CHAP. VII. SUPERHEATED STEAM. PROPERTIES OF STEAM. -COMMON STEAM. (93.) SINCE the application of the expansive action of steam involves the consideration of its properties when it ceases to be in contact with the water from which it was produced, and likewise the variation of its pressure in different states of density and at different temperatures, it is necessary here to explain some of the most important of these properties of vapour. Steam may exist in two states, distinguished from each other by the following circumstances :— 1st. It may be such that the abstraction from it of any portion of heat, however small, will cause its partial condensation. 2d. It may be such as to admit of the abstraction of heat from it without undergoing any other change than that which air would undergo under like circumstances, viz. a diminution of temperature and pressure. (94.) We shall call, for distinction, the former Common Steam, and the latter Superheated Steam. To explain the circumstances out of which these properties arise, let B (fig. 29.) be imagined to be a vessel filled with Fig.29. . water, communicating by a pipe and stopcock A D with another vessel A, which in the commencement of the process may be conceived to be filled with air. Let D be a pipe and stopcock at the top of this vessel. If the vessel в be heated, and the two cocks be opened, the steam proceeding from the water in в will blow the air out of the vessel A through the open stopcock D, in the same manner as air is blown from a steam engine. When the vessel a by these means has been filled with pure steam, let both stopcocks be closed. If the steam in A, under these circumstances, have a pressure of 15 lbs. per square inch, its temperature will be found to be 213°. Now, if any heat be abstracted from this steam, its temperature will fall, and a portion of it will be reconverted into water. B Again, suppose the vessel a to be filled with pure steam which has been produced from the heated water in B, the stopcock c being open. Let the stopcock c be then closed, and the water in в be heated to a higher temperature, the temperature and pressure of the steam in a being observed. If the stopcock c be now opened, the steam in a will be immediately observed to rise to the more elevated temperature which has been imparted to the water in B, and at the same time it will acquire an increased pressure. The increase of temperature which it has received would of itself produce an increased pressure; but that this is not the sole cause of the augmented pressure in the present case might be proved by weighing the vessel A. It would be found to have increased weight, which could only arise from its having received from the water in в an additional quantity of vapour. The increased pressure therefore, which the steam in a has acquired, is due conjointly to its increased density and its increased temperature. In general, if the water in the vessel B be raised or lowered in temperature, the steam in the vessel A will rise and fall in temperature in a corresponding manner, always having the same temperature as the water in B. If the weight of the vessel A were observed, it would be found to increase with every increase of temperature, and to diminish with every diminution of temperature, proving that the augmented temperature of the water in B produces an augmented density of the steam in A. The same pressure would be found always to correspond to the same temperature and density, so that if the numerical amount of any one of the three quantities, the temperature, the pressure, or the density, were known, the other two must necessarily be determined, the same temperature always corresponding to the same pressure, and vice versa. And in like manner, steam produced under these circumstances of the same density cannot have different pressures. It must be observed that the steam here produced receives all the heat which it possesses from the water from which it is raised. Now it is easily demonstrable, that this is the least quantity of heat which is compatible with the steam maintaining the vaporous form; for if the stopcock c be closed so as to separate the steam in a from the water in B, and that any portion of heat, however small, be then abstracted from the steam in A, some portion of the steam will be reconverted into water. This then, according to the definition already given, is Common Steam. (95.) Let us now suppose that the vessel a, being in communication with the vessel B by the open stopcock, has been filled with pure steam of any given temperature. The steam which it thus contains will be common steam, and, as has been shown (94.), it cannot lose any portion of heat, however small, without being partially condensed; but let the stopcock c be closed, and let the steam in a be then exposed to any source of heat by which its temperature may be raised any required number of degrees. From the steam thus obtained heat may be abstracted without producing any condensation; and such abstraction of heat may be continued without producing condensation, until the steam is cooled down to that temperature at which it was raised from the water in B, when the stopcock c was opened. Any further reduction of temperature would be attended with condensation. If after increasing the temperature of the steam in A, the stopcock c being shut so as to render it superheated steam, its pressure be observed, the pressure will be found to be increased, but not to that amount which it would have been increased had the steam in a been raised to the same temperature by heating the water in в to that temperature, and keeping the stopcock open. In fact, its present augmented pressure will be due only to its increased temperature, since its density remains unchanged. But if in these circumstances the stopcock c be suddenly opened, the pressure of the steam in a will as suddenly rise to that pressure which in common steam corresponds to its temperature; and if the vessel A were weighed, it would be found to have increased in weight, proving that the steam contained in it has received increased density by an increased quantity of vapour proceeding from the water in a. In fact, by opening the stopcock the steam which was before superheated steam, has become common steam. It has the greatest density which steam of that temperature can have; and consequently, if any heat be abstracted from it, a partial condensation will ensue. To render these general principles more intelligible, let us suppose that the water in B is raised to the temperature of 213°, the stopcock c being open; the vessel A will then be filled with steam of the same temperature, and having a pressure of 15 lbs. per square inch. This will be common steam. If the stopcock be now closed, and the whole apparatus be exposed to the temperature of 243°; the steam in A will preserve the same density, but its pressure will be increased from 15lbs. to a little more than 16lbs. per square inch. Let the stopcock c be then opened and while the temperature of the steam in a shall continue to be 243°, the pressure will suddenly rise from 16lbs. to about 26lbs. per square inch. The weight of the steam in A will be at the same time increased in the same proportion of 16 to 26 as its pressure. The steam thus produced in a will then be common steam, and any abstraction of heat from it would be attended with partial condensation. (96.) The law, according to which the pressure of elastic fluids in general, whether gases or vapours, increases with their temperature, was simultaneously discovered by Dalton and Gay Lussac. If the pressure which the gas or vapour would have at the temperature of melting ice, were expressed by 10,000, then the increase of pressure which it would receive for every degree of temperature by which it would be raised, its volume being supposed to be preserved, would be expressed by 2083. Thus, if the pressure of gas, or vapour, on a surface of a certain magnitude at the temperature of 32° were 10,000 ounces, then the same gas or vapour would acquire an additional pressure of 208 ounces for every degree of temperature which would be imparted to it above 32°. This law is common to all gases and vapours. It may be objected that water cannot exist in the state of vapour under the usual pressures at so low a temperature as melting ice. This, however, does not hinder the application of the above law, for that law will equally hold good by computing the pressure which the vapour would have if it were a permanent gas, and if it could therefore exist in the elastic form at that low temperature. riotte. (97.) Another law, common to all elastic fluids, and of equal importance with the former, was discovered by MaBy this law it appears that every gas or vapour, so long as its temperature is unchanged, will have a pressure directly proportional to its density. If therefore, while we compress steam into half its volume, we could preserve its temperature unaltered, we should increase its pressure in a two-fold proportion; but if the process of compression should cause its temperature to to increase, |