TREATMENT OF STEAM.

8l

Fig. 40.—Cylindrical Superheater.

wise the dry steam hardens the oil, to the detriment of the pistonand slide-valve rubbing surfaces.

Some engineers consider that when the steam is superheated,it should be mixed with the steam in the boiler; little advantageexists, however, in this arrangement, for it appears a very doubtfulproceeding to heat up the steam, and then rob it of a portion ofthe heat by mixing it again with steam from the boiler. The mainthing to be studied is to give the steam a sufficient degree of super-heat, so that in its passage to the cylinder it may not be cooleddown below the temperature existing in its primary state in theboiler; thus steam in a dry state is passinginto the cylinders, whereas without somecontrivance for drying the steam in theboiler, or in the superheater, an admixtureof steam and water presses on the piston,tending to diminish the power and increasethe consumption of fuel.

The various forms of superheaters maybe classed just as are steam-boilers. Theplain cylindrical form has an outer shell,containing a single large tube, the inner tube being stayed with ringsof angle-iron; where double round boilers are used, firing fore andaft, the part fitting on to the boiler is bevelled, whilethe other end that joins on to the funnel is quitesquare; these superheaters, four in number, convergeto one point, to which a single funnel is fitted, thebottom part of the funnel or the uptake beingbevelled to suit; this form of superheater is simplyeffective, and easily constructed, while the scale canbe readily cleaned out, and as it lies at a consider-able angle the heat acts better on the surfaces.

Some superheaters of this dass, however, are placedvertically, and it is an object with the designer toarrange passages so that the steam travels up anddown within the superheater, time being requiredto dry the steam thoroughly. The passages areformed with plating rivetted vertically between theinner and outer shells, one of these plates is rivettedto the bottom and sides, another to the top and sides, and soon alternately; but at the top and bottom alternately the vertical

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Fig. 41.—CylindricalSuperheater, withdivision plates.