The Locomotive Of Today

22 The Boiler: Brick Archy Circulation of Water, &c. When the fire burns with a long flame the tubes should be larger; but in locomotive boilers by using the brick arch, which retards the passage of the gases to the tubes and so gives more time for the proper combustion of the more volatile parts of the fuel, it is possible to make use of much smaller tubes than would otherwise be the case, and of course get more of them in the barrel and so increase the heating surface. The reduction of the diameter of the tubes is restricted by the flue way it is found necessary to maintain, and which greatly depends upon the strength of the draft. The usual proportion of the collective sectional area of the tubes to grate area is about i to 4 or 5. The ratio of the length to the diameter of the tubes is sometimes as much as 120 to 1, and the heating surface is usually about 70 times the grate area. The general direction of circulation in a locomotive boiler is upwards at the firebox end and forwards in the upper parts of the boiler, downwards at the front end, and backwards at the bottom, and it is obvious that the more freely this natural movement takes place the more rapidly steam will escape, this being also facilitated by the jar of the engine when running. The comparative amount of hard scale is generally a fairly sure index of the value of the heating surface and State of the circulation in the boiler. Where the ebullition is’ greatest the amount of hard and tenacious scale will be found to be least, though this cloes not altogether apply to the Crown plate of a locomotive boiler where the boiling off of the deposit is impeded by the stays, or to a nest of small tubes closely packed. In locomotive boilers about 70 Ibs. of water can be evaporated from 4^ sq. ft. of grate area in one minute, or 13*5 Ibs. of water from 1 sq. ft. of heating surface per hour, and an average of 3 sq. ft. of total heating surface per indi- cated horse-power may be taken as an approximation. These quantities, however, largely depend on quality of fuel, condition of boiler, rate of combustion, and skiil in stoking. The fire-grate, which forms the bottom of the firebox, consists of a series of bars arranged longitudinally, and with spaces between them for the passage of the air necessary for the combustion of the fuel placed above. The grate may be either horizontal or inclined at the front or back, so that the fuel put on at the back shakes down to the front by the jar of the engine. When the firebox is placed between the frames, as is the general practice here, and the size of grate is restricted by the distance between the frames for width, and the distance between the driving and trailing axles for length, it is only by the help of the draught produced by the exhaust steam that it is possible to burn