Imágenes de página



[ocr errors]

EFORE entering upon the consideration of the historical and mechanical details of the steam-engine, it will be necessary to explain as briefly as possible the nature and properties of steam. It is but just, however, to state, that the new theory of heat, now being submitted to the test of experiment, will modify very much the theory of the steam-engine. Until the new views, however, have been conclusively affirmed, it would be premature here to specify them; we shall therefore confine ourselves to a statement of the theory of the steam-engine as generally received.

When a quantity of water is heated until it arrives at a certain fixed temperature, an elastic fluid or aqueous vapour is evolved; this is called steam, and resembles in many of its properties common air. Like air, it is elastic, capable of being reduced in bulk by compression; the pressure which it exerts in the vessel into which it is compressed being exactly in proportion to the amount of compression. (See volume on Natural Philosophy in this series.) Like air, steam is also capable of an increase of volume or bulk; this expansion reducing the pressure on the vessel in which it is allowed to expand just in proportion to the amount of expansion. The first property of steam now mentioned is termed its elasticity, the second its expansibility; although, having these properties in common with fluid gases, steam is distinguished by the term of aqueous vapour, inasmuch as it differs from a gas, which retains permanently its gaseous condition under ordinary circumstances; while steam requires to be kept

at a uniform temperature, otherwise it changes its condition, and returns to its original form of liquid. This property, by which steam is readily converted into water, is of immense importance; by virtue of this property the power of steam is made available in what are termed condensing engines, and offers the readiest means of forming a vacuum in the cylinder, into which comparatively little force is required to press the piston.

When a quantity of water is placed in a boiler, and heat applied to it, a circulation of the particles of water immediately commences s; those nearest the bottom, receiving a certain quantity of heat, expand and rise upwards; the colder portion taking their place, and becoming heated in their turn, rise; small air-bubbles become formed at the bottom after the above process has gone on for a certain time; these bubbles contain aqueous vapour; these rise upwards, and on coming in contact with the colder portions above, are cooled or condensed, and give out their heat to the surrounding particles. This process continues until the whole mass becomes of a uniform heat, and a fixed temperature has been arrived at. After this, however long the heat might be applied to the boiler, the water would not increase in temperature; but the rising of the steam-bubbles would become so rapid that the whole mass would be in a state of agitation, and a vapour would be evolved in large quantities; this vapour, as before stated, is steam, and is invisible, like common air. The fixed temperature already alluded to is what is termed the boiling-point. Under the ordinary atmospheric pressure at the level of the sea, the boiling-point of fresh water is 212° Fahrenheit, that of salt water being somewhat higher, or about 213.2°: in the generation of steam of a high pressure the boilingpoint varies, increasing in proportion to the pressure. With a pressure of 16 lbs. to the square inch, the temperature at which the water boils is 216.3°; at 18 lbs. to the inch, 222-7°; at 20 lbs., 228.4°; at 25 lbs., 240.9°; at 30lbs., 251.4°; at 35 lbs., 260.6°; at 40 lbs. to the square inch, 268.8°; at 50 lbs., 282.7°; at 55 lbs., 288.8°. When steam is generated under a pressure of 15 lbs. to the square inch, it is termed "steam of one atmosphere;" when generated at 30 and 45 lbs. to the square inch, it is said to be of the pressure of steam of two and three atmospheres respectively.

One of the most striking peculiarities of steam is the enormous increase of its bulk, as compared with that of the water from which it is generated.

The proportion of increase will be best remembered by the statement, that under the ordinary pressure of the atmosphere, or 15 lbs. to the square inch, a cubic inch of water will produce a cubic foot of steam; and as there are 1728 cubic inches in a foot, the increase of volume of steam is 1728 times. Under an increase of pressure the volume of steam is diminished: under a pressure of 30 lbs. the volume is only one-half; taking the "relative volume of steam" raised at a pressure of 15 lbs. at 1669, steam at 20 lbs. would have a volume of 1281; at 25 lbs., of 1047; of 30lbs., or double the ordinary pressure, of 882; at 35 lbs., 766; at 40 lbs, 678; at 45 lbs., 609; and at a pressure of 50 lbs. steam would have the relative volume 554. We have before stated, that steam is capable of being reduced from a state of vapour by being reduced in temperature; this temperature is always the same as that of the water from which it is raised. By gradually reducing the temperature of steam the vapour will be condensed, and be reconverted into water: a cubic foot of steam under the ordinary pressure occupying the space of about a cubic inch of water.

In raising steam, a large amount of caloric is absorbed which is not observable by the thermometer; this is termed "latent heat." By this is meant the amount of heat required to evaporate a given quantity of water compared with that necessary to bring the water to the boiling-point. Thus it is found that to evaporate a certain quantity of water into steam at 212°, it will take 5 times as much heat as would raise the water from 32°, or the freezing-point, to 212°, the ordinary boiling-point; this excess of caloric, however, is not indicated by the thermometer,-hence the term latent heat. The latent heat of steam is generally reckoned at 1000°, the temperature of the steam being 212°; the sum therefore of the sensible heat, that is, the temperature indicated by the thermometer, 212°, and the latent heat 1000°, is equal to 1212°. The total amount of the indicated and latent constant sum" thus if the pressure is increased at which the steam is raised, so as to give a temperature of 300°, the latent heat is 912°; if 500°, 712°, and so on. It is from this property of steam that so much fuel is expended in raising it. The mechanical effect produced by the evaporation of a cubic inch of water is generally calculated as being sufficient" to raise a ton weight one foot high;" from this, however, is to be deducted loss from friction and other causes. In the body of the work will be found exemplifications of the properties of

heat at all temperatures is a

steam, and of the duty and the power of steam-engines; we therefore hasten to the consideration of the general subject.

In arranging the materials and illustrations of our work, we have been guided by the same principles which dictated the method of elucidation in our volume, Mechanics and Mechanism, in the present series. We have aimed at presenting practical details rather than elaborate theories; not deeming these unnecessary, but conceiving that for the purposes of our treatise, and for the classes for which it is more particularly designed, correct illustrations and descriptions of the mechanical arrangements constituting the different varieties of steam-engines now in use will be more generally useful than expositions of strictly theoretical rules and mathematical formulæ, which serve in many cases to confuse rather than to enlighten the uninitiated reader, to deter rather than to induce the pupil to proceed in his investigations. The work necessarily assumes the form of a mere compilation; but in addition to consulting the best authorities, and endeavouring to place the results of this in as attractive and regular a form as possible, we are indebted for some of our valuable illustrations to the editor of the Practical Mechanic's Journal, by whose courtesy we have been enabled to enrich our pages. From the number of the illustrations, and the method of arrangement, we venture to hope that our work will form in some measure a useful auxiliary in an Educational Series.

« AnteriorContinuar »