The amount of this added and abstracted heat may now be considered. First, let us assume that the cylinder Fig. 1 has an area of I square foot and that it contains 1 pound of water. The height to which the water rises in the cylinder is 0.016 foot. The pressure on the piston from the air is 14.7 pounds X 144 square inches = 2116.8 pounds. Now on applying heat to the water it will at first gradually rise in tem perature from 32° F. to 212° F. before evapo ration commences. Then 212° — 32° = ISO are the number of heat units required to raise water from 32° F. to the boiling point at atmospheric pressure. Steam now begins to form and the piston to rise until all the water is converted into steam at a temperature of 212° P. This steam, as before stated, occupies a space of 26.36 cubic feet. The heat required to perform this operation is 966 units. hence the total heat required first to raise the water from 32° F. to 212° P. and then to convert it into steam is 1S0+ 966 = 1146 units. It is quite clear how the heat required to raise the water from 32° F. to 212° F. has been expended, but it is not so clear how the 966° F. required to con vert the water at 212° F. into steam at 212° F. has been expended. It will be observed that two things have happened in this last operation. First, the water has been converted into steam, which occupies 1644 times the space occupied by the water from which it was generated. Second, the piston has been raised from the surface of the water in Fig. 1 to the surface of the steam in Fig. 3. Therefore, the heat has been expended in two ways: First, in overcoming the internal molecular resistance of the water in changing its condition from water to steam, and, second, in overcoming the external resistance of the pis ton to the increasing volume of the steam dur ing formation. The first task performed is called the internal work of the steam and the second task is called the external work. Now the share of the heat expended in each operation may he calculated as follows: The total heat expended is, as already stated, 1146 units. To raise the piston with a pressure on it of 2116.8 pounds through a height of 26.36 feet requires 55,799 foot-pounds of energy. As the energy of one heat unit is 772 foot-pounds, then 55, 799 ÷ 772 = 72.3 heat units expended in raising the piston. Adding this to ISO, the number of heat units required to raise water from 32° F. to 212° F., we have 252.3 units consumed in heating the water and raising the piston. This amount subtracted from the total heat expend ed gives 1146-252.3 = 893.7, which is the num ber of heat units expended in internal work. We can now summarize the distribution of the heat as follows: Having gone through the phenomenon of steam generation in detail, we can summarize some of the general facts that have been brourdit out. The temperature of the water gradually rises until it reaches the temperature at which steam is formed. This temperature will depend upon the pressure or the load on the piston. If this pressure is the normal atmospheric pressure of 14.7 pounds per square inch, steam begins to form at a temperature of 212° F.
As soon as 212° F. is reached, steam will begin to form and the piston will steadily rise, but no matter how hot the fire may be, the temperature of both water and steam will remain at 212° F. until all the water is evaporated. We had one pound of water at 32° F. and at 14.7 pounds ab solute pressure, and found that steam formed at a temperature of 212° F. and remained at that temperature. We added 180.9 B. T. U. (British Thermal Units), the heat of the liquid, to bring the water from 32° to the boiling point. To convert water at into steam at 212°. we added 965.7 B. T. U. more. This quantity, known as the latent heat, or heat of vaporization, makes the total heat 1146.6 B. T. U. If we should measure the volume carefully after all the water was evaporated, we should find that there was just 26.36 cubic feet of dry saturated steam. We had one pound of water. and therefore must have one pound of steam, for none of it could escape; hence one cubic foot will weigh = 0.03794 pound, which is known as the density of steam at 14.7 pounds absolute pressure or 212° F. In the table of properties of satu
rated steam all these quantities are found in the order given and at the pressure of 14.7 pounds above vacuum.
Suppose now we place a weight of 85.3 pounds on the piston. The pressure is 85.3 pounds plus 14.7 pounds, or 100 pounds absolute. We shall now find that no steam will form until a tem perature of is reached. Starting with water at it will be necessary to add 297.9 B. T. U. before a temperature of 327.58° is reached, and also we must add 884 B. T. U. more to vaporize it, making a total heat of 1181.9 B. T. U. Under this greater pressure the steam occupies a volume of only 4.403 cubic feet, or one cubic foot of it weighs = 0.2271 pound.
We have already seen that any change in the temperature of saturated steam produces a change of pressure, and that every change of pressure corresponds to a certain change in tem perature. There are several properties of sat urated steam that depend upon the temperature and pressure; and the values of all these differ ent properties when arranged for all tempera tures and pressures are called steam tables. The following are the principal items that are found in the tables: (1) The absolute pressure in pounds per square inch; it is equal to the gauge pressure plus the atmospheric pressure of 1-1.7 pounds.
(2) The temperature of the steam, or boiling water, at the corresponding pressure.
(3) The heat of the liquid; o• the number of B. T. U. necessary to raise one pound of water from 3'2° E. to the boiling point corresponding to the given pressure.
(4) The heat of vaporization, or the latent heat; this is the number of B. T. U. necessary to change one pound of water, at the boiling point, into dry saturated steam at the same tem perature and pressure.
(5) The total heat; o• the number of B. T. U. necessary to change one pound of water from 32° F. into steam at the given temperature or pres sure. The total heat is evidently equal to the sum of the heat of the liquid and the heat of vaporization.
(6) The density of the steam; that is, the weight ill pounds of one cubic foot of steam at the given temperature or pressure.
(7) The specific volume; or volume in cubic feet of one pound of steam at the required tem perature or pressure. Evidently the specific vol ume is equal to density All these properties have been calculated by means of various formulas which have been de duced from the results of actual experiment.
We have seen that a saturated vapor contains" just enough heat to keep it in the form of a vapor; if it loses heat it will condense. A super heated vapor is one that has been heated after vaporization; it can lose this extra heat before any condensation will take. place. A vapor in contact with its liquid is saturated; one heated after removal from the liquid is superheated.
For saturated steam there is a fixed tempera ture for every pressure. If we know either the pressure or the temperature, we can find the other in the steam tables. For instance, if the gauge pressure of a boiler is 60.3 pounds and we wish to know the temperature, we simply add atmospheric pressure and turn to our tallies and find it to be 307° (approximately).
With superheated steam the case is entirely different, for there is no longer the same direct relation between the temperature and pressure. In fact, the relation between temperature and pressure of superheated steam upon the amount of superheating. Superheated steam at 60.3 pounds gauge pressure may have a tempera ture considerably above 307° F. At a given pressure the temperature and volume of a given weight. of superheated steam are always greater than the temperature and volume of the same weight. of saturated steam. The properties of superheated steam at given pressure are not con stant as is the ease with saturated steam.
If superheated steam were a perfect gas, we could determine the relation of p. r. and t by the equation pi' = et; but superheated steam is not a perfect gas, hence we must modify our equa tion. By experiment it has been determined that the following equation is nearly correct: pv = 93.5 t — 971 p in which p = absolute pressure in pounds per square inch, t = absolute temperature, and r = volume of 1 pound in cubic feet.