« PreviousContinue »
three times their original bulk. The bulk, therefore, of the products of combustion which inust pass off, must be 154.814 X3=464.442 cubic feet. At a velocity of 36 feet per second, the area, to allow this quantity to pass off in an hour, is .516 square inch. In a furnace in which 13 lbs. of coal are burnt on a square foot of grate per hour, the area to every foot of grate would be .516 x 136.709 square inches; and the proportion to each foot of grate, if the rate of combustion be higher or lower than 13 lbs., may be found in the same way. This area having been obtained, on the supposition that no more air is admitted than the quantity chemically required, and that the combustion is complete and perfect in the furnace, it is evident that this area must be much increased in practice, where we know these conditions are not fulfilled, but that a large surplus quantity of air is always admitted. A limit is thus found for the area over the bridge, or the area of the fine immediately behind the furnace, below which it must not be decreased, or the due quantity could not pass off, and consequently the due quantity of air could not enter, and the combustion would be proportionally imperfect. It will be found advantageous in practice to make the area two square inches instead of .516 square inch. The imperfection of the combustion in any furnace, when it is less than 1.5 square inch, will be rendered very apparent by the quantity of carbon which will rise unconsumed along with the hydrogen gas, and show itself in a dense black smoke on issuing from the chimney. This would give twenty-six square inches of area over the bridge to every square foot of grate, in a furnace in which the rate of combustion is thirteen pounds of coal on each square foot per hour, and so in proportion for any other rate. Taking this area as the proportion for the products of combustion immediately on their leaving the furnace, it may be gradually reduced as it approaches the chimney, on account of the reduction in the temperature, and consequently in the bulk of the gases. Care must,
, however, be taken that the flues are nowhere so contracted, nor so constructed, as to cause, by awkward bends, or in any other way, any obstruction to the draught, otherwise similar bad consequences will ensue.
An idea is very prevalent that it is advantageous to make the flame, or hot gases, (as they may be termed, because we may look upon flame merely as a stream of gases heated to incandescence,) impinge upon, or strike forcibly, the plates of a boiler at any bend or change of direction in the flue. The turn in the flue is therefore made with a square end, and with square corners; but it is difficult to see on what rational grounds the idea of advantage can be upheld. The gases, if they are already in contact with the plate, cannot be brought closer to it, and any such violent action is not necessary to alter the arrangement of the particles of the gases and bring the hotter particles to the outside, while there is a great risk of an eddy being formed, and of the gases being thrown back and returned upon themselves, when they strike the flat opposing surface; thus impeding the draught, and injuring the performance, of the boiler. That circulation will take place to a very great extent among the particles of
heated gases, flowing in a stream even in a straight flue, will be apparent from those particles next the surface being retarded by the friction against the sides, and by their tendency to sink into a lower position in the stream from their having been cooled down and become more dense.
An easy curve is sufficient to cause great change in the arrangement of the particles, as those which are towards the outside of the bend have a much longer course to travel, and are thus retarded in comparison with the others. From these causes the hotter particles in the centre of the flowing 'mass are, in their turn, brought to the outer surface, and made to give out their heat. The worın of a still is never found returning upon itself with square turns, as if the vapor inside would be more rapidly cooled by its impinging on the opposite surface; yet the best form of worm is a subject which has engaged the attention of many able men, and therefore may well be taken by engineers as a guide in the management of a similar process, though carried on at a much higher temperature.
Another very prevalent practice, and which also would seem to be open to serious objections, is, that the fues are frequently made of much greater area in one part than in another. This arises from a desire to obtain a larger amount of heating surface than is consistent with the proper area of the flue, or with the amount of the heated gases which are passing through it. The flue is thus made shorter in its course than it ought to be in proportion to its sectional area. This is even sometimes done by placing a plate of iron partly across the flue, near the bottom of the chimney, thus suddenly contracting the passage for the gases. The effect of this is evidently to cause a very slow and languid current in the larger part of the flue, and the consequence is, that a deposition of soot rapidly takes place there. In niany marine and land boilers, having one internal flue in them, of too large a size, this will be found to be the case, soot being soon deposited, till the flue is so filled up that the area lest is only such as is due to the quantity of heated gases passing through it; the value of those parts of the sides of the flue which are covered with soot are thus lost. This is well exemplified in Mr. Dinnen's paper on marine boilers, in the Appendix to Weale's edition of Tredgold, where he states, that the flues of the boiler in H. M. Steamer “ African,” after she had performed a great deal of work, in the course of five weeks' time, during which period there was no opportunity of sweeping them, were found to be in exactly the same state as after a voyage of five days, or probably as they would have been found after a much shorter time, if they had been examined. These flues are about the same area throughout their whole length, but the chimney is of much less area.
In the first portion of the flue from the fire no soot was deposited, but the deposit began after the first turn that the flue took, and gradually increased in amount to the foot of the chimney. The inference that may be drawn from this fact appears to be, that the gases, at first highly heated and thereby expanded, filled the first part of the flue, but as they were cooled they became inore contracted in their bulk, regularly towards the chimney, and therefore allowed the soot to be deposited in the space not properly filled by them in their course, and all soot subsequently formed was carried out at the chimney top by the velocity and power of the current. The amount collected near the foot of the chimney, and in the portions of the flue furthest from the fire, diminished the amount of the surface of the boiler exposed to the action of the heated gases, and the efficiency of the boiler was therefore impaired to the same extent. In those boilers in which the flues, before reaching the chimney, are very much too large, and are contracted, as has been stated, by a plate put across them, the extent to which their efficiency is thus impaired must evidently be much greater and to a serious extent, as this evil exists in them to a very much greater degree.
The due amount of heating surface that ought to be given in a flue to carry off the caloric, or to cool down a given quantity of heated gases, has not yet been investigated with any great degree of accuracy, and practice varies widely under different circumstances. The largest proportion is allowed in the Cornish boilers, some of which have not less than thirty feet and even forty feet of heating surface to one foot of grate. This appears to be more than is justified by any cor. responding gain, and certainly more than would be advisable in any marine or locomotive boilers. In boilers burning thirteen pounds of coal per hour on each superficial foot of grate, a proportion of eighteen feet to each foot of grate will be found to give good results. Where slow combustion is carried on, and where an extra size of boiler is not objectionable, some advantage may be gained by increasing the amount in proportion to the amount of fuel consumed. In calculating this surface, it is usual not to include the bottoms of the square flues in marine boilers, and in circular flues from one-fourth to onethird of the surface should be deducted as bottom surface, and there. fore not efficient as heating surface. It is not usual to make any distinction between horizontal and vertical surfaces, though it is probable that the former are considerably more valuable. The efficiency, however, of some boilers which have been made with vertical tubes, would rather tend to make it doubtful whether so much difference exists between the value of horizontal and vertical surfaces as has been generally supposed. If the area, instead of being in one large flue, be sub-divided into a number of small flues, or pipes, so as to expose the gases to the required amount of surface in a short course, the distance traversed between the fire-place and the chimney does not seem to be important. The velocity of the current of gases will not be materially influenced by their sub-division, as the whole amount of the surface with which the gases must come in contact, tending to impede their course by friction, will be the same in both
It is evident that numerous small flues, by sub-dividing the large stream of gases, which in the other case flow off in one body, bring the greater proportion of the particles at once into contact with the surfaces, and therefore render it unnecessary to pay the same amount of attention to the turning of the stream and the bringing out the hotter particles from the centre of the flowing mass. If these proportions of area through the flues and of heating surface be duly attended to, the results anticipated may be depended upon, whether
Alues are of large area or are composed of a large number of small tubes.
The time occupied by the gases in passing through the boiler, from the instant of their generation to that of their leaving the boiler, and the length of the course through which they have traveled, have sometimes been looked upon as matters of great importance. Where the gases are traveling in one compact mass, it is evident that distance and consequently time (as the velocity with which the current flows is the same in all cases) must be allowed, for the different particles of this large mass so to circulate among themselves as that each may have an opportunity of coming into contact with a cooling medium, to give off its heat; but if the large mass of gases is so sub-divided that the different particles are sooner brought into contact with the due amount of cooling medium, then the time the gases remain in the boiler ceases to be of importance. When the gases have reached the foot of the chimney, in a well-proportioned boiler, they will be found to be reduced to a temperature of about 500° Fah., or below it; their bulk will, in consequence, be reduced by about onethird below their bulk on their first leaving the furnace. The reduction in the area of the flue ought not to be in the same proportion, because their velocity is no longer so great. The reduction ought to be made gradually, as has been stated before, and not by a sudden contraction at the foot of the chimney, as the effect of this is to cause a slowness of draught in the latter part of the flue, and consequently a deposition of soot; and then the surface so covered, which had been reckoned upon as effective heating surface, is lost. The area of a chimney, to allow the products of the combustion of each pound of coal consumed in an hour to pass off, should not be less than threefourths of two square inches, this latter being the area given for the flue immediately behind the fire place that is, one and a half square inch; and for a boiler burning thirteen pounds of coal per hour, on each superficial foot of its grate, the area should be three-fourths of twenty-six square inches, or nineteen and a half square inches.
Theoretical research not having as yet given us any valuable assistance in determining the proper height of a chimney, we must again refer to practice as our guide. A good draught may be obtained with a very low chimney, but at a great expenditure of fuel, from the necessity that exists in such a case for allowing the gases to pass off at a much higher temperature than would otherwise be necessary. For a chimney built of brick-work the height ought not to be less than twenty yards, and may be increased to thirty yards or forty yards, with advantage to the economy of fuel. When chimneys are carried to a still greater height, it is generally for the purpose of carrying off the smoke, or any deleterious gases, from the imme. diate neighborhood, or to create a good draught with gases at a lower temperature than those from a steam-boiler furuace. On board steam vessels chimneys are limited in their height by the size of the ship, on account of the influence the chimney has on the stability and appearance. It will generally be found advantageous to make the chimney as high as these circumstances will permit. It will be found VOLUME X, 3RD SERIES. No. 3.-SEPTEMBER, 1845.
to tend greatly to the efficiency of a boiler to allow a large space in it as a reservoir for steam. The surface for ebullition does not seem to be of much importance in comparison with this point.
In the application of the foregoing proportions to practice, no reference need be had to the form of the boiler; the same results will be obtained whether the boiler be circular, wagon-shaped, or any other form, if all the other circumstances be made the same. By due management in the process of firing, when these proportions are given to the furnace and flues, the combustion will be found to be such that but little carbon will pass off to be converted into smoke, and the results will show great economy in the consumption of fuel.
Civ. Eng. and Arc. Jour.
Remarkable Properties of Water and other Fluids, and their con.
nexion with Steam-Boiler Explosions, being a Lecture read before the Members of the Royal Manchester Institution, February 17th, 1845. By Jour EDDOWES BOWMAN, Esq.
The lecturer commenced with a few preliminary remarks upon the relation which subsists between the philosophy and the applicution of science. It often happens, he observed, that theory follows in the wake of discovery, and that experience grows familiar with important details of practice, before the abstract principle involved, is sought out and clearly recognised. In what he advanced in the present lecture, however, a different sequence will be apparent, and we shall have an illustration of the value of a scientific application of common every day facts, in the solution of problems of the utmost moment to human life. He alluded to the property which liquids possess, of assuming the form of a globe or spheroid, when thrown upon any substance which is at a high temperature. Of this property, a familiar instance is afforded by an experiment performed every day in our laundries. When it is required to know whether a smoothing iron is sufficiently hot for her purpose, the laundress, on taking it from the stove, applies extemporaneously a drop of moisture from her mouth, and if this at once rolls off in the form of a globule, she knows by experience that the iron has reached a proper temperature: while if the drop of water bubbles and boils, however violently, the iron is condemned as not hot enough, and returned to the stove.
Once, then, in the flight of ages past it was discovered that water, though it so readily boils when thrown upon a moderately heated iron, does not boil at all when in contact with metal considerbly more heated.
This fact, like many others equally familiar, has been allowed 10 lie unexplained and uninterrogated, during a long lapse of time ; and it is only within the last few years that it has attracted any attention from the physical philosopher.
During his stay in Paris, he had an opportunity of seeing, in the laboratory of Dumas, some experiments performed by M. Boutigny, of Evreux, who has devoted a great deal of time to the subject, and