In all three types of boiler the brickwork is constructed to form one central flue passing along the bottom of the boiler and two side flues extending up the side nearly to the water-level. A cross section of the brickwork is shown in fig. 2. The usual arrangement is for the flue gases to be divided as they leave the internal flue; one-half returns along each side flue to the front of the boiler, and the whole then passes downwards into the central flue, travelling under the bottom of the boiler until the gases again reach the back end, where they pass into the chimney. In a few cases the arrangement is reversed, the gases first passing along the bottom flue and returning along the side flues. This latter arrangement, whilst promoting a more rapid circulation of water, has the disadvantage of requiring two dampers, and it is not suitable for those cases in which heavy deposits form on the bottoms of the boilers.
Where floor space is limited and also for small installations, other forms of cylindrical boilers are used, most of them being of the vertical type. That most commonly used is the simple vertical boiler, with a plain vertical fire-box, and an internal Vertical. smoke stack traversing the steam space. The fire-box is made slightly tapering in diameter, the space between it and the shell being filled with water. In all but the small sizes cross tubes are generally fitted. These are made about 9 in. in diameter of 3⁄8-in. plate flanged at each end to enable them to be riveted to the fire-box plates. They are usually fitted with a slight inclination to facilitate water circulation. and a hand-hole closed by a suitable door is provided in the outer shell opposite to each tube for cleaning purposes. A boiler of this kind is illustrated in fig. 4. This form is often used on board ship for auxiliary purposes. Where more heating surface is required than can be obtained in the cross-tube boiler other types of vertical boiler are employed. For instance, in the “Tyne” boiler (fig. 5) the furnace is hemispherical, and the products of combustion are led into an upper combustion chamber traversed by four or more inclined water-tubes of about 9 in. diameter and by several vertical water-tubes of less diameter. In the “Victoria” boiler made by Messrs Clarke, Chapman & Co., and illustrated in fig. 6, the furnace is hemispherical; the furnace gases are led to an internal combustion chamber, and thence through numerous horizontal smoke-tubes to a smoke-box placed on the side of the boiler. In the somewhat similar boiler known as the “Cochran,” the combustion chamber is made with a “dry” back. Instead of a water space at the back of the chamber, doors lined with firebrick are fitted. These give easy access to the tube ends.
| Fig. 3.—Lancashire Boiler (Messrs Tinker, Ltd.). |
| Fig. 4.—Simple Vertical Boiler. |
The cylindrical multitubular return tube boiler is in almost universal use in merchant steamers. It is made in various sizes ranging up to 17 ft. in diameter, the usual working pressure being from 160 to 200 ℔ Marine. per sq. in., although in some few cases pressures of 265 ℔ per sq. in. are in use. These boilers are of two types, double- and single-ended. In single-ended boilers, which are those most generally used, the furnaces are fitted at one end only and vary in number from one in the smallest boiler to four in the largest. Three furnaces are the most usual practice. Each furnace generally has its own separate combustion chamber. In four furnace boilers, however, one chamber is sometimes made common to the two middle furnaces, and sometimes one chamber is fitted to each pair of side furnaces. In double-ended boilers furnaces are fitted at each end. In some cases each furnace has a separate combustion chamber, but more usually one chamber is made to serve for two furnaces, one at each end of the boiler. The two types of boilers are shown in figs. 7 and 8, which illustrate boilers made by Messrs D. Rowan & Co. of Glasgow, and which may be taken as representing good modern practice. The furnaces used in the smaller sizes are often of the plain cylindrical type, the thickness of plate varying from 3⁄8 in. up to ¾ in. according to the diameter of the furnace and the working pressure. Occasionally furnaces with “Adamson” joints similar to those used in Lancashire boilers are employed, but for large furnaces and for high pressures corrugated or ribbed furnaces are usually adopted. Sketches of the sections of these are shown in fig. 9. The sections of the Morison, Fox and Deighton types are made from plates originally rolled of a uniform thickness, made into a cylindrical form with a welded longitudinal joint and then corrugated, the only difference between them being in the shapes of the corrugations. In the other three types the plates from which the furnaces are made are rolled with ribs or thickened portions at distances of 9 in. These furnaces are stronger to resist collapse than plain furnaces of the same thickness, and accommodate themselves more readily to changes of temperature.
| Fig. 5.—Vertical Boiler with Water-tubes (the “Tyne,” by Messrs Clarke, Chapman & Co.). |
There are two distinct types of connexion between the furnaces and the combustion chambers. In one, shown in fig. 8, the furnace is flanged at the crown portion for riveting to the tube plate, and the lower part of the furnace is riveted to the “wrapper” or side plate of the combustion chamber. In the other type, shown in fig. 7, and known generally as the “Gourlay back end,” the end of the furnace is contracted into an oval conical form, and is then flanged outwards round the whole of its circumference. The tube plate is made to extend to the bottom of the combustion chamber, and the furnace is riveted to the tube plate. The advantage of the Gourlay back end is that in case of accident to the furnace it can be removed from the boiler and be replaced by one of the same design without disturbing the end plates, which is not possible with the other design. The Gourlay back end, however, is not so stiff as the other, and more longitudinal stays are required in the boiler.
| Fig. 6.—Vertical Boiler with internal combustion chamber (the “Victoria,” by Messrs Clarke, Chapman & Co.). |
| Fig. 7.—Single-ended Marine Boiler. |
The flat sides and backs of the combustion chambers are stayed either to one another or to the shell of the boiler by numerous screw stays which are screwed through the two plates they connect, and which are nearly always fitted with nuts inside the combustion chambers. The tops of the chambers are usually stayed by strong girders resting upon the tube plates and chamber back plates. In a few cases, however, they are stayed by vertical stays attached to T bars riveted to the boiler shell. A few boilers are made in which the chamber tops are strengthened by heavy transverse girder plates. The end plates of the boiler in the steam space and below the combustion chambers are stayed by longitudinal stays passing through the whole length of the boiler and secured by double nuts at each end. The tube plates are strengthened by stay tubes screwed into them.