Steam.—Steam engines are made in very many forms, and it would be impossible for us to describe even a small proportion of those made; for small purposes they are most generally made having engine and boiler combined, but where moderately high powers are needed, and space has to be considered, it is found more economical and convenient to keep them separate; the supply of steam from boiler to engine being conveyed by a pipe. It might be here mentioned that it is necessary that all steam chambers and pipes be coated or covered with some non-conducting material, to prevent loss of heat and consequent condensation of steam, and it is found advantageous to keep the steam at as high a temperature as possible, to increase its efficiency; with most large engines the steam is superheated (i.e. heated to higher than its ordinary temperature) as it passes from boiler to engine.
It is not our intention, neither would it be possible in our limited space, to give a practical treatise upon the steam engine, but it will doubtless be interesting and instructive to many if a general description of the chief features be given. They practically consist of the “boiler,” to which is attached the “feed pump,” “water gauge,” “steam gauge,” and “safety valve.” The “engine,” which consists of the cylinder and piston, “governor,” “cranks,” “eccentrics,” and flywheel.
Boilers take many forms, but in actual principle consist of two sorts, both being cylindrical, one being clear inside, and the other nearly filled with flue tubes, which very greatly increase the heating surface. The first mentioned, which is generally used for large works, and is known as a Cornish boiler, Fig. 190, has a cylindrical outer shell, within which is a smaller cylinder, the space between the two, which is closed at both ends, containing the water as shown. This boiler is fixed in brick-work, and the furnace is situated within the inner cylinder. To increase the power of these boilers, large water tubes are carried across the inside of the inner cylinder (opening into the water chamber at each end) where the flame and heat pass after leaving the furnace.
190. Cornish Boiler. 191. MULTITUBULAR BOILER. (SECTION)
Multitubular Boilers are generally those that are attached to or combined with the engine, where space is a primary object (locomotive engines have multitubular boilers). Fig. 191 shows the arrangement of this boiler in a vertical position with horizontal tubes (shown without the engine); they are also very commonly made with vertical tubes. All boilers are, or should be, provided with ample accommodation for removing the incrustated deposit, which forms with moderate rapidity, as the water is continually boiling, and as evaporation in a steam boiler is very rapid, the supply of water is constantly being renewed, and each successive charge of water brings its proportion of lime to be deposited. (For fuller details upon incrustation and causes, see Bathroom.)
The Feed Pump is a small pump of ordinary shape and construction, situated near and worked by the engine, its purpose being to supply water to the boiler when needed. Mechanism is provided for throwing it in and out of gear as the water gauge indicates; it will be readily understood that no means can be provided for filling a steam boiler by hand, that is to say, it must be done mechanically, as no loose cover can be provided.
192. WATER GAUGE.
193. STEAM GAUGE.
The Water Gauge (Fig. 192) consists of two suitably constructed cocks, both being screwed into the boiler one above and one below the correct or average water level, a strong glass tube extending between them as shown; the water level is necessarily the same in the glass tube as in the boiler and consequently the attendant can see at a glance when water is needed; the object of having cocks at each end of the tube is to prevent the escape of water and steam by closing the cocks should the tube be broken.