Blackwood's Magazine.
THE STEAM ENGINE SIMPLIFIED.
It is a universal property of matter, that by the application of heat, so as to raise its temperature, it suffers an increase in its magnitude. Also in different substances, when certain temperatures are attained by the application of fire, or other methods of heating, they undergo a change of form. Solids, at certain temperatures, are converted into liquids; and liquids, in like manner, when heated to certain degrees, become aeriform fluids or gases. These changes are familiar to every one in the ordinary phenomena attending water. Below the temperature of 32° of the common thermometer, that substance exists in the solid form, and is called ice. Above that temperature it passes into the liquid state, and is called water; and when raised to the temperature of 212°, under ordinary circumstances, it passes into the aeriform state, and is called steam. It is to this last change that we wish at present principally to call the attention of the reader. In the transition of water from the liquid state to the state of vapour or steam, an immense change of bulk takes place. In this change, a solid inch of water enlarges its size about 1,700 times, and forms 1,700 solid inches of steam. This expansion takes place accompanied with a certain force or pressure, by which the vapour has a tendency to burst the bounds of any vessel which contains it. The steam which fills 1,700 solid inches at the temperature of 212°, will, if cooled below that temperature, return to the liquid form, and occupy only one solid inch, leaving 1,699 solid inches vacant; and, if it be included in a close vessel, leaving the surfaces of that vessel free from the internal pressure to which they were subject before the return of the water to the liquid form. If it be possible, therefore, alternately to convert water into vapour by heat, and to reconvert the vapour into water by cold, we shall be enabled alternately to submit any surface to a pressure equal to the elastic force of the steam, and to relieve it from that pressure, so as to permit it to move in obedience to any other force which may act upon it. Or again, suppose that we are enabled to expose one side of a movable body to the action of water converted into steam, at the moment that we relieve the other side from the like pressure by reconverting the steam which acts upon it into water, the movable body will be impelled by the unresisted pressure of the steam on one side. When it has moved a certain distance in obedience to this force, let us suppose that the effects are reversed. Let the steam which pressed it forwards be now reconverted into water, so as to have its action suspended; and at the same moment, let steam raised from water by heat be caused to act on the other side of the movable body; the consequence will obviously be, that it will now change the direction of its motion, and return in obedience to the pressure excited on the opposite side. Such is, in fact, the operation of an ordinary low-pressure steam-engine. The piston or plug which plays in the cylinder is the movable to which we have referred. The vapour of water is introduced upon one side of that piston at the moment that a similar vapour is converted into water on the other side, and the piston moves by the unresisted action of the steam. When it has arrived at the extremity of the cylinder, the steam which just urged it forwards is reconverted into water, and the piston is relieved from its action. At the same moment, a fresh supply of steam is introduced upon the other side of the piston, and its pressure causes the piston to be moved in a direction contrary to its former motion. Thus the piston is moved in the cylinder alternately in the one direction and in the other, with a force equivalent to the pressure of the steam which acts upon it. A strong metal rod proceeds from this piston, and communicates with proper machinery, by which the alternate motion of the piston backwards and forwards, or upwards and downwards, in the cylinder, may be communicated to whatever body is intended to be moved.
The power of such a machine will obviously depend partly on the magnitude of the piston or the movable surface which is exposed to the action of the steam, and partly on the pressure of the steam itself. The object of converting the steam into water by cold, upon that side of the piston towards which the motion takes place, is to relieve the piston from all resistance to the moving power. This renders it unnecessary to use steam of a very high pressure, inasmuch as it will have no resistance to overcome, except the friction of the piston with the cylinder, and the ordinary resistance of the load which it may have to move. Engines constructed upon this principle, not requiring, therefore, steam of a great pressure, have been generally called "low-pressure engines." The re-conversion of the steam into water requires a constant and abundant supply of cold water, and a fit apparatus for carrying away the water which becomes heated, by cooling the steam, and for supplying its place by a fresh quantity of cold water. It is obvious that such an apparatus is incompatible with great simplicity and lightness, nor can it be applied to cases where the engine is worked under circumstances in which a fresh supply of water cannot be had.
The re-conversion of steam into water, or, as it is technically called, the condensation of steam, is however by no means necessary to the effective operation of a steam-engine. From what has been above said, it will be understood that this effect relieves the piston of a part of the resistance which is opposed to its motion. If that part of the resistance were not removed, the pressure of steam acting upon the other side would be affected in no other way than by having a greater load or resistance to overcome; and if that pressure were proportionately increased, the effective power of the machine would remain the same. It follows, therefore, that if the steam upon that side of the piston towards which the motion is made were not condensed, the steam urging the piston forwards on the other side would require to have a degree of intensity greater than the steam in a low-pressure engine, by the amount of the pressure of the uncondensed steam on the other side of the piston. An engine working on this principle has therefore been called a high-pressure engine. Such an engine is relieved from the incumbrance of all the condensing apparatus and of the large supply of cold water necessary for the reduction of steam to the liquid form; for instead of being so reduced, the steam is in this case simply allowed to escape into the atmosphere. The operation, therefore, of high-pressure engines will be readily understood. The boiler producing steam of a very powerful pressure, is placed in communication with a cylinder furnished in the usual manner with a piston; the steam is allowed to act upon one side of the piston so as to impel it from the one end of the cylinder to the other. When it has arrived there, the communication with the boiler is reversed, and the steam is introduced on the other side of the piston, while the steam which has just urged the piston forwards is permitted to escape into the atmosphere. It is evident that the only resistance to the motion of the piston here is the pressure of that portion of steam which does not escape into the air; which pressure will be equal to that of the air itself, inasmuch as the steam will continue to escape from the cylinder as long as its elastic force exceeds that of the atmosphere. In this manner the alternate motion of the piston in the cylinder will be continued; the efficient force which urges it being estimated by the excess of the actual pressure of the steam from the boiler above the atmospheric pressure. The superior simplicity and lightness of the high-pressure engine must now be apparent, and these qualities recommend it strongly for all purposes in which the engine itself must be moved from place to place.
The steam-engine therefore consists of two distinct parts,—the boiler, which is at once the generator and magazine of steam, and the cylinder with its piston, which is the instrument by which this power is brought into operation and rendered effective. The amount of the load or resistance which such a machine is capable of moving, depends upon the intensity or pressure of the steam produced by the boiler, and on the magnitude of the surface of the piston in the cylinder, upon which that steam acts. The rate or velocity of the motion depends, not on the power or pressure of the steam, but on the rate at which the boiler is capable of generating it. Every stroke of the piston consumes a cylinder full of steam; and of course the rate of the motion depends upon the number of cylinders of steam which the boiler is capable of generating in a given time. These are two points which it is essential should be distinctly understood, in order to comprehend the relative merits of the boilers used in travelling steam-engines.
The motion which is primarily produced in a steam engine is a reciprocating or alternate motion of the piston from end to end of the cylinder; but the motion which is necessary to be produced for the purposes to which the engine is applied, is rarely or never of this nature. This primary motion, therefore, is almost always modified by some machinery interposed between the piston and the object to be moved. The motion most generally required is one of rotation, and this is accomplished by connecting the extremity of the piston-rod with a contrivance constructed on the revolving axle, called a crank. This contrivance does not differ in principle from the common winch, or from the key which winds a clock. The motion of the piston-rod backwards and forwards turns such a winch. At each termination of the stroke, the piston, from the peculiar position of the crank, loses all power over it. To remedy this two cylinders and pistons are generally used, which act upon two cranks placed on the axle at right angles to each other; so that at the moment when one of the pistons is at the extremity of its stroke, and loses its power upon one crank, the other piston is at the middle of its stroke, and in full operation on the other crank. By these means an unremitting force is kept in action.
Edinburgh Review.