Table of Properties of Steam and Water (cont.) 0 32 4 1 10 20 68 40 50 1091.7 1091.7 1091.7 1074.1 39.2 1093.9 1086.7 1071.7 50 1097.2 1079-2 1079-3 1063.3 8.83 108,731 1102-7 1066.7 1067.1 1052.5 8.89 58,868 30 86 1108-2 1054.2 1055.0 1041.6 8.94 33,385 104 1113.7 1041.5 1042.9 1030-8 8.98 19,723 122 1119-2 1029.0 8.74 211,131 216,600 ... ... 59,300 ... 19,810 1030.9 1020-0 9.04 12,079 9.06 7,672 993.4 9.10 7,725 5,027 14; column 16 is found by subtracting column 6 from 14; column 17 is calculated by Griffiths from various data; columns 18 and 19, see p. 54. Table of Properties of Steam and Water (cont.) 50 122 60 140 70 158 : : : : : : : ... : 9.95 9.67 9:53 9:44 9.36 9.83 9-61 9:48 9:39 9.33 9.74 9.55 9.44 9-36 9:30 9-69 9-56 9:44 9-36 9-29 9-23 110 230 9-93 9.84 9.64 9.52 9.42 9.34 9.28 9-22 120 248 9.80 9.63 9.59 9:49 9:40 9-33 9-27 9-22 130 266 9-67 9-42 9:58 9:48 9.39 9.33 9.27 9.22 140 284 9.54 9-21 957 9:47 9:39 150 302 9-67 9.51 9:40 9.33 9.27 9.24 9-83 9-619-47 9:37 9.30 9.25 9.22 9.73 9.56 9:44 9.35 9.28 9.23 9-20 9.65 9.51 9:40 9:32 9.26 9.22 9-19 9-16 9-76 9-60 9.47 9-38 9.30 9.24 9-20 9-17 9-15 9-19 9-17 9:15 9-19 9.16 9.14 9-19 9-16 9.14 9.12 Column 21 gives Fairbairn and Tate's experiments recalculated; column 22 gives Ramsay and Young's experiments calculated by author. surrounding boiler. In their summary Fairbairn and Tate also depart from the principles laid down by them, and instead of assuming the saturation point to have been reached when the first change of level was observed, they waited until a considerable difference of pressure showed itself. The fluctuations recorded by them of the levels of the two mercury columns are also not satisfactory; but the general principle on which the experiments were carried out is a good one, and, in spite of the above objections, sufficient reliance was placed on their results to cause these to be recalculated. The low-pressure tests were not made on the same principle and have been rejected; of the high-pressure ones only Nos. 1, 4, 7, 8, 9, 13 gave marked indications as to when the point of saturation had been reached, and only these have been used in col. 21. The values against 266° F. are the mean results. Ramsay and Young's experiments extend over a much wider range of temperature, but the volume of steam experimented on was only a little larger than one cubic centimeter, or say one-thousandth of the volume employed by Fairbairn and Tate. It would, therefore, not be wise to rely on them absolutely, even although they may help to explain the great difference to be found between the weight of steam as measured on the indicator cards and the weight of feed water. These experiments are, however, of great comparative value, in so far as that they show a gradual decrease of density as the super-heat is increased. Their experiments extend from 284° F. to 518° F., and also from 59° F. to 122° F., but the lower temperature experiments are few, and not quite in accord with each other, and certainly are in direct conflict with Rankine's deductions. They were made in another apparatus. It is probable, therefore, that the increase of density with decreasing low temperature shown in the table may not be correct. Their experiments made at 392° F. and 464° F. are quite out of harmony with the rest, and have not been taken into account in the compilation of the table. = The densities in cols 21 and 22 are expressed in terms of hydrogen gas = 1 at 0° C. = 32° F. 491° F. absolute, and 760 mm. pressure 147 lbs. per square inch. The weight of hydrogen is 0089551 gram per liter. The specific volume is the volume in c.c. occupied by 1 gram steam and is easily found from the table. At According to the chemical analysis one would expect the density of steam to be 8.98. This low value is probably only reached when the steam is thoroughly superheated and under a low pressure. any rate in Ramsay and Young's experiments the value was still above 90 when the pressure was 52 lbs., and the temperature 518° F., this being a superheat of 234° F. Total Heat of Steam.-Regnault's experiments on the total heat of steam, from 145° F. to 381° F., adapt themselves very closely to the formula H=1101·5 – 0·305 T where T is the temperature F. This formula has been used in the compilation of column 14. by subtracting the total heat in the water from the total heat in the steam. Regnault carried out experiments on the specific heat of water. Col. 1, the total heat in water, is the sum of these values reckoned from 32° F. (see col. 3). These values have to be subtracted from col. 14, the difference being the latent heat of steam. In recent years the specific heat of water has been much enquired into, and Regnault's values are not accepted as correct; another series by Velten, cols. 4, 5, 6, is therefore embodied in the table, and the latent heats, col. 17, found by subtraction from col. 14. As will be seen there is a considerable difference between cols. 15 and 16, even enough to affect the results of economic trials when reduced to evaporation from and at 212° F. Recently Griffiths has recalculated these values (see col. 17). As Regnault's results are those generally adopted, these have been accepted throughout this work. Water Density and Compressibility.-Up to a temperature of 212° F. experiments on density of water have usually been made under a pressure of one atmosphere, whereas above this point the pressures have been various. It has therefore been necessary to correct the densities found by Hirn, and by Ramsay and Young, for pressure, and this involves a knowledge of the elastic compressibility of water at all temperatures. Col. 7 gives this value up to 500° F., from 32° F. to 140° F. being values by Pagliani and Vicentini, and above this point the values are taken from Ramsay and Young's experiments. With the help of these values col. 8 has been compiled. It contains the densities of water at the various boiling temperatures. It should, however, be noted that these values are not direct observations, and it is more than probable (see retarded ebullitions, p. 62) that just before bursting into steam there is a relatively large increase of water volume. Critical Temperature and Pressure. When a volatile fluid like water is heated under pressure, it will be noticed that at a certain temperature, called the critical temperature, water and steam cannot be distinguished from each other. No increase of pressure will effect condensation. The lowest pressure at which this change takes place is called the critical pressure. According to Cailletet and Colardeau the critical point is reached at 2005 atmospheres and 680° F. (see table, p. 53); above this temperature water does not exist, only steam. At the critical temperature it is evident that there is no latent heat of evaporation, because there is no evaporation. This means that if the values of the latent heats in cols. 15, 16, 17 were extended to 690° F., they would be reduced to nothing. Regnault's values seem far more likely to fulfil this condition than the other two. It is also evident that at the critical temperatures the volume of water must exactly equal the volume of steam, for the two are then identical; this means that if cols. 8 and 19 and 20 were extended, their values ought to grow equal to each other when a temperature of 690° F. is reached. Seeing that water on being heated under pressure approaches nearer and nearer to the conditions of steam, and that steam and water are undistinguishable from each other at the critical temperature, it is more than likely that the change from steam to water or vice versá even at low temperature is not so abrupt as is generally believed. The following further properties of water may on occasion be of service. The Boiling Point of Salt Water is higher than that of pure water, so that steam evolved from brine must be slightly superheated; but it is only with the greatest difficulty, even in laboratory experiments, that this has been verified. Boiling Temperatures of Salt Water Gerlach, Zeitschr. f. analyt. Chemie,' 1887, vol. xxvi. p. 413. The Specific Heat of Salt Water is less than that of pure water, as will be seen from the following table. As the density of water increases with the added salt, the heat capacity of water does not sink quite at the same rate as the specific heat. J Thomsen, Pogg. Ann.,' 1871, vol. exlii. p. 337; A. Winkelmann, Wied. Ann.,' 1873, vol. cxlix. p. 1; Marignac, Ann. de Chim.,' 1876, vol. viii. p. 41. Air absorbed by Water.-Air and all gases are absorbed by water and change its density. Thus H. J. Chaney (Trans.,' 1892, vol. 183a, p. 334) finds that 1 cubic foot of water saturated with air is 321 grains lighter than pure water, or about 0.075 per cent. (1 cubic foot weighs about 435,933 grains). It was formerly believed that all air could be expelled by boiling; more recent experiments show that this is not the case; but the view is still held that at a given temperature a fluid absorbs a given volume of gas quite independent of the pressure. The weight of absorbed gas is therefore proportional to the gas pressure. Experiences as regards occluded gases in metals throw doubts even on this theory. In the following table the volumes absorbed at various temperatures are given, and also the weight of absorbed gas if the pressure is one atmosphere. When a mixture of several gases presses on a fluid, the effective pressure of one of these gases is the product of the combined pressure into the volumetric proportion of the gas. Thus for air, where the volumetric percentages of oxygen and nitrogen are as 208 to 79-2, oxygen only exerts a pressure of 0.208 atmosphere, and nitrogen 0·792 atmosphere, and the weight of oxygen absorbed by water when in contact with air is only 0-208 of what it would be if pure oxygen were in contact with the water. Air and steam behave in the same way. |