All About Engines

Raising Steam 79 modern form it is the outcome of many years of prac- tical experience and exact scientific experiment. From the illustration (Fig. 43) on Plate 5 and the section in Fig. 44 it will be seen to consist of a steam drum con- nected by a large number of tubes with two water drums, situated on a level with the grate. In their passage from the grate to the up-take, the hot gases pass through the tubes on each side. Those which are nearer to the fire become the hotter, some of the water they contain is converted into steam, and the mixture of steam and hot water flows up into the steam drum. Meantime, cooler water flows down the outer tubes into the water drums, and then follows the hot water and steam up the inner tubes. A constant circulation is thus kept up, and the steam is raised very rapidly. The up-take, lead- ing to the chimney, has a partition down the centre, and a damper on each side enables only one set of tubes to be used when less steam is required. An improvement of very considerable importance con- sists of angle-iron baffles laid between pairs of the outer row of tubes, thus securing a more equable dis- tribution af temperature. The outer casing consists of a double thickness of sheet steel with a layer of asbestos between, and the whole boiler is extra- ordinarily light, considering the large amount of water which it is capable of evaporating per hour. Provisions for Safety The pressure inside a modern boiler may be any- thing up to 250 lb. on the square inch, If the fire