4 (fl. 78) shows an old-fashioned way of boiler setting. The gases pass from the combustion-chamber, II, over the bridge wall, along the flame-bed, down under and along one side of the lower cylin drical shell, then back again along the other side, under the lower shell, to the flue. The feed is introduced in the lower vessel, where the temperature is lowest. This arrangement makes of the lower drum little more than a feed-heater, with the additional disadvantage that it has to bear the same high pressure as the main shell, above.
Boiler boiler requires a fire-door to close the fire-box, a grate upon which the fuel is spread, and (if of considerable size) a man hole by which it may be entered for the purposes of inspection, cleaning, and repair. Small boilers have no man-holes, but are supplied with hand holes. Man-holes and hand-holes are closed by plates of elliptical form, and of such proportions and dimensions that they can readily be inserted in the holes and turned around, so that the pressure of the steam or of the water inside holds them tightly in place and their blowing out is impossible. The tightness of the steam joints is further insured by a screw-bolt working through an arched bridge-piece, with ends bearing upon the boiler at the sides of the man-hole or the hand-hole.
The Safety AAbliances usually attached to a boiler are the safety-valve, the gauge-cocks or try-cocks, the water-gauge, and the pressure-gauge. There are also, though less generally, recording-gauges having fusible plugs, and low-water alarm- and sounding-gauges.
The is a disc so held down to its seat by weights or springs that it will lift and discharge steam only when the pressure under it reaches a certain predetermined point. It may have either a flat or a conical seat, and the disc may be held to its seat by a plain weight, a spring, or a lever and weight. In any case it is well to have upon each boiler at least one safety-valve, so locked up or otherwise constructed that the pressure at which it will lift cannot be accidentally or intentionally increased. In Figure 8 the safety-valve is shown blowing into the stack —a bad plan, because it increases the draught and urges the fire. Besides this, every one should know when the safety-valve blows. Safety-valves should be lifted twice each day, to be sure that they are not stuck upon their seats; and when they blow off, the pressure at which they do so should be noted by reference to the pressure-gauge, in order to see that they arc not blowing at a point above that intended. The pressure at which they quit blowing should also be noted.
The is a vertical glass tube connected at top and bottom with two horizontal fittings, one of which is intended to be in the steam room and the other in the water-space. The level of the water in the boiler is shown with fair degree of certainty in the tube.
The is simply any form of cock that will show by its discharge whether there is water or steam at the level at which it enters the side of the boiler. Try-cocks should be tested two or more times a day, and from
time to time should have a steel rod passed through them into the boiler, to free them from incrustation. The same may be said of the connections of the glass water-gauge or column.
The Device consists of a float which, when the water gets below a certain level, gives an alarm and also causes au increase in the feed and in the water-gauge glass. There are also gauge-cocks or try cocks, the opening of which will show with reasonable accuracy where the water-line is. In Figure 6 is a float, to which there is attached a rod, passing through a stuffing-box in the shell and intended to show by the position of the weight attached to the chain or cord passing over the wheel V%) what is the water-level. A fusible plug is a brass fitting placed in the crown-sheet of a boiler and having drilled through it a hole in which there is a fusible metal rivet, which when the temperature gets above a cer tain point (determined by the melting-point of the alloy), by the water-level getting below the crown-sheet, will melt and allow steam to escape into the fire-box, putting out the fire and giving an alarm. It must be remembered that the melting-point of fusible plugs rises with use and with time, so that after more than a year or so they are not to be depended upon.
dial pressure-gauge (p1. 78, fig. 6) shows by the distortion of a metal diaphragm or tube the amount of steam pressure put upon it, and this is indicated, through levers and pinions, by an index hand. By the height of a column of mercury in a tube the mercury pressure-gauge shows the pressure which exists under it. In the dif ferential mercury-gauge (fig-. 5) there is a piston, having a small head at the lower end to receive heavy pressures, while .a larger upper head takes the pressure of mercury in a reservoir communicating with a graduated tube. A very small rise of the piston makes a corresponding great rise of the mercury in the tube. The recording- and alarm-gauge (figs. 9, co) has a diaphragm, which works a crank-shaft and through it an index hand showing the pressure, and at the same time operating a lever having a pencil-point which records upon a graduated paper ribbon moved by clockwork the pressures shown by the index. Tice paper strip is marked with transverse lines corresponding to hours, so that the pencil records the pressure for each minute. The whole is enclosed in a glass case, to prevent any one from tampering with it; and when the pressure passes a certain limit, an electrical attachment rings an alarm-bell. It is intended that this appliance shall be kept in the office as a check upon the engineer or the fireman, it being required that it shall be maintained continually in steam communication with the boiler.