Steam, Its Generation and Use

by Babcock & Wilcox Company

Vacuum gauges indicate the difference, expressed in inches of mercury, between atmospheric pressure and the pressure within the vessel to which the gauge is attached. For approximate purposes, 2.04 inches height of mercury may be considered equal to a pressure of one pound per square inch at the ordinary temperatures at which mercury gauges are used. Hence for any reading of the vacuum gauge in inches, G, the absolute pressure for any barometer reading in inches, B, will be (B - G) ÷ 2.04. If the barometer is 30 inches measured at ordinary temperatures and not corrected to 32 degrees Fahrenheit and the vacuum gauge 24 inches, the absolute pressure will be (30 - 24) ÷ 2.04 = 2.9 pounds per square inch.

[Pg 118]

[TABLE 19] FACTORS OF EVAPORATION CALCULATED FROM MARKS AND DAVIS TABLES Feed Temp- erature Degrees Fahren- heit Steam Pressure by Gauge 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 32 1.2143 1.2170 1.2194 1.2215 1.2233 1.2233 1.2265 1.2280 1.2292 1.2304 1.2314 1.2323 1.2333 1.2342 1.2350 1.2357 1.2364 1.2372 1.2378 1.2384 1.2390 40 1.2060 1.2087 1.2111 1.2131 1.2150 1.2168 1.2181 1.2196 1.2209 1.2221 1.2231 1.2241 1.2250 1.2259 1.2267 1.2274 1.2282 1.2289 1.2295 1.2301 1.2307 50 1.1957 1.1984 1.2008 1.2028 1.2047 1.2065 1.2079 1.2093 1.2106 1.2117 1.2128 1.2137 1.2147 1.2156 1.2164 1.2171 1.2178 1.2186 1.2192 1.2198 1.2204 60 1.1854 1.1881 1.1905 1.1925 1.1944 1.1961 1.1976 1.1990 1.2003 1.2014 1.2025 1.2034 1.2044 1.2053 1.2061 1.2068 1.2075 1.2083 1.2089 1.2095 1.2101 70 1.1750 1.1778 1.1802 1.1822 1.1841 1.1859 1.1873 1.1887 1.1900 1.1911 1.1922 1.1931 1.1941 1.1950 1.1958 1.1965 1.1972 1.1980 1.1986 1.1992 1.1998 80 1.1649 1.1675 1.1699 1.1720 1.1738 1.1756 1.1770 1.1785 1.1797 1.1809 1.1819 1.1828 1.1838 1.1847 1.1855 1.1863 1.1869 1.1877 1.1883 1.1889 1.1895 90 1.1545 1.1572 1.1596 1.1617 1.1636 1.1653 1.1668 1.1682 1.1695 1.1706 1.1717 1.1725 1.1735 1.1744 1.1750 1.1760 1.1766 1.1774 1.1780 1.1786 1.1792 100 1.1443 1.1470 1.1493 1.1514 1.1533 1.1550 1.1565 1.1579 1.1592 1.1603 1.1614 1.1623 1.1633 1.1642 1.1650 1.1657 1.1664 1.1671 1.1678 1.1684 1.1690 110 1.1340 1.1367 1.1391 1.1411 1.1430 1.1448 1.1462 1.1477 1.1489 1.1500 1.1511 1.1520 1.1530 1.1539 1.1547 1.1554 1.1562 1.1569 1.1575 1.1581 1.1587 120 1.1237 1.1264 1.1288 1.1309 1.1327 1.1345 1.1359 1.1374 1.1386 1.1398 1.1408 1.1418 1.1427 1.1436 1.1444 1.1452 1.1459 1.1466 1.1472 1.1478 1.1484 130 1.1134 1.1161 1.1185 1.1206 1.1225 1.1242 1.1257 1.1271 1.1284 1.1295 1.1305 1.1315 1.1324 1.1333 1.1341 1.1349 1.1356 1.1363 1.1369 1.1375 1.1381 140 1.1031 1.1058 1.1082 1.1103 1.1122 1.1139 1.1154 1.1168 1.1181 1.1192 1.1203 1.1212 1.1221 1.1230 1.1239 1.1246 1.1253 1.1260 1.1266 1.1272 1.1278 150 1.0928 1.0955 1.0979 1.1000 1.1019 1.1036 1.1051 1.1065 1.1078 1.1089 1.1099 1.1109 1.1118 1.1127 1.1136 1.1143 1.1150 1.1157 1.1163 1.1169 1.1176 160 1.0825 1.0852 1.0876 1.0897 1.0916 1.0933 1.0948 1.0962 1.0975 1.0986 1.0997 1.1006 1.1015 1.1024 1.1033 1.1040 1.1047 1.1054 1.1060 1.1066 1.1073 170 1.0722 1.0749 1.0773 1.0794 1.0813 1.0830 1.0845 1.0859 1.0872 1.0883 1.0893 1.0903 1.0912 1.0921 1.0930 1.0937 1.0944 1.0951 1.0957 1.0963 1.0969 180 1.0619 1.0646 1.0670 1.0691 1.0709 1.0727 1.0741 1.0756 1.0768 1.0780 1.0790 1.0800 1.0809 1.0818 1.0826 1.0834 1.0841 1.0848 1.0854 1.0860 1.0866 190 1.0516 1.0543 1.0567 1.0587 1.0606 1.0624 1.0638 1.0653 1.0665 1.0676 1.0687 1.0696 1.0706 1.0715 1.0723 1.0730 1.0737 1.0745 1.0751 1.0757 1.0763 200 1.0412 1.0439 1.0463 1.0484 1.0503 1.0520 1.0535 1.0549 1.0562 1.0573 1.0584 1.0593 1.0602 1.0611 1.0620 1.0627 1.0634 1.0641 1.0647 1.0653 1.0660 210 1.0309 1.0336 1.0360 1.0380 1.0399 1.0417 1.0432 1.0446 1.0458 1.0469 1.0480 1.0489 1.0499 1.0508 1.0516 1.0523 1.0530 1.0538 1.0544 1.0550 1.0556

[Pg 119]

The temperature, pressure and other properties of steam for varying amounts of vacuum and the pressure above vacuum corresponding to each inch of reading of the vacuum gauge are given in [Table 20] .

[TABLE 20] PROPERTIES OF SATURATED STEAM FOR VARYING AMOUNTS OF VACUUM CALCULATED FROM MARKS AND DAVIS TABLE Vacuum Ins. Hg. Absolute Pressure Pounds Temperature Degrees Fahrenheit Heat of the Liquid Above 32 Degrees B. t. u. Latent Heat Above 32 Degrees B. t. u. Total Heat Above 32 Degrees B. t. u. Density or Weight per Cubic Foot Pounds 29.5 .207 54.1 22.18 1061.0 1083.2 0.000678 29 .452 76.6 44.64 1048.7 1093.3 0.001415 28.5 .698 90.1 58.09 1041.1 1099.2 0.002137 28 .944 99.9 67.87 1035.6 1103.5 0.002843 27 1.44 112.5 80.4 1028.6 1109.0 0.00421 26 1.93 124.5 92.3 1022.0 1114.3 0.00577 25 2.42 132.6 100.5 1017.3 1117.8 0.00689 24 2.91 140.1 108.0 1013.1 1121.1 0.00821 22 3.89 151.7 119.6 1006.4 1126.0 0.01078 20 4.87 161.1 128.9 1001.0 1129.9 0.01331 18 5.86 168.9 136.8 996.4 1133.2 0.01581 16 6.84 175.8 143.6 992.4 1136.0 0.01827 14 7.82 181.8 149.7 988.8 1138.5 0.02070 12 8.80 187.2 155.1 985.6 1140.7 0.02312 10 9.79 192.2 160.1 982.6 1142.7 0.02554 5 12.24 202.9 170.8 976.0 1146.8 0.03148

From the [steam tables] , the condensed [Table 21] of the properties of steam at different pressures may be constructed. From such a table there may be drawn the following conclusions.

[TABLE 21] VARIATION IN PROPERTIES OF SATURATED STEAM WITH PRESSURE Pressure Pounds Absolute Temperature Degrees Fahrenheit Heat of Liquid B. t. u. Latent Heat B. t. u. Total Heat B. t. u. 14.7 212.0 180.0 970.4 1150.4 20.0 228.0 196.1 960.0 1156.2 100.0 327.8 298.3 888.0 1186.3 300.0 417.5 392.7 811.3 1204.1

As the pressure and temperature increase, the latent heat decreases. This decrease, however, is less rapid than the corresponding increase in the heat of the liquid and hence the total heat increases with an increase in the pressure and temperature. The percentage increase in the total heat is small, being 0.5, 3.1, and 4.7 per cent for 20, 100, and 300 pounds absolute pressure respectively above the total heat in one pound of steam at 14.7 pounds absolute. The temperatures, on the other hand, increase at the rates of 7.5, 54.6, and 96.9 per cent. The efficiency of a perfect steam engine is proportional to the expression ( t - t ₁ )/ t in which t and t ₁ are the absolute temperatures of the saturated steam at admission and exhaust respectively. While actual engines only approximate the ideal engine in efficiency, yet they follow the same general law. Since the exhaust temperature cannot be lowered beyond present practice, it follows that the only available method of increasing the efficiency is by an increase in the temperature of the steam at admission. How this may be [Pg 120] accomplished by an increase of pressure is clearly shown, for the increase of fuel necessary to increase the pressure is negligible, as shown by the total heat, while the increase in economy, due to the higher pressure, will result directly from the rapid increase of the corresponding temperature.

[TABLE 22] HEAT UNITS PER POUND AND WEIGHT PER CUBIC FOOT OF WATER BETWEEN 32 DEGREES FAHRENHEIT AND 340 DEGREES FAHRENHEIT Temp- erature Degrees Fahren- heit Heat Units per Pound Weight per Cubic Foot Temp- erature Degrees Fahren- heit Heat Units per Pound Weight per Cubic Foot Temp- erature Degrees Fahren- heit Heat Units per Pound Weight per Cubic Foot Temp- erature Degrees Fahren- heit Heat Units per Pound Weight per Cubic Foot 32 0.00 62.42 89 57.00 62.13 146 113.86 61.27 203 170.95 60.05 33 1.01 62.42 90 58.00 62.12 147 114.86 61.25 204 171.96 60.02 34 2.01 62.42 91 59.00 62.11 148 115.86 61.24 205 172.96 60.00 35 3.02 62.43 92 60.00 62.09 149 116.86 61.22 206 173.97 59.98 36 4.03 62.43 93 60.99 62.08 150 117.86 61.20 207 174.97 59.95 37 5.04 62.43 94 61.99 62.07 151 118.86 61.18 208 175.98 59.93 38 6.04 62.43 95 62.99 62.06 152 119.86 61.16 209 176.98 59.90 39 7.05 62.43 96 63.98 62.05 153 120.86 61.14 210 177.99 59.88 40 8.05 62.43 97 64.98 62.04 154 121.86 61.12 211 178.99 59.85 41 9.05 62.43 98 65.98 62.03 155 122.86 61.10 212 180.00 59.83 42 10.06 62.43 99 66.97 62.02 156 123.86 61.08 213 181.0 59.80 43 11.06 62.43 100 67.97 62.00 157 124.86 61.06 214 182.0 59.78 44 12.06 62.43 101 68.97 61.99 158 125.86 61.04 215 183.0 59.75 45 13.07 62.42 102 69.96 61.98 159 126.86 61.02 216 184.0 59.73 46 14.07 62.42 103 70.96 61.97 160 127.86 61.00 217 185.0 59.70 47 15.07 62.42 104 71.96 61.95 161 128.86 60.98 218 186.1 59.68 48 16.07 62.42 105 72.95 61.94 162 129.86 60.96 219 187.1 59.65 49 17.08 62.42 106 73.95 61.93 163 130.86 60.94 220 188.1 59.63 50 18.08 62.42 107 74.95 61.91 164 131.86 60.92 221 189.1 59.60 Temp- erature Degrees Fahren- heit Heat Units per Pound Weight per Cubic Foot Temp- erature Degrees Fahren- heit Heat Units per Pound Weight per Cubic Foot Temp- erature Degrees Fahren- heit Heat Units per Pound Weight per Cubic Foot Temp- erature Degrees Fahren- heit Heat Units per Pound Weight per Cubic Foot 51 19.08 62.41 108 75.95 61.90 165 132.86 60.90 222 190.1 59.58 52 20.08 62.41 109 76.94 61.88 166 133.86 60.88 223 191.1 59.55 53 21.08 62.41 110 77.94 61.86 167 134.86 60.86 224 192.1 59.53 54 22.08 62.40 111 78.94 61.85 168 135.86 60.84 225 193.1 59.50 55 23.08 62.40 112 79.93 61.83 169 136.86 60.82 226 194.1 59.48 56 24.08 62.39 113 80.93 61.82 170 137.87 60.80 227 195.2 59.45 57 25.08 62.39 114 81.93 61.80 171 138.87 60.78 228 196.2 59.42 58 26.08 62.38 115 82.92 61.79 172 139.87 60.76 229 197.2 59.40 59 27.08 62.37 116 83.92 61.77 173 140.87 60.73 230 198.2 59.37 60 28.08 62.37 117 84.92 61.75 174 141.87 60.71 231 199.2 59.34 61 29.08 62.36 118 85.92 61.74 175 142.87 60.69 232 200.2 59.32 62 30.08 62.36 119 86.91 61.72 176 143.87 60.67 233 201.2 59.29 63 31.07 62.35 120 87.91 61.71 177 144.88 60.65 234 202.2 59.27 64 32.07 62.35 121 88.91 61.69 178 145.88 60.62 235 203.2 59.24 65 33.07 62.34 122 89.91 61.68 179 146.88 60.60 236 204.2 59.21 66 34.07 62.33 123 90.90 61.66 180 147.88 60.58 237 205.3 59.19 67 35.07 62.33 124 91.90 61.65 181 148.88 60.56 238 206.3 59.16 68 36.07 62.32 125 92.90 61.63 182 149.89 60.53 239 207.3 59.14 69 37.06 62.31 126 93.90 61.61 183 150.89 60.51 240 208.3 59.11 Temp- erature Degrees Fahren- heit Heat Units per Pound Weight per Cubic Foot Temp- erature Degrees Fahren- heit Heat Units per Pound Weight per Cubic Foot Temp- erature Degrees Fahren- heit Heat Units per Pound Weight per Cubic Foot Temp- erature Degrees Fahren- heit Heat Units per Pound Weight per Cubic Foot 70 38.06 62.30 127 94.89 61.59 184 151.89 60.49 241 209.3 59.08 71 39.06 62.30 128 95.89 61.58 185 152.89 60.47 242 210.3 59.05 72 40.05 62.29 129 96.89 61.56 186 153.89 60.45 243 211.4 59.03 73 41.05 62.28 130 97.89 61.55 187 154.90 60.42 244 212.4 59.00 74 42.05 62.27 131 98.89 61.53 188 155.90 60.40 245 213.4 58.97 75 42.05 62.26 132 99.88 61.52 189 156.90 60.38 246 214.4 58.94 76 44.04 62.26 133 100.88 61.50 190 157.91 60.36 247 215.4 58.91 77 45.04 62.25 134 101.88 61.49 191 158.91 60.33 248 216.4 58.89 78 46.04 62.24 135 102.88 61.47 192 159.91 60.31 249 217.4 58.86 79 47.04 62.23 136 103.88 61.45 193 160.91 60.29 250 218.5 58.83 80 48.03 62.22 137 104.87 61.43 194 161.92 60.27 260 228.6 58.55 81 49.03 62.21 138 105.87 61.41 195 162.92 60.24 270 238.8 58.26 82 50.03 62.20 139 106.87 61.40 196 163.92 60.22 280 249.0 57.96 83 51.02 62.19 140 107.87 61.38 197 164.93 60.19 290 259.3 57.65 84 52.02 62.18 141 108.87 61.36 198 165.93 60.17 300 269.6 57.33 85 53.02 62.17 142 109.87 61.34 199 166.94 60.15 310 279.9 57.00 86 54.01 62.16 143 110.87 61.33 200 167.94 60.12 320 290.2 56.66 87 55.01 62.15 144 111.87 61.31 201 168.94 60.10 330 300.6 56.30 88 56.01 62.14 145 112.86 61.29 202 169.95 60.07 340 311.0 55.94