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MODERN STEAM PRACTICE.
the introduction of Steel the question has been raised as to the rela-tive resistance of the iron and Steel plates to this action. So far asexperiment or experience has gone, the action seems to be prettymuch the same in both materials. The influence of scale upon theSteel plates is prejudicial, as a galvanic action is set up between thepart covered with scale and any parts not so covered, which causespitting of the latter. This scale of black oxide can be removed byexposure to acid, and in ships building at present for H. M. Navythe plates are immersed in a solution of sulphuric acid and water soas to clear away any scale which may adhere to them.
Where certain kinds of peaty water is used for feeding, the boilerseems to be quite unaffected by corrosive action. This is notablythe case in the boilers of the Loch Lomond Steamers, some of whichafter very many years’ Service are unaffected by corrosive action.It appears that a kind of coating, of a dark or brownish colour, isdeposited on the iron, which protects it, and does not appear toaffect the conducting power of the plate.
One method of bringing about the much to be desired lighteningof boilers is to adopt the locomotive type of boiler with forced com-bustion. This method is now tried in steam launches, or torpedoboats, where as much as 150 lbs. of fuel appears to have beenburned per foot of grate per hour.
In reference to this question of economy of weight, The Engineer,in a leading article, July 8, 1881, says—“The modern high-pressuremarine boiler is by no means all that a boiler should be. We may takeas a type a three-furnaced boiler to carry 70 lbs. Such a boiler will beabout 12 feet in diameter by io’ö feet long. It will contain threefurnaces, each three feet in diameter, and a little more than 7 feetlong, and each furnace will have a separate back uptake, and sixty3-inch tubes 7 feet long. A boiler of this kind, if fitted with a largesteam dome, will steam well, and may be depended upon, with faircoal, to work a pair of compound engines up to 500 indicated horse-power. Its shell plates will be nearly 1 inch thick, and its totalweight without water will be roughly 28 tons, and it will hold 14tons of water. Its gross weight therefore will be, under steam andallowing for grate-bars, &c., not far short of 45 tons. It will havea grate surface of about 57 square feet, a tube surface of 900 feet,the crowns of the furnaces will amount to about 100 square feet,and the uptakes may be taken as 120 feet more. The total heatingsurface will be therefore a little over 1100 square feet.