It would be inconsistent with our limits to enter.into any description of the construc tive details.of steam-engines; we can only afford to give a general notion of the way in which the motion is originated, and to explain the chief principles on which the motive power and economy of engines depend. We shall consider the latter first, and may say that the article STEAM should be read as an introduction to what follows, as we must assume a familiarity with the statements there made.
The common mode of employing steam in an engine is by causing it to press alter nately on the two surfaces of a movable diaphragm or piston inclosed in a fixed, steam tight, cylindrical box. In fig. 2, A is the piton, and 13 a section of the box: The piston, by means of a rod E, passing through the end of the box, is made to communi caie motion to the rest of the machinery. The steam is first admitted to one end of the cylinder throughan open ing D, and forces the piston along to the other end. The current of steam from the boiler is then allowed to pass into the other end of the cylinder through the opening C, and forces the piston back again to its original position, and so on. But it is obvious that while this • motion is going on, the steam previously admitted at D must be allowed some exit, or the piston could not be forced back. The manner of this exit constitutes the difference between the two principal classes of engines. according as the steam is allowed simply to rush out into the atmosphere, or is conducted into a sepa rate vessel, and there "condensed." The simplest way in which steam can be used in a cylinder is at the same time the most wasteful. It consists in filling each end of the cylinder alternately full of steam direct from the boiler, and having the full boiler pressure, and thus forcing the piston along in exactly the same way as that in which it would have to be forced were water the fluid used instead of steam. We have said this is wasteful; let us examine the. reasons. If we imagine the cylinder to have a capacity of 7 cubic ft.. then, if it be filled entirely with steam from the boiler at GO lbs. pressure, it will contain just one pound weight of steam.* The total heat in this pound of as given in the table. is equiv alent to 1171 thermal units.} When the piston has reached' the end of its stroke, the steam contained in the cylinder is thus in itself a great storehouse of work, for each of these thermal units is equivalent to 772 "foot-pounds" of mechanical energy. But instead of utilizing this force, at the moment when the cylinder is full of steam the one opening is put into communication with the boiler, the other opening with the atmosphere, and the steam immediately rushes out of the cylinder, and dissipates its contained energy thrmigh the air.
It niOst be remembered that although the steam, when allowed to go into the atmos phere. is immediately reduced to the pressure corresponding to the temperature of the air (which in ordinary eases won Id be only a fraction of a pound per square inch), still the full pressure of the atmosphere itself will Plways be acting on the back of the piston during its stroke; and that therefore, to find the force with which the piston is being pushed along, we must subtract that pressure from the steam-pressure. On the one side of the
piston will be the atmosphere with its uniform pressure of nearly 15 lbs. per square Inch. and on the other side the steam pressure of GO pounds. The effective pressure thus will lie or 45 lbs. per square inch only.
Let us now consider the somewhat more economical case of an engine in which the steam is first used as described above, but afterward, instead of being allowed to pass into the atmosphere, is conducted through a pipe into a closed vessel, and there con densed. The process commonly called condensation, and associated with the idea of liquefaction, consists in essence merely of the subtraction from steam of a portion of its sensible heat. This reduction of temperature has a double effect on the steam—tirst, the liquefaction of a part of it; and then, the reduction of the rest to the. pressure corre sponding to the reduced temperature. (It will lie rontembered that steam exists at all temperatures.) It is not possible to do one of these things without the other, and this fact lies at the bottom of a correct conception of what is called by engineers a " vacuum." What is commonly called " vacuum" simply menus pressure less than the atmospheric pressure; and, in the case of steam-engines, a vacuum generally implies a pressure of between 2 and 4 lbs. per square inch—that is, from a seventh to a fourth of the ordinary pressure of the air. The most common way of condensing steam is by bringing it into contact either with a jet of cold water, or with surfaces kept continually cool by it current of water. In either ease, directly the steam is brought into contact with the wafer, or cooling surface, it transfers to it the larger portion of its sensible heat. During this process, the greater part of the steam is liquefied, and the remainder retains only such a pressure as corresponds to its greatly reduced temperature! The advantages possessed by a condensing over a non-condensing engine will now be obvious. When the piston is being forced from one end of the cylinder by steam entering through the other, the force on the back of the piston resisting its motion in that direction, instead of being equal to the pressure of the atmosphere, is only the pressure of the steam in the condenser, or about 1 lb. per square inch. The net effective force is therefore G0-1, or 59 lbs., instead of C0-15, or 45 pounds. In actual practice these figures would be modified, because, from various causes, such a low back pressure as 1 or 15 lbs. above zero (in condensing and non-condensing engines respec tively) is never obtained, but the principle remains the same.