A. From 32 to 72 gross tons of water per hour, for an engine weighing 40 tons and having two cylinders 18 inches in diameter and 24 inches stroke. Q. What is the average amount of coal required to evaporate 6 to 8 tons of water per hour in such an engine? A. One ton per hour, as one pound of the usual run of coal will make from 6 to 8 pounds of steam with the boiler in average condition. Q. What are the disadvantages of high boiler pressures? A. Heavier and dearer boiler, and increased maintenance expense all over boiler and cylinders and their appurtenances. Q. What are the advantages? A. Smaller cylinders necessary; better thermal utilization, both because the steam carries more heat per unit of weight and because it can be cut off earlier if desired. Q. Does this heat advantage extend indefinitely? A. No; where expansion is to less than back pressure it is a disadvantage. CHAPTER IV THE FIRE-BOX Q. Describe in a general way the construction of the fire-box? A. There is an inner and an outer shell, forming a double bottomless box of boiler-plate, and having in front, through both walls, a doorway closed by a furnace-door. The bottom is formed by the grate, upon which the fuel is placed, and below which is the ash-pan which receives the ashes that fall through the grate, and which is supplied with suitable dampers to regulate the amount of air admitted under the grate. The top of the fire-box inner wall is usually flat, and is called the crownsheet; the top of the outer shell or wall over this is sometimes convex, sometimes flat-usually the former. (See Figs. 27, 28, 29, 30 and 31.) Q. What materials are used for fire-boxes? A. In this country, wrought iron, wrought steel, and Bessemer steel; in Europe, principally copper. Q. What is the advantage of copper fire-boxes? A. They let the heat pass through more readily than either iron or steel does. Q. Will the same fire-box do for all kinds of fuel? A. No; there should be a special design and construction for each kind of fuel. Q. What fire-box is usually employed for hard coal? A. One with a very thick grate, and having less provision for letting air in above the fire. Q. Describe the Milholland fire-box for hard coal? A. It is shown in Figs. 29 and 31. The furnace top slopes downward from the boiler barrel, and the crownsheet is stayed with screw stays, except for a short dis tance back of the tube-plate; water grates, B, B, are used, as shown. Q. What fire-box is ordinarily used for bituminous or soft coal? A. Quite deep and rectangular, with vertical walls and a slightly sloping flat top; the top of the box is flared out larger than the bottom, to permit the combustion gases to enter rows of tubes more nearly throughout the entire boiler-barrel width. Q. Why is it permissible, even necessary, to give a small deep fire-box for soft coal? A. Because the soft coal first burns into coke, which is spongy and easily broken up, and admits the air. Q. What is the objection to extending the fire-box too far lengthwise of the engine? A. It makes firing difficult. Q. What class of fire-box is necessary for burning wood? A. One that is very deep. 1. Boiler. 2. Fire-box. 3. Dome. 4. Dome-ring. 5. Dome-cap. 6. Dome- pipes, L. and R. 24. Smoke-box Ring. 25. Smoke-box Front. 26. Smoke-box Door. Door-liner. 33. Corner 40 Safety |