Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/318

302 I R N The blowing engines in ordinary use in England are worked by steam, but in other countries, e.g., Sweden, where water-power is available, this is frequently utilized. In principle cylinder blowing ma chines are precisely like inverted steam engines, the air taking the place of the steam ; the single- acting machines are the reciprocal analogues of the atmospheric engine (saving of course in the means by which the return stroke is effected), whilst the double-acting machines are high-pressure steam engines in verted. In clack valve machines the motion of the piston in one direction causes a diminished pres sure behind it, and consequently air rushes in through the intake valve at that end, whilst the com pression of the air in front of it opens the outlet valve of the other end and causes the air to escape ; on the return motion this outlet valve is closed and the intake valve of the same end opened, whilst the intake valve of the other end is closed and the outlet valve opened. In "slide valve" machines the moving clack valves are replaced by sliders connected with the piston rod by means of an eccentric on the shaft driving it, so that, when the piston begins to make its stroke, the appropriate valves are closed or unclosed as the case may be. In order to equalize the intermittent blast thus produced, a regulator is sometimes interposed between the blowing cylinder and the furnace, consisting of a reservoir or chamber of considerable size which acts in much the same way as the air cham ber of an ordinary force pump, the whole mass of air becoming some what compressed when air is blown in, and the expansion during the momentary cessation of the supply keeping up a sufficiently equable stream of air issuing from the reservoir until the next cylinder- ful of air is blown in. To econo mize space, a piston box with a piston loaded with weights, or a loaded gasometer in a water tank, may be substituted for the reservoir; the latter expedient is objectionable, causing the air to be saturated with moisture. If the furnace is at some distance from the blowing engine, the large mass of air in the blast main and superheaters serves to render uniform the current supplied to the furnace without any other regulator being requisite. 15. HotUast Stoves. The oldest form of blast heating apparatus, applied by Neilson, consisted of a tubular rivetted boiler plate heating vessel (h, h, fig. 15), mounted in a brick chamber OOOO, and heated by a fire underneath fed through the door D, the waste gases from the fire passing out at the far end to the chimney. Crescent- shaped partitions p, p, p inside the heater caused the current of air from the blowing engine which entered at B to take a serpentine course as indicated by the arrows, finally passing off at S to the furnace. This 1 6), consisting of two parallel tubes L, L running along the base of the stove above the firebars d, d, communicating the II. I. Section. II. Elevation. III. and IV. Plans. was speedily superseded by the " Calder pipe stove " (fig. one with the other by a series of inverted U or horse-shoe shaped tubes; the blast being introduced into the cold