STEAM BOILER

There are other methods of indicating the quantity of water to the eye and the ear (see Lardner on the Steam Engine, p. 2?>G.etseq.); but that which is most deserving of notice is one in which the indicator is made to govern the feeder, so as to supply water to the boiler in the same proportion as that in which it is evaporated. The wire supporting the float on the surface of the water above described, represented at N in the figure, is attached to the arm of a lever, of which the opposite arm is connected with a common puppet valve placed in the bottom of the small feeding cistern. When the surface of the water falls, the float falls with it, and draws down the arm of the lever, which raises the opposite arm and ojjens the valve. Water from the feeding cistern flows into the boiler through the tube P, and continues to flow into it until the level of the surface has been raised so high that the float no longer pulls down the arm of the lever, and therefore the valve in the bottom of the cistern is closed. In the actual operation of this self-acting feeder, however, the surface of the water so adjusts itself that the float opens the valve just so much as is nece ary, and sufficient to admit as much water from minute to minute as is consumed by evaporation, and therefore the level of the water is maintained in a fixed position. To secure the safety of boilers it is nece ary to provide means for preventing the pre ure of the steam contained in them from exceeding a certain limit, and also for indicating its pre ure at all times. The former object is attained by safety valves, which are of various forms. The safety valve is a common puppet valve placed on the top of the boiler, which opens upwards, and which is held down by a pre ure proportional to the limit within which the steam is intended to be kept. When the pre ure of the steam exceeds that limit, this valve is raised by it, and the steam escapes until its pre ure is reduced to below that of the valve. The magnitude of the aperture of the valve should be sufficient to enable the steam to escape faster than under any po ible circumstances it could be generated. Safety valves diflTer one from another, chiefly in the manner in which the required pre ure upon them is produced. In some weights of the requisite amount are placed directly upon the valve, as represented in the figure under N. In some the valve is held down by the elasticity of one or more springs. In others a lever pre es on the valve by a sliding weight, the pre ure being varied at discretion by shifting the position of the weight; and in others the lever which acts on the valve is held down by a spring similar to that of the common spring balances, which may be screwed up to any required tension. In all boilers true safety valves should be provided, one of which should be placed out of the reach of the engineer, so as to impose a major limit to the pre ure which he can give to the steam. The most ordinary indicator of the pre ure of the steam in low-pre ure boilers is the mercurial steam gauge, represented at O in the figure, which consists of a gla tube bent into the siphon form, the bent part and a portion of each leg being filled with mercury^ One leg communicates with the steam within the boiler, and the other is open to the atmosphere. The difference between the levels of the columns of mercury in the two legs is always a measure of the difference between the pre ure of the steam and that of the atmosphere. If the tube be of iron, which is most generally the case, the position of the surface of the mercury in it is indicated by a float having a wooden rod rising from it. This indicator cannot be conveniently applied to high pre ure boilers, since the column of mercury supported by the pre ure of the steam would have too great a height. In these boilers a thermometer is immersed, by which the temperature of the steam is indicated; and this becomes an indicator to its pre ure, since the pre ure varies with the temperature. See Steam. The safety of high-pre ure boilers is sometimes provided for, by inserting in them a plug of metal capable of being fused at that temperature above which the boiler is exposed to the danger of explosion. If the water and steam exceed that temperature, the metal of the plug or of its solder will melt, and it will fall out, and the steam will Qscape from the opening which it leaves. An alloy composed of one part of lead, three of tin, and five of bismuth, will fuse at the common temperature of boiling water; and alloys of the same metals in various proportions will fuse at diff'erent temperatures from 200° to 400O. When a boiler ceases to be worked and the furnace is extinguished, the space within it appropriated to steam '; will be left a vacuum by the condensation of the steam with which it was previously filled. The external pre ure of the atmosphere acting on the boiler would, under such circumstances, have a tendency to crush it inwards. To prevent this a safety valve is provided, opening inwards, and balanced by a weigiit to keep it closed until it be relieved from the pre ure of the steam within. To render the strength of the fire proportional to the evaporating power, a plate called a damper is placed with its plane at right angles to the flue, so that by raising and 11.59 STEAM CARRIAGE. lowering it like the sash of a window the space for the pa age of air through the flue is increased or diminished. The force of draft and that of the fire can, therefore, be varied merely by shifting the position of the damper. Such a damper can be regulated by hand, but more uniformly by connecting it with a float placed on a column of water or mercury, which measures the force of the steam, as represented in the figure, where the chain suspending the damper is seen at O and the float at P. If the force of the steam be suddenly increased, such a column will raise the float and lower the damper; and if the force of the steam be too feeble, the contrary effect will be produced. Such an apparatus is called a selfre^ulating damper. The form of boilers is sometimes cylindrical, having an oval flue conducted through them from end to end. The ends are hemispherical. Such a form is favourable to strength, and is better adapted than the waggon shape to high-pre ure boilers. (For the boilers of railway engines, see Locomotive Engine; and for those of marine engines, see Steam Navigation. For more minute details respecting steam boilers, see Lardner on the Steam Engine, 7th edition; also Tredgold, second edition, Appendix.) [s. 1172]