BOILERS FOR STATIONARY ENGINES.

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centre of the boiler, there being no tubes at the centre, but merelyall round the opening in the top of the pot, which is bolted to thetube-plate by means of projecting flanges on the pot and tube-plate.

In some cases, such as in the Fire-engine, it is a desideratumto have a rapid steam-producing boiler; a good example is simplyan ordinary vertical boiler, having the tubes suspended inside ofthe fire-box, arranged with an internal tube in each, loosely sup-ported from the tube-plate, these small inside tubes leaving annularspaces between them and the larger tubes, so that only a thin filmof water is exposed to the heating surface. By this means steam israised rapidly; but it must be borne in mind that, as the evapora-tion of the water is very great, care must betaken that a sufficient quantity of water is keptup in the boiler, which would otherwise soonboil dry. The circulation is very rapid in thetubes. The bottoms of the tubes are hermeti-cally sealed; and as the steam is generated,it ascends, displacing the water in the annularspace between the inner and outer tubes, andthe water from the top circulates down theinner tubes and fills up the cavity. The smoke-pipe is connected to the top of the fire-box,passing through the steam space, and is rivettedto an angle-iron ring on the top of the boiler;and there is an open part left in the centre ofthe fire-box where there are no tubes, thisopening being blocked up with a lump of fire-brick suspended by a rod from the top of theboiler, thus the flame is prevented from going directly up thechimney, as it impinges against the fire-clay lump, and by thismeans it is distributed beneficially amongst the small tubes.

Instead of a number of annular tubes, one large tube has been suc-cessfully adopted, the arrangement consisting of an internal fire-box,having an annular 1 water space all round. On the outside of thiswater space there is an annular flue, and the whole is contained inan ordinary vertical boiler, having a hemispherical top; the flameand the gases, after doing duty in the fire-box, find their way

Fig. 12. —Boiler with SuspendedAnnular Tubes. a, Shell.b, Fire-box. c, Tubes. d, Fire-brick lump, e, Smoke-pipe.F, Fire -door

1 The space between a small inner and large outer tube is called annular.