All About Engines

J Raising Steam 91 The heating of the feed water is accomplished by passing it through a nest of tubes placed in the flue leading to the chimney. There are several dif- ferent forms, each claiming to offer some special advantage in cheapness, efficiency, or ease of repair ; but the principle of all of them is the same. They are not employed on locomotives, but are to be found in all large land and in marine installations, and on account of the purpose they serve are usually called economisers. A smaller, but not negligible, economy is effected by taking the control of the feed water out of the hands of the man in charge and rendering it auto- matic. There is one proper height for the surface of the water in the boiler, and only one. At this level steam is disengaged most readily from the surface with the least splashing, and it is freer from particles of water, which are useless for driving the engine. Moreover, in water-tube boilers it is very important that the supply of water should be maintained, in view of their small capacity. In power stations, where the load varies very greatly, there are corre- sponding variations in the demand for steam from the boilers, and—in the absence of any automatic arrangement—the water gauges have to be watched very closely. There are a number of devices for regulating the feed water and maintaining a constant level. Most of them depend upon a float inside the boiler, or outside, in a separate chamber, which opens or closes a valve as it falls or rises; but there is one,