Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/319

IRON 303 maiu B6, the air passes over into the hot main S through the curved tubes h, h, becoming heated in so doing, the flames from the fire Dd enveloping the horse-shoe tubes and then passing by the flue / to the chimney C. Many modifications of this stove have been introduced, several of which are still in use : thus in some the air is compelled to pass alternately from a portion of the one main to the other T> Fig. 15. and vice versa several times so as to be more completely heated, passing through several horse-shoes before emerg-, ing; the horse-shoes are sometimes altered into inverted V s, and made rectangular or flatly elliptical in section instead of circular so as to expose greater heating surface. To avoid the liability to fracture through unequal expansion, the U s are sometimes made of two parallel vertical tubes united by 11. Fro; 16. Calder Pipe Stove. I. End Elevation. III. Plan. II. Elevation. a horizontal connecting tube socketed into each, just as each vertical tube is socketed into the main (fig. 17). The " pistol pipe " stove, still largely used (fig. 18), chiefly differs from this in having the limbs of the U tube closely adjacent, so as to consist in fact of a single tube with a partition D in the centre, the cold air passing up one side of the partition and down the other so as to become heated in passing ; to accommodate the pipe to an arched roof, the upper end is bent inwards, thus giving the form of a pistol stock and barrel to the double pipe, two ranks of pipes facing one another being built in the same stove. Another modifica tion of the older tubular superheater consisted of a serpen tine or coil of piping made of cast iron pipes bent into half circles and socketed together, so as to form a continuous worm tube which was mounted inside a brickwork stove and heated by a fire in much the same way as the Calder pipe stove. The " Wasseralfingen superheater" consists of a kind of serpentine of which the curved parts lie outside FIG. 18. the stove, only straight tubes being exposed to heat. The "Blaina oven" differs from this chiefly in the serpentine being vertical instead of horizontal, and in the whole of the serpentine (the curved ends inclusive) being exposed to the heating flame. Ths substitution of the waste gases from the blast furnaces as fuel for the coal or coke-fired fireplaces originally used for these and various other forms of superheater not only works advantageously in saving the fuel that would otherwise be requisite, but also keeps up a more regular heating effect, and diminishes the liability to breakage through unequal expansion, &c. All these forms of super heater, however, are open to the same objection, viz., that it is impracticable to heat the blast continuously by means of them to a higher temperature than about 450 C., otherwise the iron pipes get speedily burnt away. In order to obtain a higher temperature, the principle of the Siemens regenerative furnace is employed in the Siemens-Cowper stove, the flame from the combustion of the waste gases from the blast furnaces being made to traverse piles or stacks of brickwork loosely heaped together or regularly laid so as to heat up the brickwork, the products of combustion finally passing, off to the chimney at a comparatively low temperature. After the lapse of a certain time the flame is shut off, and the cold air blast made to traverse the heated brickwork in the reverse direction, entering at the cooler chimney end, and leaving at the hottest point near where the furnace gases and the air to burn them originally entered ; two such regenerators aroused together alternately, the flame heat ing up one whilst the blast is being heated in the other, and vice versa, the shifting of the blast and furnace gases from the one to the other being accomplished by opening and shutting suitable valves. Owing to the presence of dust in the blast furnace gases, the cavities between the piled bricks are apt to become filled up with deposit; to remedy this inconvenience in the " Whitwell stove" the piles of brickwork are replaced by a series of parallel firebrick walls about 9 inches apart, each wall being perforated by arched openings at the top and bottom respectively in each alternate wall, so that the flame passes alternately up and down between each wall and the adjacent one, thus heating up the surfaces of the walls (figs. 19, 20). By means of manholes at the top and sides scrapers can be introduced from time to time, and the deposit of flue dust scraped off the walls and removed from the stove without rendering it necessary to take down the internal brickwork at all. With regenerative stoves of this description worked in pairs it is easy to obtain a continuous blast at a temperature of 750 to 800 C. Determination of Temperature of Blast. A rough and ready method often employed is to take out a plug from the blast main, or "gooseneck," supplying the tuyere, and to hold in the issuing stream of hot air a rod of zinc or other test metal for a determinate time, noting how long it takes to melt its end. More accurate methods consist in the use of specially constructed pyrometers. Certain forms containing a compound silver and platinum spiral, working on much the same principle as that of Breguet s thermo meter, and others in which the expansion of a bar of metal moves an index by means of multiplying wheels or levers, although useful for comparatively low temperatures (below 500 C.), are out of the question for intensely heated blasts ; in such cases two forms of pyrometer invented by Siemens are applicable. One of these is an adaptation of the method employed by Pouillet for determining