Fig. 35.
Plain cylindrical and wagon boilers have for many years been made with internal tubes of various shapes and arrangement, through which the flame passes to add to the heating surface. These are shown in dotted lines on the previous drawings of wagon boilers, [Fig. 11] and [Fig. 12]. They are also shown in [Fig. 32], where a tube passes from over the fire to the front of a plain cylindrical boiler; in [Fig. 33] two tubes pass from the sides to the front: in [Fig. 34] the tube passes from the back, but returns over the fire and passes again to the back: and in [Fig. 35] a tube from the back passes out through a cross tube in each side. The boilers in all these cases are fired externally. This addition of tubes has tended very much to increase the size of these boilers in order to make room for the tubes. These boilers are now found of 9, 10, and even 11 feet diameter; and this large shell being fired externally is exposed to the same dangers as those described in the plain cylindrical boiler, while it is not so easy to keep clean on account of the obstruction offered by the internal flues. When the flame has passed under the whole length of the bottom of these large boilers before going through any tube, it is doubtful whether the heating surface of the tube helps much in the generation of steam; but the tube is of use in reducing the quantity of water in the boiler, as it occupies a considerable space.
Fig. 36.
Explosions of these boilers have sometimes taken place by collapse of the tubes, but much more generally by the failure of the shell over the fire, as shown in the sketch [Fig. 36], representing an explosion that occurred at Wolverhampton in 1865, in which the first rent took place in a seam over the fire where frequent repair had led to a considerable length of longitudinal seam being in one continuous line. The four plates over the fire parted and opened out until they had ripped two seams completely round the boiler; and the plates were thrown in one flat piece, as shown, upon a bank behind. The main body of the boiler with the tubes was turned over, and the front end blown away.
Fig. 37.
A modification or amalgamation of several of the forms of boilers already mentioned led to the construction known as the Butterley boiler, shown in [Fig. 37], with a wagon-shaped end over the fire, continued in a single tube within a plain cylindrical shell beyond. This boiler has been found to generate steam very rapidly; but the extreme weakness of the construction over the fire and along the tube, especially at the part where the front end of the tube widens out in a bell mouth to meet the wagon-topped fireplace, has led to so many explosions that few boilers are now made of this form. A very early explosion that occurred at Edinburgh in 1821 was of a boiler somewhat of this shape, only that the wagon-topped fireplace was much longer. Other explosions of this form of boiler occurred at Ashton-under-Lyne in 1845, at Wolverhampton in 1854, and at Tipton in 1856.
