Fig. 105.
The tube-plates (as those parts of the boiler ends in which the tubes are inserted are called) are bored with holes in corresponding positions, truly cylindrical, and corresponding in magnitude to the tubes, so that the tubes, when passed into them, will be just in contact with them. The length of the tubes is so regulated, that when extending from end to end of the boiler, and passing through the holes, they shall [Pg369] project at each end a little beyond the holes. The manner of fastening them so as to be water-tight is as follows:—A steel hoop or ferrule, made slightly conical, a section of which is exhibited at C. [fig. 105.], the smaller end of which is a little less than the internal diameter of the tube, but which increases towards the outer end, is driven in as represented in the figure. It acts as a wedge, and forces the tube into close contact with the edges of the hole in the tube-plate.
When particular tubes in a boiler are worn out, and require to be replaced, their removal is easily effected. It is only necessary to cut the steel ferrule on the inside, and to bend it off from contact with the tube, by which means it can be loosened and withdrawn, and the tube removed.
In the engine to which this description refers there were one hundred and twenty-four tubes, the external diameter of which was 15⁄8 inch. The distance between tube and tube was 3⁄4 of an inch. The number of tubes vary in different engines, some having so many as one hundred and fifty, while the number in some is less than ninety. The evaporating power of an engine greatly depends on the proper number and magnitude of its tubes; and the experience which engineers have had on railways have led them gradually to increase the number of tubes, and diminish their magnitude. In the Rocket, already mentioned as having gained the prize on the opening of the Liverpool and Manchester Railway, the number of tubes was twenty-four, and their diameter three inches; but in all the engines subsequently made their number was augmented, and their diameter diminished. The practical inconvenience which limits the size of the tubes is their liability to become choked by cinders and ashes, which get wedged in them when they are too small, and thereby obstruct the draft, and diminish the evaporating power of the boiler. The tubes now in use, of about an inch and a [Pg370] half internal diameter, not only require to be cleared of the ashes and cinders, which get fastened in them after each journey, but it is necessary throughout a journey of any length that the tubes should be picked and cleaned by opening the fire door at convenient intervals.
The substitution of brass for copper tubes, which has been already mentioned as so great an improvement in the construction of locomotive engines, is ascribed to Mr. Dixon, who suggested them in 1833, being then the resident engineer of the Liverpool and Manchester Railway. They are said to last six or eight times as long as copper tubes of the same dimensions.
When tubes fail, they are usually destroyed by the pressure of the water crushing them inwards: the water enters through the rent made in the tube, and flowing upon the fire extinguishes it. When a single tube thus fails upon a journey, the engine, notwithstanding the accident, may generally be made to work to the end of its journey by plugging the ends of the broken tube with hard wood; the water in contact with which will prevent the fire from burning it away.
Tubes of the dimensions here referred to weigh about sixteen pounds, and lose from six to seven pounds before they are worn out. Their cost is about one pound each.
The tubes act as stays, connecting the ends of the boiler to strengthen them. Besides these, there are rods of wrought iron extended from end to end of the boiler above the roof of the internal fire-place. These rods are represented at o in their length in [fig. 97.], and an end view of them is seen in [fig. 102.] The smoke-box F, [fig. 97.] [ 104.], containing the cylinders, steam-pipe, and blast-pipe, is four feet wide, and two feet long. It is formed of wrought iron plates, half an inch thick on the side next the boiler, and a quarter of an inch elsewhere. The plates are riveted in the same manner as those of the fire-box already described. From the top of the smoke-box, which, like the fire-box, is semi-cylindrical, as seen in elevation in [fig. 103.], and in section [fig. 104.], rises the chimney G, fifteen inches diameter, and formed of 1⁄8 inch iron plates, riveted and bound round by hoops. It is flanged to the top of the [Pg371] smoke-box, as represented in [fig. 104.] Near the bottom of the smoke-box the working cylinders are placed, side by side, in a horizontal position, with the slide valves upwards. In the top of the external fire-box a circular aperture is formed fifteen inches in diameter, and upon this aperture is placed the steam-dome T (figs. 97. 101, 102.) two feet high, and attached around the circular aperture by a flange and screw secured by nuts. This steam dome is made of brass 3⁄8 inch thick. In stationary boilers, where magnitude is not limited, it has been already explained, that the space allowed for steam is sufficiently large to secure the complete separation of the vapour from the spray which is mixed with it when it issues immediately from the water. In locomotive boilers sufficient space cannot be allowed for this, and the separation of the water from the steam is effected by the arrangement here represented. A funnel-shaped tube d′ (figs. 97. 102.), with its wide end upwards, rises into the steam-dome, and reaches nearly to the top of it. This funnel bends towards the back of the fire-box, and is attached by a flange and screws to the great steam-pipe S, which traverses the whole length of the boiler. The steam rising from the boiler fills the steam-dome T, and descends in the funnel-shaped tube d′. The space it has thus to traverse enables the steam to disengage itself almost completely from the priming. The wider part of the great steam-pipe a is flanged and screwed at the hinder end to a corresponding aperture in the back plate of the fire-box. This opening is covered by a circular plate, secured by screws, having a stuffing-box in its centre, of the same kind as is used for the piston-rods of steam-cylinders. Through this stuffing-box the spindle a″ of the regulator passes, and to its end is attached a winch h′, by which the spindle a″ is capable of being turned. This winch is limited in its play to a quarter of a revolution. The other end of the spindle a″ is attached to a plate e′ seen edgeways in [fig. 97.], and the face of which is seen in [fig. 102.]: this circular plate e is perforated with two apertures somewhat less than quadrants. That part of the plate, therefore, which remains not pierced forms two solid pieces somewhat greater than quadrants. This plate is ground so as to move in steam-tight [Pg372] contact with a fixed plate under it, which terminates at the wide end of the conical mouth of the steam-pipe S. This fixed circular plate is likewise pierced with two nearly quadrantal apertures, corresponding with those in the movable plate e′. When the movable plate e′ is turned round by the winch h′, the apertures in it may be made to correspond with those of the fixed circular plate on which it moves, in which position the steam-pipe S communicates with the funnel d′ by the two quadrantal apertures thus open. If, on the other hand, the winch h′ be moved from this position through a quarter revolution, then the quadrantal openings in the movable plate will be brought over the solid parts of the fixed plate on which it moves, and these solid parts being a little more than quadrants, while the openings are a little less, all communication between the steam-pipe S and the funnel d′ will be stopped, for in this case the quadrantal openings in the fixed and movable plates respectively will be stopped by the solid parts of these plates. It will be evident that as the winch h′ of the regulator is moved from the former position to the latter, in every intermediate position the aperture communicating between the funnel d′ and the steam-pipe S will be less in magnitude than the complete quadrant. It will in fact be composed of two openings having the form of sectors of a circle less than a quadrant, and these sectors may be made of any magnitude, however small, until the opening is altogether closed.
By such means the admission of steam from the boiler to the steam-pipe S may be regulated by the winch h′.