54

MODERN STEAM PRACTICE.

best plan for superheating the steam; far before the complicatedarrangements of separate superheating boxes, with the extra stop-valves, &c. In fact, dry superheaters soon get out of Order, moreespecially when there is no steam in the boilers, as must be the case

Longitudinal Section. Front Elevation and Transverse Section.

Fig. 28.-—High Double Boiler.

B, Combustion chamber. cc, Tubes. d d, Smoke-boxes. ee, Uptakes.

aa, Furnaces.

for a considerable time when the fires are first kindled. Any onecan fancy the flame acting on a thin tube, roasting, as it were, thesteam, which subsequently dries up the lubricants, and soon playshavoc with the slide-valves, pistons, and cylinder faces of the engine.Steam is only partially dried in the best modern practice, and canbe done in the boiler itself. It will be understood, in the boilerdescribed, that two ends and two furnace backs are saved, thematerial being better disposed in the uptakes.

As we are dealing at present with low-pressure steam-boilerssuited for the merchant Service, we will draw attention to overheadflue arrangements. All boilers of this dass should be so designedthat every part is easily accessible for repairs; and, when properlyconstructed, we do not see why the flues should not last as long asany other part, and certainly boilers can be designed so that theflame and heated gases will pass up and down over a greater lengthof surface than in the plain tubulär boilers. The flues in this ex-ample are the entire width of the boiler, leaving 6 inches of waterspace at the sides; the flame passes to the top of the combustionchamber at the back of furnaces, then dips downwards, and so on,