small fire is kindled on one of the grates and the others lighted from it. In either case the whole of the fire bars are first covered with coal, so as to restrict the draught to that point where the fuel is burning; gradually, as the fire increases, the hot fuel is raked back, igniting the remainder. If the coal is very dusty it may be necessary to cover the bars with old matting or with paper.
If steam has to be raised quickly, the fires are started in all the furnaces at once; but this should never be done unless there is steam in the donkey boiler, and appliances are available for producing artificial circulation. Even where this is the case, and steam has to be raised with the utmost possible despatch, it is safer to force only one boiler, or such a number as are sufficient to drive the engine; for in all cases of great hurry, things are likely to go wrong, and it has happened over and over again that circumferential seams of boiler bottoms have cracked when not sufficiently warmed during this period. The circulating appliances used to be hydrokineters; now it is customary to fit a donkey suction to each boiler bottom, by which means the cold water is drawn off there and reintroduced through the feed near the water level. Where none of these arrangements are fitted, the heating of the boiler must progress slowly, taking about twelve hours. Many engineers start fires only in one furnace of each boiler, usually the lowest one, in the hope that the cold water, which remains undisturbed
· These pipes should be so fitted that water cannot accidentally be forced from one boiler into another if their pressures should differ.
at the bottom, will get warmed. A better plan is to light fires in one of the wing furnaces, F, fig. 1, of each boiler, which sets up a natural circulation, the water rising on one side and descending on the other.
If steam must be raised quickly, a good plan is to fill the boilers to nearly full glass, to light all the fires at once, and to blow off all the cold water below the furnaces as soon as that above them is boiling. From fear of subsequent leakage, the safety valves are usually kept closed, and the result is that the heated
air in the steam space shows a F
pressure on the gauge before steam has been generated, which is most misleading, because it all disappears as soon as the engines are started.
This air pressure on the water Fig. 1
surface also prevents the formation of steam bubbles when 212° F. has been reached, which would materially assist the circulation, by carrying water with them as they rise. It is, therefore, better to keep the main or safety valves full open till steam is blowing off freely, and only to close them when the boiler bottom has grown hot. If closed earlier, the chances are, even with slow firing, that the boiler bottoms remain cold long after the full working pressure has been reached, and boiler shells being constructed mainly to resist the steam pressure, are then sometimes unable to bear the additional straining to which they are subjected by the very serious differences of temperature between the upper two-thirds and lower one-third of their circumference. (See fig. 93, p. 57.)
Cracked Shell Plates.—This difference of temperature can amount to as much as 270° F., and, as iron expands about one-thousandth of its length between 32° F. and 212° F., the bottom of the boiler shell would tend to be about one-seven-hundredth of its length shorter than the top, which, in a double-ended boiler of 17 feet length, would amount to more than inch. Of course the upper two-thirds of the boiler would be slightly compressed, but the amount of metal in the lower third being the smaller, suffers the severest stress, probably quite two-thirds of that which the difference of temperature would warrant. But if iron or steel is prevented from contracting onethousandth of its length, which is the same thing as stretching the metal by that amount, a stress of 13 tons per square inch is set up, and this, then, is approximately the extra stress which the lower third of the boiler shell has to resist.
As the percentage of strength of the circumferential joints is comparatively low, it is not to be wondered at that either the rivets shear or that the metal tears. In either case there is no immediate danger, for the leakage is slight and the water cold, at least at first. However, sometimes the solid plate cracks circumferentially, and then the rush of water is considerable; but it is only amongst iron boilers that this
happens, which is doubtless due to the fact that this material is decidedly weaker across the grain than with it. As might be expected, double-ended boilers are more often injured in this way than single-ended ones.
Those who wish to obtain numerical results on this subject should fit the following arrangement to the backs of a boiler, one near the water level and the other near the bottom, or wherever they think it most convenient. A short iron tube (fig. 2), about 4 of an inch in diameter, and closed at one end, is screwed at an angle of about 30° into the back plate, and a little mercury or oil poured into it. When desired a thermometer can be inserted, and the temperature measured.
Fig. 2 If mercury is used the tubes must not be of brass or gun metal, otherwise they will be eaten away.
Stoking.-A perfect knowledge of stoking can only be gained by a practical experience, which does not fall to the lot of most engineers. It is not difficult to throw shovelsful of coal through the comparatively small fire doors, even when they are 4 feet above the floors, but generally inexperienced hands will not be able to place the fuel on those parts of the grate where it is wanted, even when the conditions are favourable—and that is one of the chief secrets of good stoking, particularly with Welsh and similar coal, which may not be disturbed after it has been put on the grate. North-country coal has a tendency to cake, whereby the air passages are choked. While breaking up this fuel with a rake or slicer an excellent opportunity is afforded for levelling it.
While burning Welsh coal the case is different. The depressions, where the air meets with least resistance, are burnt away quickest, even to such an extent that the upper edge of the bars becomes exposed, admitting a damaging excess of air (fig. 3). On account of this short cut very much less air finds its way through the thick fuel, and the combustion is reduced there. Un
FIG. 3 covered bars and thin fires are readily discovered on trials by holding an anemometer in each ashpit. On recoaling, the hollows will most likely be more than filled up; but even if levelled, the combustion will be fiercest where the hot fuel was lying thickest, for there it is
all aglow (fig. 4), and these are the parts which will now burn away quickest. Continued care is therefore necessary.
What is true of a single fire is true of a number, and unless all the fires connected to one funnel are kept equally thick, the thin ones will burn away fastest, and if there are several stokers on one watch,
he will have the lightest job who chokes his furnace with fuel. When steaming under strong forced draught with closed stoke holes, speed in coaling is of the utmost importance, so as not to keep the
doors open too long, FIG. 4
and it is customary to have a right- and left-handed stoker for each fire, with a leading man to direct them where to throw the fuel. In order to see better which parts of the grate are bare he should wear green-glassed spectacles.
Thickness of Fires. The question as to how thick the fires ought to be kept is a difficult one to answer. Less resistance will be offered to the passage of the air, and the combustion will be more rapid, the thinner they are. Thick fires, or rather heavy firing, offer the advantage that the furnace doors have to be opened less frequently, and the loss and injury due to inrushes of cold air are not so great; but such fires produce an enormous amount of volatile products, which are often so cold that they cannot ignite, as they should do, when mixed with the air which is admitted through the doors. This loss is, of course, greater with bituminous coal.
* Under natural draught the fires should be kept about 6 and 8 ins. thick respectively for North-country and for Welsh coal. Under forced draught they are sometimes 12 ins. thick.
Smoke Consumption.-In order to consume the volatile gases some firemen coal first one side of a grate and then the other, half of the upper surface being thus always in a glowing condition. Another plan-but this can only be carried out with caking coal—is to throw the green coal on the front end of the grate, and to rake it back when it is well alight. The combustible gases which are at first given off are thoroughly mixed with the air which reaches them through the doors, and passing over the red-hot coal the mixture is bound to ignite.
This plan has the advantage of allowing very long grates to be used. However, the ashes and clinker will be driven to the back ends, against the firebrick, whence it is most inconvenient to remove them. With dirty coals this trouble is so serious that firemen prefer to throw them as far back as possible, and to rake them forward when partly consumed, accumulating the ashes at the front end. This is an uneconomical proceeding.
Air Admission above the Grate. It is conceded on all hands that a certain amount of air must be admitted above the grate, and that it is difficult to determine how much ; but it will not be out of place to illustrate the subject by drawing attention to the behaviour of the flame of an ordinary paraffin lamp. If the wick is turned too low disagreeable smells are produced, filling the room in a very few minutes ; if too high, other smells and much soot are produced. In the one case we have incomplete combustion, due to the cooling action of an excess of air; in the other case too little was supplied. Smoke and smells are emitted from funnels, showing that here too unconsumed gases are escaping; for carbonic acid, as well as steam, is without odour.
With present arrangements it is almost impossible to regulate the air supply, or at any rate to fix it, so that the ratio of the excess to the total shall have a definite value, and consequently we find in practice that it varies from 25% to 200%. (See pp. 95 and 102.)
It is clear that when the fires are thick less air passes through them, less fuel is consumed, and relatively more air is drawn in through the doors. With thin fires the draught through them is stronger, more fuel is burnt, and less excess air drawn in. Now, although the products from thick fires contain more inflammable gases than those from thin ones, and consequently require more excess air than these, it is difficult to believe that they will receive just their correct share, whatever the sizes and number of holes in the doors, whatever the state of the grate, and whatever the intensity of the draught or the quality of the coal; and it is only reasonable to assume that the best results will be obtained I. If no air is admitted through the doors when the fires are
very thin and all aglow. II. If much air is admitted immediately after coaling and when
the fires are thick. III. If more air is admitted with North-country than with
tion is reduced. But even with the most correct proportions the combustion will not be perfect, if for no other reason than that the flame comes in contact with the boiler plates, and gets cooled, before it is burnt out.
As regards the excess air, it is perhaps of more importance to decide where to admit it than how much to admit, provided it be enough. If it were possible to keep the fires very thin, it might be well to admit all the air through the bars, and none above the fire; but this is impossible. To admit air through the furnace doors stimulates combustion at this point, so that, before mixing with the distilled furnace gases, it has already been robbed of much of its oxygen. Just after coaling this is not the case; but being even colder than the products of distillation, it will cool instead of igniting them. As already mentioned, this result can be evaded by throwing the green coal only on the front ends; but another plan is to have much brick-work at or behind the bridge, which soon grows very hot, and the mixed gases ignite on coming in contact with it, and burn in the combustion chamber. Unfortunately, the distance to the tubes being short, the flame is immediately extinguished on entering them.
The plans have also been tried of admitting air at the bridge, or through a tube passing from the shell direct into the combustion