Superheated steam has a lower thermal conductivity than saturated steam, and this quality reduces the leakage of heat to the cylin der walls by 12 per cent. The radiation from pipes carrying superheated steam is less, also. The fuel expense of superheating steam is not large: the amount of fuel required to raise steam to 100° of superheat—that is, to 312°, being an increase of only 5.69 per cent over that required to produce the satu rated steam of 212°. The limit to which superheat may be added to water confined in a boiler is about equivalent to a pressure of 1,574 pounds per square inch. Above this point the proper lubrication of the cylinders of reciprocating engines, and the maintenance of packings of valves and condens ing apparatus in normal condition is impracti cable. The practical limit of safe pressure in modern boilers is 200 pounds per square inch, equivalent to a temperature of about 380°, or 168° of superheat. In the United States the range of superheat practice does not exceed 500 degrees, and there are few instances in which it exceeds 450°. In Europe, however, few plants are installed without superheaters, and the temperatures range from 600° for ordinary to 850° as the top limit. For these very high pressures specially built boilers are required, and the extra cost of such work offsets the advantages which may be gained. °Live steam" is steam which has performed no work, or rather which is available for the performance of work. Steam which has performed work is called exhaust steam. When a pound of water at 212° is turned into steam at 212°, 965.7 heat units are ab sorbed in the change of form. This energy reappears when the steam returns to the form of water. Herein lies the vast economic profit in the utilization of exhaust steam in the heating of feed water for the boiler, warming the shop, etc. Every pound of exhaust steam the change from a liquid to a gaseous state takes place beneath the surface, the steam ing to the surface and escaping with ebullition.
The boiling point — that is, the temperature at which steam is formed, depends on the pres sure under which it is generated. If the water is confined to a limited space, as in a boiler, if the pressure above it is relieved with an airpump, the boiling point will be lowered. If pressure is brought to bear on the water in the boiler, the boiling point is raised. This in effect, what happens in ordinary boiler practice: the pressure is increased by making more steam than there is room for in the boiler above the water. The steam being comnressible, accumu lates in a state of compression, and the boiler pressure rises, as shown by the steam-gauge. The following table gives the relations between pressure, volume and total heat of steam for temperatures between 32° F. and 428° F. Dur ing the first stage of heating, all the heat sen sibly goes into increasing the internal energy of the fluid. This is represented in the last column of the table. During the second stage the heat taken in is known as latent heat of steam. The total heat of both stages is repre sented by the numbers in the fourth column of the table. The latent heat of the steam is de by subtracting the heat content of the liquid from the heat content of the steam.
has within itself the potency of 751,314.6 foot pounds of heat energy, imparted to it by the fuel. If even but a part of this latent heat energy can be used, an equivalent amount of fuel is saved, or, what practically amounts to the same thing, so much of the fuel is used twice.
Vaporization.— When heat is applied to water, a point is reached at which the heat overcomes the cohesion and the pressure of the atmosphere, then the water passes into vapor. Evaporation takes place at the surface of the water. Its rapidity varies with the temper ature and the pressure upon that surface.
When a flask containing water (see Fig. I) is placed over the flame of a lamp, the air dis solved in the water is first driven off, then as the temperature of the water rises, the liquid molecules in contact with the bottom of the flask become so hot that the heat is able to overcome their cohesion, the weight of the overlying water and the pressure of the atmos phere above the water. At this temperature Dulong and Arago determined the tension of steam many years ago by means of the apparatus shown in Fig. 2.
In the figure (k) is a copper boiler, with a tube (a) containing a thermometer (t), which meas ures the temperature of the water, and its vapor. The tension of the steam is measured by a manom eter (m). The steam passing through the tube (c) exerts a pressure on a column of water in the tube (1). This pressure is further transmitted to the mer cury in the vessel (d), and thence to the manometer. By taking the manometer readings correspond ing to each degree of the ther mometer, a direct measurement of tension was obtained up to a pres sure of 24 atmospheres, and from this on by calculation. The fol lowing is a table of results: Regnault, 14 years later, devised a method by which the vapor of water could he measured at temperatures above or below boiling point. By this method the following tensions were obtained for temperatures ranging from 10° below to 101° above zero, of the Centigrade scale.
• In the making of steam in boilers it is of the greatest importance that clean, soft water be used. The condensation of the salts in boiler water eventually reaches a point where they are deposited as scale on the inside of the plates and tubes, a condition in which much fuel is wasted. (Sec WATER SOFTENING).
The Energy of Steam.— Water has the greatest specific heat of any known substance, except hydrogen. By this we mean that more heat enters into it, in order to raise its tem perature one degree, than into any other sub stance, with the one exception mentioned. Its stored energy as noted above is 965.7 thermal units per pound of water, Fahrenheit scale. It is easily condensed, giving out this energy. These facts, together with its universal and abundant presence in large quantities, have ren dered steam, tip to this time, a favorite means for the generation of mechanical power. The process of changing steam into mechanical power may be briefly outlined as follows: If we start with water at 32° F. and apply heat, the temperature of the water will rise one degree for each thermal unit, but expansion does not begin until 38° to 40° of tem perature are reached. When 180/2 units of heat have been absorbed the temperature of the water will be found to be 212° F. and its expansive force (into a vacuum) equal to 14.7 pounds to the square inch, or that of the atmos phere at sea-level. At this point the water is incapable of becoming any hotter tinder that pressure. The heat added, after that point is reached, is used in converting the water into steam, and 965.7 thermal units are required for each pound of water thus converted. This so called latent heat is stored energy, to be given back again in mechanical work and heat, as the steam is condensed. If we enclose both water and steam in a boiler of suitable con struction, and continue heating, part of the water will be vaporized, but being prevented by the confined space from expanding, it crowds the steam already enclosed into a smaller vol ume and the pressure upon the surface of the water is increased so that the heat now added increases temperature again. When we have added 1,188.6 total thermal units (including temperature and latent heat) the pressure or energy of the steam will be equal to 101.6 pounds to the square inch and the temperature will have risen to 329° F. (See table I, columns 1, 2 and 4).