Fig. 123.
The form and arrangement of the water-spaces and flues in marine boilers may be collected from the sections of the boilers used in some of the government steamers, exhibited in figs. 121, 122, 123. A section made by a horizontal plane passing through the flues is exhibited in [fig. 121.] The furnaces F communicate in pairs with the flues E, the air following the course through the flues represented by the arrows. The flue E passes to the back of the boiler, then returns to the front, then to the back again, and is finally carried back to the front, where it communicates at C with the curved flue B, represented in the transverse vertical section, [fig. 122.] This curved flue B finally terminates in the chimney A. There are in this case three independent boilers, each worked by two furnaces communicating with the same system of flues; and in the curved flues B, [fig. 122.], by which the air is finally conducted through the chimney, are placed three independent [Pg449] dampers, by means of which the furnace of each boiler can be regulated independently of the other, and by which each boiler may be separately detached from communication with the chimney. The letters of reference in the horizontal section, [fig. 121.], correspond with those in the transverse vertical section, [fig. 122.], E representing the commencement of the flues, and C their termination.
Fig. 124.
A longitudinal section of the boiler made by a vertical plane extending from the front to the back is given in [fig. 123.], where F, as before, is the furnace, G the grate-bars sloping downwards from the front to the back, H the fire-bridge, C the commencement of the flues, and A the chimney. An elevation of the front of the boiler is represented in [fig. 124.], showing two of the fire-doors closed, and the other two removed, displaying the position of the grate-bars in front. Small openings are also provided, closed by proper doors, by which access can be had to the under side of the flues between the foundation timbers of the engine for the purpose of cleaning them.
Each of these boilers can be worked independently of the others. By this means, when at sea, the engine may be worked by any two of the three boilers, while the third is being cleaned and put in order. In all sea-going steamers multiple boilers are at present provided for this purpose.
In the boilers here represented the flues are all upon the same level, winding backwards and forwards without passing one above the other. In other boilers, however, the flues, [Pg450] after passing backwards and forwards near the bottom of the boiler, turn upwards and pass backwards and forwards through a level of the water nearer its surface, finally terminating in the chimney. More heating surface is thus obtained with the same capacity of boiler.
The most formidable difficulty which has been encountered in the application of the steam-engine to sea-voyages has arisen from the necessity of supplying the boiler with sea-water instead of pure fresh water. The sea-water is injected into the condenser for the purpose of condensing the steam, and it is thence, mixed with the condensed steam, conducted as feeding water into the boiler.