Q. What about the necessary height of brick arch? A. It should be higher than the fire-door, so as to deflect the air down on the fire when the door is open. Q. Are all master mechanics, engine-runners, and firemen agreed as to the value of the brick arch? A. No; opinions differ diametrically on this as on many other subjects; often because similar conditions do not exist. Brick Arch Fig. 64. Brick Arch. Q. Describe the Security fire-box arch, as seen in Figs. 65 and 66. A. Four inclined water tubes run from the back inner fire-box sheet to the front; upon and in between them rest fire-brick with recesses in the bottom for increasing the mixing effect of the gases, deflecting cinders better, and holding back the molten slag, which might otherwise honeycomb the flue sheet. The bricks may readily be removed to permit access to tubes and stay-bolts. Their ends are so shaped as to prevent their fusing together. Q. What is the effect of such an arch on the fire-box and flue gas temperatures? A. Tests on a Mikado engine showed an increase of fire-box temperature of 100° F. with 70 lbs. of coal fired per square foot of grate per hour, up to 300° F. with 30 lbs. and the same at 140 lbs. The effect on the superheat was practically nil. Q. On what does the amount of heat that a given area of surface transmits depend? A. On the conductivity of the material, the character of the surface, the temperature difference between the fluids on the two opposite sides, the location and arrangement of the surface, the density of the gas, its velocity, and the time allowed for transmission. Q. What limits the minimum evaporation? A. The amount of fuel that can be burned. Q. What is the maximum for locomotive boilers? A. Locomotives have attained 12 pounds per square foot per hour, but at the expense of efficiency. CHAPTER XI THE BRIDGE OR BRIDGE WALL Q. What is the bridge or bridge-wall in a fire-box? A. A fire-brick wall built across a fire-box in front of the tube-sheet; often used to support the front end of the brick arch. It forms a combustion chamber in front of the tube-sheet and protects the tube ends from the direct heat of the fire. Q. Where arch tubes are used, are they of the same material as other locomotive boiler tubes? A. No, they are heavier. Q. Why? A. Because tubes used in fire-boxes to carry arch brick are exposed to the boiler pressure on the inside, while in the case of the other tubes, the boiler pressure is on the outside. Q. What are the functions of the brick arch? A. Steam producing, fuel saving, and smoke prevention. Q. How does the brick arch aid in steam producing? A. If rightly situated it prevents cold air from the fire-box door going direct to the flues, forces the flame up and back against the fire-box sheets, and stops the light coal from being lifted by the exhaust into the flue ends. Q. How should an arch be located to get good results? A. The front end should be at least 5 inches from the flue sheet, below the bottom of lower boiler tubes, and not nearer than 18 inches from the crown sheet. Q. If it is right to keep the top of the arch that close to the sheet, why not go up still closer? A. Any position closer than that is liable to cause such an intense heat, on that portion of the crown sheet directly above the high part of the arch, that the water would be driven away and the crown sheet blistered. Q. Is the plan of setting arches on studs, screwed into the side sheets, better than setting them an arch tubes? A. No; because where arch tubes are used the benefit of their heating surface is a great advantage in steaming. Q. How many square feet would three arch tubes, two inches diameter, nine feet long, add to the heating surface? A. About 14, or perhaps 8 to 10 per cent. Q. Do you take into consideration that arch tubes sometimes burst, causing injury to men on the engines? A. That does happen, but if the tubes are washed out every time the boiler is washed there is little danger of that. Q. Do you mean that tubes fail oftener because of being allowed to fill with mud, than from any other cause? A. Yes; other failures can be avoided by careful inspection. Q. What is a water table? A. A device for improving the combustion in the firebox. The form invented by William Buchanan, of the N. Y. C. & H. R. R. R., consists of two flat, parallel plates, extending diagonally upward from the tube sheet to the back fire-box sheet. These plates, which are about 412 inches apart, are strengthened with stay-bolts in the same manner as the inner and outer fire-box sheets, and form an inclined water leg connecting the front and back legs. A hole 18 or 20 inches in diameter is made through the center of the table for the passage of the combustion gases to the upper part of the fire-box on their way to the tubes. Q. Is it extensively used? |