The Locomotive Of Today

The Boiler: Evaporative Power, HeatingSur/acey&c. 21 The evaporative power of a boiler mainly depends upon the efficiency of its heating surface to transfer the heat from the products of combustion within to the water without. For this it relies on both radiation and contact, from two or three hot masses in the boiler, solid incandescent fuel in the firebox, and flame and hot gases in the tubes. The radiation of heat from the solid fuel is greater than that from the flame, while the hot transparent gases scarcely radiate any heat at all. In estimating the heating surface it is customary to take the area of the firebox and tubes in contact with heat on one side and water on the other, and consider the evaporative power of the boiler as proportionale to the total number ol sq. ft. thus found. As all parts of the heating surface, how- ever, do not possess the same efficiency, the heating surface of the firebox being much more effective than that of the tubes, the result so obtained is misleading in practice ; adding length to the tubes cloes not increase the evaporative efficiency of the boiler nearly so much as increasing the size of the firebox. A flat horizontal surface not too far above the fire, is con- sidered most favourable, and by being made concave to the fire it has the further advantage of being better able to receive and transmit the radiant heat, of boiling off the matters deposited from the water, and so to some extent preventing formation of scale, and of being stronger and more durable. Next in conductive power come the flat sides of the box which are made sloping as they then receive the rays of heat at a more favourable angle, and allow the steam bubbles to escape more freely ; they also increase the size of the water space and admit of the use of longer stays at the top of the box where most expansion takes place. The tube plate, although made vertical owing to the rapid impingment of the flame is as effective as the crown, which is too often hampered by the stays, etc. The great superiority of firebox heating surface is owing to the radiant heat being principally given off there, also to the faet that the more violently the flame impinges on a sur- face, the greater the ebullition and consequent formation of steam on the opposite side of that surface. . The effective area of tubes internally heated is only half their total area, as heat is mostly given off at the upper surface of the tube, owing to the faet that hot gases being light rise, then on giving off their heat they fail and are replaced by hotter ones; also by the difficulty steam has in escaping from the under side of a tube, and the thickness of soot that too often collects inside the tube at the bottom.