Fig. 31.
In order to avoid having a large diameter for plain cylindrical boilers, especially where exposed to the fire, boilers have been used that have supplied the required steam power by a combination of several cylinders of small diameter. One of these known as the Elephant boiler, has been so much used in France that it is sometimes called the French boiler; it is shown in [Fig. 29], and consists of two cylinders of small diameter connected by upright conical tubes to a large cylinder above. Another form called the Retort Boiler, shown in [Fig. 30], has been described at a previous meeting of this Institution (see Proceedings Inst. M. E. 1855 page 191). The disadvantages of these two combinations of plain cylinders are that they are not easy to clean or examine internally, and also there is not free exit for the steam, which has to find its way along small channels, and carries the water away with it, causing priming, and also retarding the generation of steam and endangering the boiler plates. With a view to strengthen the plain cylinder made of wrought-iron plates, the seams are sometimes made to run diagonally, as shown in [Fig. 31], on the principle that, as the longitudinal is the weakest seam and the transverse the strongest, a diagonal between them gives the greatest amount of strength to the boiler as a whole.
Fig. 32.
Fig. 33.
Fig. 34.