All About Engines

Raising Steam 99 the steam is caused to pass through tubes fixed in the flues or up-take so that the hot gases circulate round them. Not only are drops of water evaporated and condensation in steam pipes and cylinder avoided, but the steam increases in volume and a smaller weight is taken by the engine to produce the same power. For every 1000 Fahr, of superheat—that is, for every ioo° Fahr, by which the steam which reaches the engine is hotter than that which comes from the boiler—a reduction of 10 per cent, in the steam consumption is obtained. In the case of Lancashire boilers the flue at the end of the boiler is the place usually chosen for the superheater. Reference to Plate 4 will show that the Babcock and Wilcox superheater is placed be- tween the inclined tubes and the steam and water drum. In the Yarrow boiler it is fixed on the left- hand side between the circulating tubes and the up-take, as shown in Fig. 44. On that side, more- over, the number of circulating tubes is reduced in order to secure the same draught on each side of the furnace. Even if the steam has to be superheated in a separate furnace the gain is often sufficient to justify the expense. Supplying the Feed Water As the engine uses up steam, water must be con- tinually supplied to the boiler. In Watt’s days a small hand pump was used, but this has given place to pumps worked from the engine shaft, or separate independent steam pumps, or injectors.