The mechanical power exerted is the effect of the physical changes produced in water, by great and sudden alterations of its temperature. By an addition of heat, it is changed from a dense incompressible liquid, into an invisible, highly elastic fluid, the bulk of which exceeds, by many hundred times, that of the water of which it is formed. This change of state, however, is not permanent ; an abstraction of the heat from the steam reducing it again to water. These changes, either separately or combined, are the cause of motion in every modification of the steam engine.
The effects of heat upon water are of two kinds ; it first raises the temperature until it reaches a certain point, when it becomes stationary ; and the continued action of heat then converts the water into steam ; the temperature of which remains the same as that of the water from which it is formed. But although the intensity of the heat of steam, as indicated by the thermometer, is the same as that of the boiling water; the quantity of heat in a given weight of steam, exceeds considerably the quantity of heat in the same weight of boiling water. This heat is that which is necessary to maintain the water in the state of vapour; and if any portion of it be withdrawn, a corresponding portion of steam is rendered liquid. From the heat not being apparent by the thermometer, it is termed latent heat. The quantity of latent heat combined with steam, is variously given by different authors; the best experiments appear to fix it at about 1000, or about 5 1/2 times the heat of boiling water; it being found that l lb of water converted into steam, will raise 5 1/21bs of water from 32° to 212o Fahr. From various experiments it likewise appears, that the latent heat is nearly the same, whatever be its temperature as indicated by the thermometer.
The temperature at which water boils or is converted into steam depends upon the pressure upon the surface of the water. Exposed simply to the common pressure of the atmosphere, water boils at 212° Fahr.; but upon the top of lofty mountains, where the mercury in the barometer indicates a much smaller pressure, the temperature of the boiling point is proportionately lower; and on the other hand, if water be confined in a vessel from which there is no escape for the steam until it attains a certain pressure, the temperature of the boiling point will be proportionately raised.
In like manner the pressure of the steam varies with and is proportional to the temperature at which it is formed. The determination of the elastic force of steam of different degrees of temperature, is of the greatest importance to the practical engineer, and the subject has consequently undergone much investigation. The experiments of Watt, Dalton, Robins, Southern, Ure, Arsberger, and Philip Taylor have all been of great service in determining this question.
The two tables given for greater perspicuity in the next page - the first being the result of a series of experiments made by Dr. Dalton, the second supplied by the Royal Academy of France, in their report upon the comparative degrees of safety between high and low pressure engines - are inserted as being not only essential when contained in a close Vessel, taken at every 10° of Temperature from 212o Fahrenheit, (the Boiling Point,) up to 320°.
Pressure of Steam, or the Force which it will exert to enter into a vacuous Space.
Pressure of the Steam against the Atmosphere, when the Barometer is at 30 Inches, or the Force it will exert to escape from the closed Vessel into the open Air.
Column of Mercury.
Column of Water.
Column of Mercury.
equal to the
Height of Mercury in
Temperature of Fahrenheit.
Pressure per Square Inch, in lbs. Avoirdupois.
essential to the practical engineer, but of the greatest interest to the scientific engineer. They differ in no very material point from other calculations that have been made, and are quite near enough to be adopted as a standard for guidance in mechanical operations.
The volume of steam into which a given quantity of water expands, depends upon the pressure of the steam. At the temperature of 212° , one volume of water furnishes nearly 1,800 volumes of steam, of a pressure equal to that of the atmospheric, or about 151bs on the square inch. With an increased pressure its bulk is diminished, but in what ratio is not quite settled. From the experiments of Mr. Southern and also those of MM. Clements and Desorme, all conducted with great care, the same law appears to hold good with respect to steam, as with other aeriform fluids, that the density is directly as the pressure, or the volume inversely as the pressure. Thus, if at the pressure of the atmosphere or 151bs per square inch 1 volume of water will furnish 1,800 volumes of steam; at the pressure of 2 atmospheres or 30lbs, it will give 900 volumes; at4 atmospheres450 volumes, etc. and this is the theory most generally adopted.
The motion of steam engines is derived from the following causes, namely: -
First, From the direct pressure of steam upon the piston. Second, From the condensation of steam on one side of a piston, and by the vacuum thus effected, obtaining the pressure of the atmosphere on the other side. Third, From the combined action of the pressure of steam on one side of a piston, and of a vacuum on the other side effected by condensation. To these may be added a fourth, which, for want of a more appropriate term,hasbeencalled the "reaction" of steam. The theory of the action in each case we will now briefly explain.
It has already been observed that water when converted intosteam is increased in its bulk about 1800 times, at atmospheric pressure. It follows that a cubic inch of water will furnish a cubic foot of steam. Let us suppose a cylinder whose area is equal to one square foot, to contain a cubic inch of water; on the surface of which rests a steam tight piston, but which is free to move without friction, and that the weight of the piston is counterbalanced by a weight suspended from a line running over a pulley. If heat be now applied to the bottom of the cylinder, and the water be thereby converted into steam, the piston will be raised one foot high in the cylinder. In this case, however, no mechanical power is obtained, as the steam has merely overcome the pressure of the atmosphere. But let us suppose that in addition to the pressure of the atmosphere upon the piston, a load equal to 15lbs upon each square inch of its area, be placed upon it; the pressure of the steam will in this case be doubled, and will consequently raise the piston six inches in the cylinder; and the mechanical effect willbe, 144 times 151bsraised through the space of 6 inches.